This started out as a small chiding letter to Dirt Bike Magazine about how unbelievable it was that their EFI four stroke race bike could waste so much fuel. The letter however ballooned out into a 30 page rant and rave session. After considerable editing and revision over a five day period though it turns out to actually be one of my better pieces of writing. I have included it here on saileva.com both because the letter is just a nice piece of writing, but also because a few new concepts are introduced that have not previously been addressed in my technical articles. For the most part these new ideas are just three things. One is that smaller cylinder port two strokes have tended to require higher compression ratios than larger two strokes simply because the larger two strokes are over square engines with more room for wider and shorter cylinder ports. Another new concept I introduce in this letter is that an earlier time of late compression ignition up at very high engine speeds in excess of perhaps about 10,000RPM can allow higher mean piston speeds than would normally be possible on the same fuel. I also introduce a list of ten parameters of gasoline engine operation, which is a more concise synopsis of how a gasoline engine works than saileva.com previously had. Most of the letter is however redundant in that the same information and observations have been published in my previous technical articles.
An Open Letter to Dirt Bike Magazine
A 3.2 gallon tank to go 50 miles on an EFI 250F! I just can’t believe that high of fuel consumption. That is what carbureted two strokes suck down, four strokes just aren’t that inefficient. I once rode with a guy on a KX500 and he seemed to need most of the big five gallon IMS oversize tank he had to do the approximately 50 mile single track ride. That was a carbureted 500cc two stroke on tight trails though. Another guy on the same ride was on a Husky 360 two stroke with a 3.5 gallon tank, and he was able to finish the ride easily with gas to spare. Out on more open desert trails both of those bikes would have gone a whole lot farther on the gas they carried. Two strokes do tend to suck down lots of fuel when they are jetted rich, tuned for maximum power output and ridden hard with the clutch slipping at the top of the (narrow) power band all the time. I could only just barely finish 45 mile sections of ISDE qualifier enduro races on my CR125 with the stock 1.9 gallon tank. And that was with the needle clip position leaned out one notch for 6,000 foot elevations. Even on more wide open dirt roads that fuel hungry 125 two stroke got hardly better than 25mpg, which seemed horrible for such a small engine. Many 250 two strokes were able to lay down much more power than the 125 while using hardly any more fuel, still though I have heard it said that about 17mpg is all that can be expected from a 250 two stroke being ridden hard on tight single track trails. Two strokes are just extremely inefficient at the top of the power band with the clutch slipping. Not only is the combustion efficiency poor for such a rich jetted engine, but a whole lot of the fuel just gets blown out the exhaust unburned when a two stroke is kept at the top of the power band all the time. Just how much fuel a two stroke uses depends a lot on how it is tuned. My 1986 Husqvarna WR400 for example seems well able to get down to less than 1 GPH on casual trail rides. It is not exactly fuel efficient, but compared to other two strokes it seems to do remarkably well. The WR400 has quite a bit of range on it’s stock 3.4 gallon tank, the main problem is that it is so darn expensive to fill that big 3.4 gallon tank with gas and premix.
Four strokes just don’t use that much fuel. My 1991 Husqvarna WMX 610 has been known to do as poorly as 25mpg on rides with lots of tight first and second gear single track when it was running poorly with lots of hesitation, but it always delivered 40 to 50mpg on big fast roads. Even up to very high speeds on the highway it always kept on delivering 50mpg. When running better with less hesitation tight trails don’t seem to cut into the fuel mileage nearly so much and I have gotten 45 to 50mpg in mixed rides with lots of small dirt roads and some single track. My 1991 Husqvarna WXE 350 usually also got 50mpg, but it would deliver that mileage over a much wider range of conditions. Even entirely on very tight first and second gear single track it would do 40 mpg when it was running well and 50mpg in mixed rides with lots of single track and some small dirt roads was a common occurrence. If it was hesitating badly the mileage would drop off to 35 to 45mpg in mixed rides or even all the way down to 25mpg on tight single track trails. Out on the open highway the WXE 350 was known to do as well as 60mpg at quite high speeds. That’s not very good for a four stroke at all, but dirt bikes just always seem to be extremely fuel hungry.
A smaller 250cc engine does even better on tight trails. Even a carbureted 250F uses less fuel than the WXE 350 on tight trails. A 250F would tend to see less improvement in mileage at higher speeds, but still it is going to get the best mileage when it is in higher gears going faster. Some 250F dirt bikes have been known to do 70 or even 80mpg in casual cruising on small and twisty highways. A race bike geared low, tuned to run overly rich and being run hard certainly is going to suck down more fuel, but it seems totally inconceivable that the stock two gallon tank on the WR250F would not go 50 miles.
I would estimate that running very rich the WR250F wide open at 11,000RPM at 94mph would be able to burn no more than 4GPH which would be 23mpg, and obviously any time it goes slower the mileage is going to spike up above 50mpg. It just does not seem that there is any way that any amount of over richening the mixture and ridding fast and hard would be able to use up the stock tank on the WR250F in just 50 miles.
For comparison a 1000cc sport bike going fairly fast around a road race circuit is said to get about 35mpg. That’s four times the displacement, four times the maximum power output and much much higher speeds. Of course there is the fact that the transmissions on both a 250F and a four cylinder liter bike are about the same size and capacity. Said another way the transmission losses are going to be just as high for the 250F as for the liter bike. It is the abysmal transmission efficiency on single cylinder dirt bikes that makes them unable to do well on fuel. The thing is though that these high transmission losses are a much larger problem in lower gears and at lower power output such as ridding on a tight single track trail. Up in higher gears at higher power output levels the transmission losses on a single cylinder engine are less of a problem. This is why good running dirt bikes get so much better mileage at higher speeds. Even a 250F will go the most miles on a gallon of gasoline when it is in higher gears running at higher power output levels.
Obviously a single cylinder engine is very compact and fits nicely into a dirt bike chassis, but the transmission losses are a problem for efficiency and range. These problems are made much worse when the transmission is incorrectly sized. A single cylinder two stroke would tend to have only half as much trouble with low transmission efficiency, but that is only true if the transmission is correctly sized. The transmission and primary reduction on my 1986 WR400 are exactly the same as on my 1991 WMX 610. Obviously that same transmission that works well for the 577cc four stroke is quite a bit oversized for the 396cc two stroke. In fact that very same transmission and clutch was also used on the 125 and 250 two strokes, just with a bit more primary reduction so that it did not have to spin so fast.
I used to think that transmission was too small for the big thumper, after a few hundred hours my old five speed began to get clunky feeling and would no longer shift as reliably. I put that same five speed 610 motor in my 1992 TE chassis with just a bit of valve work and a new piston pin and it has been working pretty well. I have only put 25 hours on the new 610 powered Showa chassis, but the motor seems to be doing great. The fact that the transmission now seems to be quieter and smoother probably has something to do with the full synthetic oil I switched to. The 15-50 full synthetic just seems much tougher than the old 20-50 conventional oil I always used to run. I also used to think that running the small transmission on the big 577cc engine required the removal of the rather low 2.6:1 first gear. With no more than 1.8:1 reduction in the five speed transmission it would indeed be able to handle much higher torque throughput from a big single cylinder four stroke engine. As it turns out though the six speed does in fact work just fine with the big thumper. I have over 100 hours now on the rebuilt 1997 six speed motor that I put in my 1991 WMX 610 chassis, and I have not been going particularly easy on it either. I have it geared up with 15/50 sprockets and I often do a bit of rock climbing in first gear. That 15-50 full synthetic oil just seems like some really tough stuff.
In Europe they sold the six speed 610 geared up with 17/48 sprockets for autobahn cruising and the transmission apparently held up fairly well to this service as well. I’ll bet not many of the 610 Husqvarnas did much hard core off road ridding with the 17/48 gearing, but just pulling out up steep paved streets in first gear with that big powerful motor would have been test enough. For me the 15/50 gearing is high enough. It likes to cruise down the highway at just 50 to 60mph instead of 70 or 80mph, but it does also pull up to 115mph at 8,500RPM quite easily. The stock 14/50 gearing I was running for a while probably is the ideal gearing. It was a bit better for tight trails because the jump between first and second did not seem so large. In the higher gears though the spacing was still too tight with the 14/50 sprockets, and the 15/50 gearing actually delivers faster acceleration at higher speeds because there is less shifting. Of course I tend to place a premium on less shifting because I usually let off and pull in the clutch to shift. The Husqvarna transmissions will just bang gears under a bit of a load like any good dirt bike, but after doing this for a while the corners of the engagement pawls get roughened up and the transmission won’t shift as easily anymore. I have always found that I can bang some gears under a load every once in a while to make a pass or something without doing much long term damage to the transmission. If I start banging the gears all the time when I ride though then within just a few hours the transmission becomes harder to shift. What is amazing though is that clutching all the shifts for another period of several hours smoothes the corners out and the transmission once again will shift easily and reliably. Just one more interesting combination of a barely functional race bike design and long lasting casual cruiser rolled into one.
The other barely functional race bike design on the early Husqvarna four strokes is of course the reed valve lubricating system. It sort of works for a dirt bike, but it does not like sustained high engine speeds. Revving out briefly to 8,000 and even 8,500RPM for maximum power on the 610 works fine, but the big four stroke will in fact sometimes rev even higher after the power levels off. It would be very easy to toast a 610 motor by ridding it like a two stroke. Leave it pinned and use the clutch to modulate power delivery and you’d be lucky to make it through a practice lap let alone a desert race. If someone managed to develop a technique of slipping the clutch on the 610 so that they did not destroy the rod bearing then they would also be faced with a rather small clutch, the same size as on a 125 two stroke. The clutch on the 610 does hold up to some slipping, being an old 125 rider I do tend to slip the clutch when I get into tricky situations. The reality though is that I tend to slip the clutch on tough technical single track trails where the amount of power required is the 30 or 35hp that a 125 two stroke can make as opposed to the much larger amount of power that the 610 is capable of making.
With the throttle just cracked open the 610 will happily stay around 4,000RPM as the clutch is modulated to deliver rather small amounts of power. Interestingly this works a whole lot better with the flywheel removed and a points ignition installed than it does with the stock ignition system. Part of this is that the big four pounds of flywheel weight is simply excessive for anything other than 1200 to 1600RPM tractoring. Being an old 125 rider I am used to laying down much more power than the 610 can make at 1200 to 1600RPM so I tend to slip the clutch up at 3,000 or 4,000RPM anyway. Another aspect of why clutch slipping at 4,000RPM works so much better with the points ignition system is the crankshaft wiggle advance. The additional three or four degrees of spark advance that comes on all of a sudden when the inertia of the upward moving piston and rod overcomes the compressive force on the piston is an interesting phenomenon and can be used to advantage on a dirt bike in a number of different ways. Normally the crankshaft wiggle advance coming up at 6,000RPM (or as high as 7,000RPM with a radically lightened 332g piston) just helps the engine to more easily rev out to make maximum power without such excessive spark advance that 3,000RPM operation is extremely compromised.
Since the compressive force on the piston is smaller when the throttle is closed and some intake vacuum is being pulled the crankshaft wiggle advance actually does some other interesting things as well. One is a sort of a load dependent advance mechanism. With the throttle partially closed and some intake vacuum being pulled the crankshaft wiggle advance comes at a somewhat lower engine speed. What this means is that over some range of engine speeds around 5,000 to 6,000RPM the engine appears to run better over a range of loads than would normally be the case with a fixed advance curve. At least within that narrow range of engine speeds and over a certain range of engine loads the points ignition Husqvarna can be made to run like a bike with a throttle position sensor. The other interesting thing that the load dependence of the crankshaft wiggle advance does is allow more precise engine speed holding when slipping the clutch to modulate power delivery. With the throttle just cracked open a slight bit the crankshaft wiggle advance comes at a lower engine speed perhaps as low as 4,000 or 4,500RPM. If the engine is running crisply enough that it is able to enter late compression ignition mode with that very small throttle opening then it will hold steady at one engine speed over a range of light loads. The way this works is that the crankshaft wiggle advance is giving it’s additional three or four degrees of advance at say 4,500RPM, but when a load is applied the engine speed drops down and the spark timing is then three or four degrees later at the static timing setting. If the engine is running crisply enough then the later spark timing will actually produce more power and the engine speed then hovers right around the speed at which the crankshaft wiggle advance comes on. When the throttle is opened wider the crankshaft wiggle advance does not come on until a rather much higher engine speed where the larger amount of spark advance is necessary for the engine to run in late compression ignition mode under a heavy load. If on the other hand the engine is not running crisply enough then it will hesitate badly around 3,500 to 4,000RPM with small throttle openings and then when the crankshaft wiggle advance hits at 4,000 or 4,500RPM it will take off and make much more power with the same small throttle opening.
When I first got my new 11.3:1 Husqvarna 610 motor together last April it seemed to be working pretty well on the stock ignition with the spark timing backed off as far as it would go (about 25 degrees BTDC). It was making lots of power and often it would even rev all the way out to 8,500RPM despite the absolutely fixed spark timing from 2,000RPM all the way up. I did notice that the flywheel inertia was making clutch modulation difficult, but other than this slight problem on tight trails and hard starting it seemed to be working great. When that 1992 SEM ignition system also completely failed in June I switched back to a points ignition and I was just blown away by the difference. On the first ride out with the new points I was climbing a rather easy but twisty and small single track trail through the woods and I was so impressed with how much better the power delivery was with the points ignition that I swore I would never go back to a fixed advance curve CDI again.
I have ridden some bikes with fixed advance curve CDI ignition systems that appeared to work just great though, so I know they can be made to work well. The 1997 Yamaha WR250 is a good example of a great two stroke with a wide power band and easy power modulation. I rode one that was a year old once late in the summer of 1997. I was riding with a couple of guys that I had met at the parking area, and this guy with the WR 250 just could not stop talking about how great his bike was. Every time we stopped for a little rest it was just on and on about how it was an off-road specific model but aside from a little bit weak compression damping it worked great out on the motorcross track as well, and on and on about how the power delivery was better than a ZY even though it might not pull quite as hard up at the top end but that actually it did pull as hard as most 250 two strokes despite the wider power band and better power delivery. Finally he asked if he could ride my 125, and I rode his WR 250 for a while. That was it, no more about the WR after that. Then all he could talk about was the surprising top end power of the decade old CR 125.
When it was running well my old 125 did pull hard way up high at above like 9,000RPM, but I doubt if it was more than a stock mid 1990s CR 125 was able to do. What was unique about the 1987 CR 125 with the Pro Circuit pipe and the needle clip position leaned out was that it just kept revving. Across the entire low end and midrange it pulled about like a Honda 50, in other words smooth and consistent but with very little power. Then when the power hit around 8,000RPM it hit pretty hard and built up to even more power at 10,000RPM before flattening off and pulling even longer. The surprising thing was just that it pulled hard up top but then also kept pulling long enough to actually get into the next gear without slipping the clutch. Part of this was the Pro Circuit pipe, but the leaner needle clip position also played a roll. If the engine was able to run crisply on the leaner mixture at 8,000RPM then dumping more fuel with the main jet got more revs than some other 125 motorcrossers were able to do. As for the 1997 WR250 the two things that made a big impression on me were that it was very comfortable to ride either sitting or standing and that the power came on at such a low engine speed. The fact that the WR250 made somewhat more power way down at like 5,000RPM than the 125 did screaming over 10,000RPM was very impressive. What really stands out though is the fact that the 1997 WR 250 actually had substantial modulateable power over a range of engine speeds.
I also thought that the Kawasaki KDX 200 was a great trail bike aside form it’s irrationally high weight and less than race spec suspension. The Honda XR’s also often ran pretty well, but again the high weight and overall lack of inspiring performance severely detracted from anything good that the tune of the motor was able to deliver. I also rode a street legal early eighties Yamaha XT350 once that had great power delivery and a surprising amount of midrange get up and go, again though it weighed so much that it was a hazard on the trail. The reason I got to ride it was that my friend Luke could not get it up the tough climbs. Despite great power delivery the weight was just too much for him. With a little bit of coaching he was however able to blast my 125 up those same moderately tough climbs while I stood up on the XT and yanked it this way and that with all my might. Actually how I started ridding the XT was that the second time Luke crashed and got it stuck in the bushes it was so far down the hill that I told him I was not going to help him yank it out a third time. It was either turn around or he rode the 125 which was much easier to extract from the bushes. He did crash and get the 125 stuck in the bushes a few times, but amazingly he caught on very quickly to the 125 ridding style and we were able to finish out the rest of the ride without much difficulty. I have always remembered the great power delivery of that XT350, and the Honda XR 250 that I had for a few years did pretty well also. The weight of those things was just way too much though and top end power output was not impressive either. The XR 250 would keep revving and revving but the power just never materialized. I could never understand how a 250cc four stroke that would rev so high could not make anywhere near as much power as the 125 two stroke. It gave a great low end grunt from way down at 3,000RPM and pulled pretty hard through the midrange also, up on top though it was just flat and dead even though there seemed to be no end to the revs. I see now that the XR 250 was rated by Honda to do 19hp maximum output at 8,000RPM, half of what even the weaker 250F bikes can do.
In fact that XR 250 was able to rev so high that a friend who had learned to ride on my 125 managed to bend an intake valve when he rode it. Oh yeah, my instruction to him for ridding the two stroke had been “Just keep revving it until it stops making power!” Not necessarily such good advice for a crappy four stroke.
Luke and his father (who rode some sort of extremely expensive Harley that was always in the shop for repairs) thought that the XT350 was very weak on power. I thought they were nuts. I never rode it faster than about 50mph on a small dirt road, but it was obvious to me that the only thing really wrong with it was that it just weighed so darn much. It pulled great in the midrange from 5,000 to 7,000RPM, felt like it was very willing to rev higher and was usable with pretty good torque all the way down to 3,000RPM. I mean what did they want from an air cooled four stroke? Melted pistons like the Harley reliably delivered.
For the most part the fixed advance curve CDI bikes just ran like crap. I remember loads of various hunks of junk people had that would hardly run bellow 3,000RPM and then would become hideously loud and harsh above 6,000RPM without making any power at all. Probably the worst CDI bike I ever saw was the 2002 Yamaha TTR 225 that I had for about a year. The motor itself was a screamer, despite the fact that it was just an air cooled two valve mill it pulled pretty hard all the way up to 9,000RPM. The ignition system and carburetor on the TTR 225 were just horrendous though. The stupid vacuum slide Mikuni would hesitate at first, and then it would get super rich and the engine was loud and harsh and did not make much power in the midrange around 4,000 to 5,000RPM. I scraped that vacuum slide Mikuni and put a 34mm DellOrto on the TTR225 and this helped a whole lot. The DellOrto was not as overly rich so the engine did not make quite as much screaming power to 9,000RPM, but the much higher level of control over the entire range of engine speeds was well worth the power sacrifice. The DellOrto could have been richened up to do the same thing on the top end as the Mikuni, but I had already gotten the piston to melt just a bit to where it was using oil, smoking and hard starting. With the leaner DellOrto the top end worked itself in and in 10 hours of ridding the motor was working well again. The only problem with the TTR225 motor then was that the ignition system just flat out had the wrong advance curve. Even when it was running well in the midrange and able to rev out it sometimes would barely run down at 2,000 to 3,000RPM. It could have been sort of fixed by richening up the DellOrto down low, but that is not a repair, it is just causing more problems. I sold the Yamaha at least able to handle reasonably good throttle modulation over a range of engine speeds even if it was rather “cold blooded” down at 2,000 to 3,000RPM sometimes. The girl that bought it was just thrilled to have a bike that fired up easily with a button (push start as she called it), and I showed her boyfriend how to richen up the DellOrto if the need arose.
Of course now that the gasoline is nearly always for 13:1 and 14:1 engines none of those 9:1 and 9.5:1 motors would even run. My 9.7:1 rebuilt 1991 Husqvarna WXE 350 motor with the XR400 rod was running really great for a while on a points ignition with the static timing setting at 23 degrees BTDC (about 26 or 27 degree BTDC spark timing at high engine speeds). Then for a while I had to bump the static timing setting up to 31 and 33 degrees BTDC to get it to make power, and now recently it just won’t make power on the super high pressure fuel that comes out of the pumps. It will rev to 7,000RPM and move the bike along a bit in full flame front travel mode, but there is just no late compression ignition and no power. Even with the static timing setting all the way up at 40 degrees BTDC it just would not go. Then another day on different gasoline it was able to make some power from 5,000 to 8,000RPM with the static timing setting at 33 degrees BTDC. If I backed off a few degrees though again it was just in full flame front travel mode and would not make much power.
It was the same way with the stock 10.2:1 1991 WXE 350 all through 2014. A few times I got fuel that ran great at static timing settings around 23 to 25 degrees BTDC (spark timing of 27 or 29 degrees BTDC at higher engine speeds with the crankshaft wiggle advance) and would rev out to over 10,000RPM. I even had it lighting off on late compression ignition a bit at 4,000 feet of elevation down to a static timing setting of 18 degrees BTDC once, but of course it was hesitating horribly at lower engine speeds, had to be well warmed up before it would make power from 5,000 to 7,000RPM and it would not rev all the way out. Most of the time though I had to go all the way up to a static timing setting of around 30 to 33 degrees BTDC to get it to light off on late compression ignition at all. With that much advance on the small 3.3 inch bore engine it was just supper loud and harsh at lower engine speeds but it did still make power up at 7,500 to 9,000RPM.
So far my trusty old stock 10.2:1 1991 WMX 610 motor has not failed me. Even though I have to go up to 29 degrees BTDC on the static timing setting sometimes it always runs and pulls hard to 8,000RPM. I have taken to putting the gasoline in the 10.2:1 motor first to make sure it will run before I put it in my new 11.3:1 motor. If it will run and make power to 8,000RPM in the 10.2:1 motor at 29 degrees BTDC then I can run the 11.3:1 motor down at around 25 to 26 degrees BTDC even though the 11.3:1 motor is jetted leaner with the needle clip in the leanest position.
This technique has actually worked well, and I have been able to get the 11.3:1 motor to run crisply much of the time with less than 27 degrees BTDC on the static timing setting. There for a while before I instituted the 10.2:1 into the 11.3:1 swapping procedure I was having to run the 11.3:1 motor up at 30 and 31 degrees BTDC on the static timing setting much of the time and it was just super loud and super harsh with dramatically reduced torque across the entire midrange.
I have also noticed that although the old stock 10.2:1 motor requires the fastest flame front travel speed fuel to rev out to 8,000RPM my new hot rod 11.3:1 motor with the bigger valves and slightly longer duration camshaft is often able to rev all the way out to 8,500RPM on slightly slower flame front travel speed gasoline. I can even run regular 87 (RON+MON)/2 octane rating gasoline with fairly good results sometimes, where the old stock 10.2:1 motor just always needed premium. What I do notice with the slightly slower flame front travel speed fuel though is that the motor is just not as efficient cruising along at low power output levels at more than 3,500RPM. On faster flame front travel speed fuel with the engine running crisply it just smoothly transitions from full flame front travel mode cruising down the highway in sixth gear at 4,000RPM to late compression ignition mode at 4,500RPM even with the static timing setting down at 23 degrees BTDC. On the slightly slower flame front travel speed fuel it does not even really help much when the static timing setting is up at 28 degrees BTDC unless the engine is really super crisp. The only thing that gets good light load operation with the slower flame front travel speed fuel is when the engine is just so crisp that it lights off on late compression ignition easily at 4,000RPM with just a very small twist of the throttle. The problem with this though is that then I completely lose the ability to deliver torque at 3,000 to 3,500RPM. Just another manifestation of the universal reality that slower flame front travel speed fuels result in narrower ranges of operable engine speeds and narrower ranges of engine loads where good efficiency can be delivered. This is even true on computer controlled port injected engines, although fancy computer compensations can go a long way to getting any fuel to run better over a wider range of operating conditions. These fancy compensations are even pretty easy to program in if the fuel remains the same all the time. If on the other hand the fuel is changing every time you go to the gas station then the only thing that is going to get a computer controlled vehicle to work at it’s best is a closed loop feedback system with something like an oxygen sensor and lots of fancy measuring and adjusting algorithms.
Just to be clear though most computer controlled port injected engines are not able to compensate for large changes in the pressure and temperature capabilities of the fuel. The only way that the requirement of a good match between the compression ratio of the engine and the temperature and pressure capabilities of the fuel can be removed is with a variable boost system like a computer controlled variable vane geometry turbocharger or with a variable volumetric efficiency system like a computer controlled variable valve timing system. This is probably why nearly all of the new automotive engines have very sophisticated variable valve timing systems. These sophisticated variable valve timing systems are however not good for much if the compression ratio of the engine is 10.5:1 and the fuel is for 14:1 engines. And all of the port injected automotive engines from recent years do in fact have very similar compression ratios, they are nearly all at 10.5:1 or 10.6:1 with a few outliers slightly off in both directions. A variable valve timing system on a 10.5:1 engine would still be good for delivering high torque over a wide range of engine speeds if the fuel was for 10.5 or 11.5:1 engines. When the fuel is for 14:1 engines though even a variable valve timing system is not going to help much with getting a 10.5:1 engine into late compression ignition mode. If the 10.5:1 engine can get into late compression ignition at all on fuel for a 14:1 engine it is going to be extremely loud, harsh and inefficient and torque production will be much less than if there was a better match between the compression ratio and the temperature and pressure capabilities of the fuel.
There are a few current models down at 9.8:1 or so and there is the Ford Fiesta normally aspirated 1.0 liter port injected three cylinder at 11.0:1 and the port injected 2.0 liter four cylinder Honda Civic at 10.8:1. For the most part though the standard compression ratio has become 10.5:1 and 10.6:1 for port injected automotive engines. This just is not at all compatible with the 13.0:1 street bike engines, is even less compatible with the 13.5:1 Japanese 250F bikes and just really looks out of this world incompatible with the new 14.4:1 KTM 250SX-F.
And full circle back to the 2015 WR250F, at 13.5:1 it’s now the highest compression ratio engine ever eligible for a green sticker in California. The 15:1 Husqvarna 125 two strokes form decades gone by don’t get this honor because a two stroke runs about the same as a four stroke with a compression ratio about 85% as high. The 15:1 Husqvarna 125 two stroke would be equivalent to perhaps about a 12.5:1 four stroke, and my 12.3:1 1986 Husqvarna WR400 is equivalent to about a 10.75:1 four stroke. This is a rather rough estimate as different cylinder port two strokes attain different levels of maximum cylinder filling. The quite significantly oversquare WR400 with short and wide cylinder ports attains a higher level of maximum cylinder filling than does the high revving square bore and stroke 125 with it’s relatively taller ports. Again though don’t get confused and think that just because a 125 two stroke needs a higher compression ratio than a 400 two stroke this then transfers over to smaller four strokes needing higher compression ratios. It does not work that way. There are some reasons why larger or smaller four stroke engines might tend to require slightly higher or lower compression ratios, but there are actually several different phenomenon that tend to cancel each other out here. Yes, a higher revving smaller engine tends to have a harder time flowing well and attaining as high of a volumetric efficiency as a slower turning engine, but very fast turning small engines also get hotter and therefore would require a slightly lower compression ratio for the same level of cylinder filling. And to confuse things farther smaller engines typically operate at somewhat lower mean piston speeds than larger engines, which can go either way in terms of what it means for the required compression ratio. What it comes down to is that there are always slight differences from one engine design to another, but maximum compression ratios for four stroke engines tend to be remarkably consistent for all high performance engines running on the same fuel.
Prior to the 2015 Yamaha WR250F getting the same 13.5:1 compression ratio that the Yamaha YZ250F and the rest of the Japanese 250F bikes have had it was the KTM 250 XCF-W that had the highest green sticker legal compression ratio at 12.8:1. That 12.8:1 was right in there with the 13.0:1 street legal sport bikes, so there was no discrepancy within the motorcycle industry. As Dirt Bike magazine was fond of printing in 2013 and 2014 the absolute maximum compression ratio for pump gas was considered to be about 13.0:1. The problem with this though was that the 13.0:1 maximum compression ratio for the port injected motorcycles was not compatible with the 10.5:1 and 10.6:1 standard compression ratio for port injected automotive engines built in the same years. Now that a few models of port injected automotive engines are up to 11.0:1 the old long standing 11.5:1 maximum compression ratio in the motorcycle industry looks more realistic. I don’t really know one way or another, but if I had to guess I would bet on the 11.5:1 absolute maximum compression ratio for fast turning motorcycle engines that attained torque peaks up around 7,000 to 8,000RPM. I believe that would tend to be about the same as the 10.5:1 compression ratio for slower turning pushrod street rods that was widely publicized throughout the 1990’s and into the first decade of the 21st century. The long stroke pushrod engines do their best with maximum volumetric efficiency down at around 3,000 to 4,000RPM where it is relatively easy to attain good flow.
The longstanding problem with three and a half and four inch stroke pushrod engines being the standard for comparison though is that those engines actually delivered the fastest times at the drag strip with much longer duration camshafts than was appropriate for any sort of other application. A 260 degree at 0.05” valve lift camshaft in a four inch stroke pushrod engine is a total mismatch. The reason those seemingly irrationally long duration camshafts did well at the strip was the simple fact that four inches is way too darn long of a stroke for any gasoline engine. Because gasoline engines need to spin up to rather high speeds to run well even a very long four inch stroke gasoline engine is going to twist some massive maximum engine speeds through the traps at the top end. It turns out that either a 260 degree at 0.05” valve lift camshaft in a two valve per cylinder big block or a 260 degree at 1mm valve lift camshaft in four valve per cylinder motorcycle is the correct duration for drag racing a gasoline engine. On a radically oversquare four valve per cylinder motorcycle the 260 degree at 1mm valve lift camshaft delivers maximum cylinder filling at around 8,000RPM and this also typically corresponds to a rather high volumetric efficiency up close to 100%. On a square bore and stroke big block Chevy pushrod engine the 260 degree at 0.05” valve lift camshaft also works best up towards 8,000RPM. The difference is that the two parallel valves per cylinder operated by pushrods on an engine that is not oversquare does not attain close to 100% volumetric efficiency at 8,000RPM. By 8,000RPM the big block Chevy is choking hard and won’t flow. The reason that the 260 degree at 0.05” valve lift camshaft wins the races is that a nice constant torque curve over a wide range of engine speeds is what is required to come out of the hole fast in a drag race. Since gasoline engines don’t make power well at all at less than 6,000RPM the big block Chevy just can’t start at much lower of an engine speed even though the four inch stroke is so long that the range of operable engine speeds would tend to be from 2,500 to 5,000RPM not 4,000 to 8,000RPM as is the case with the race winners. Of course there is also the simple fact that higher revs mean more power, and drag racing engines of any type tend to operate at rather high mean piston speeds.
What really gives away the inappropriateness of the four inch stroke gasoline engine though is the fact that in other forms of automotive racing shorter three and quarter inch strokes are preferred and shorter duration camshafts are often used to deliver big torque all the way down to 4,00RPM out of corners even though 4,000RPM is obviously far too slow to spin a gasoline engine. The reason that the engine speed is stretched down to 4,000RPM on shorter stroke engines is the same reason that the engine speed is stretched out to 8,000RPM on four inch stroke drag racing engines, to broaden the range of operable engine speeds. The point of all of this about pushrod dinosaurs is that excessively long duration camshafts have often been used in drag racing and sometimes other forms of racing not because they are appropriate for the stroke length of the engine they are in, but rather simply because racers have been trying hard to work around the severe limitations of two parallel valve per cylinder push rod engines that won’t flow well. The result is pushrod drag racing engines that don’t ever attain high cylinder filling and therefore don’t ever attain as high of torque peaks as would be expected for the displacement and the fuel being used.
With a mild stock 190 degree at 0.05” valve lift camshaft a three and a half inch stroke automotive engine attains maximum cylinder filling way down at like 3,000RPM. At that low of an engine speed it does not matter what kind of valve train the thing has, it is going to attain very close to 100% volumetric efficiency rather easily with a short duration camshaft. Drop a set of 10.5:1 pistons in that otherwise stock engine and that is the maximum compression ratio, or very close to it. At that maximum compression ratio there is not going to be much spark advance though, so it is entirely understandable why lots of people instead preferred to go down to 10.0:1 or even all the way down to 9.5:1 just to get some substantial advance for clean and efficient full flame front travel mode operation under light loads on a mechanically controlled engine. With a good functioning load dependant advance mechanism it is however not absolutely necessary to drop off much from the maximum compression ratio. If the vacuum advance on a carburetor was set up just right for a specific set of operating conditions with jetting and spark timing also aimed at a specific application and driving style then the 10.5:1 pistons worked great. What confused a lot of people though was that everything always came from the factory with distributors setup for like 35 degrees of spark advance under a full load at 3,000RPM. That sure as heck is not going to work close to the maximum compression ratio.
Running one of those otherwise stock three and a half inch stroke automotive engines at 10.5:1 would require a centrifical advance mechanism that advances only 10 degrees of crankshaft rotation instead of 25 degrees of crankshaft rotation, and it is going to have to top out at the 5,000RPM maximum engine speed instead of topping out at 3,000RPM. It would be 10 degrees BTDC on the static timing setting, 20 degrees BTDC maximum full load advance at 5,000RPM and another 10 degrees of vacuum advance as the throttle closes. Getting something like that to work is tricky, and when you are done you have a three and a half inch stroke automotive engine with extremely heavy rods and pistons and a piss poor valve train that runs well over only a very narrow range of engine speeds. Even though it might idle great down to 800RPM it will pull hard over only a very narrow range of engine speeds from 3,000 to 4,000RPM and then it flattens out up to 5,000RPM where it still does not run well because it is somewhat too high of a mean piston speed and the volumetric efficiency has dropped way off. If that sounds familiar then you probably drove one of the better running early computer controlled cars from the 1980’s. They were better only compared to the rest of the garbage being churned out by the automakers and even though they appeared to run better the extra performance ended up being useful only for pulling a big hill in high gear at 70mph. The excessively long stroke length, extremely heavy pistons and rods and generally anemic valvetrains meant that the range of engine speeds was too narrow even to get between the gears. And because all of the automotive engines were extremely radically oversized it did not matter how well they ran under a load, fuel mileage was only a function of idling performance.
I had one of those, a 1987 EFI Toyota. I’ve still got it actually but with it’s low 9.5:1 compression ratio it won’t run most of the time on the gasoline for 13:1 and 14:1 engines. Even back when my 10.2:1 Husqvarna ran well at 23 degrees BTDC fifteen years ago the 9.5:1 Toyota was extremely loud and harsh and did not do well on fuel. It would tow a 23 foot cabin cruiser at 14mpg on the highway and up big steep hills at 4,200RPM in second gear fairly well but that was about all it would do. Getting between the gears required revving out to 5,500RPM where the volumetric efficiency had dropped way off and the engine was extremely loud and harsh with a bunch of spark advance piled on. Then after the upshift the engine would just barely run at 3,000RPM making huge amounts of noise and heat and accelerating only slowly. From 3,700 to 4,200RPM though it pulled pretty hard and could drag that boat up massive hills with ease. As the fuel slowly became for higher and higher compression ratio engines the Toyota got louder and harsher until finally it just would not make power at all. I used to have to mess with the position of the distributor sometimes up or down on the spark timing to get the engine to run as well as it could, but then once the fuel was for a much higher compression ratio engine it just would not go any more. And that is the way it sits now. Occasionally by some miracle I get a tank of gasoline that will light off and the Toyota makes big torque from 3,500 to 4,200RPM again, but then on the next tank it is always back to no torque and just idling. Without late compression ignition the fuel mileage is also extremely poor much of the time. I used to always get 25 to 27mpg on smaller highways and going out on the interstate at higher speeds brought the mileage down to 23mpg. On fuel for higher compression ratio engines now I have gotten as poor as 15mpg running empty, and even on the highest flame front travel speed premium fuel it gets 20mpg to the 27 it used to get under the same operating conditions on either regular or premium. I used to think that the 2.3 liter gasoline engine was way oversized for the little Toyota truck even if it would only run well up to 4,200RPM. Now that the engine will only idle though the 4,000 pound truck is seeming like an awfully heavy hunk of scrap that does nothing but waste fuel. The engine is still so radically oversized that the truck will go 85mph just in full flame front travel mode, but it is extremely slow to get up there and gets very hot doing it. It also requires very fast flame front travel speed fuel to do much in full flame front travel mode. On slower flame front travel speed fuel it sometimes won’t pull past 60mph in any gear. On regular it used to have an annoying dead spot in fifth gear around 65mph, it was very slow to pull from 65 to 70mph in fifth gear and sometimes it just would not go past 70mph in fifth. If I downshifted to fourth though it would pull up to 90mph with authority and then I could shift into high gear and it would pull well past 100mph.
So the 9.5:1 compression ratio was always a bit too low, but it did sort of work on the same fuel that could also be run in a 10.5:1 engine. It was never clear just what the absolute maximum compression ratio was, but a lot of people thought it was up at 11.5:1 like many motorcycles from the 1990s and the first decade of the 21st century ran at. First of all it is sometimes true that much higher speed engines don’t attain quite as high maximum cylinder filling, but this is less significant than many people have believed it to be. What is really going on is that when an engine has a long duration camshaft and a torque peak way up at 7,000 to 8,000RPM it is just simply easier to run very close to the maximum compression ratio with mechanical control or a simple fixed advance curve CDI ignition as most motorcycles from the 1990s used. Way up at 8,000RPM even 20 degrees BTDC on the spark timing is in fact pretty close to the maximum compression ratio, and down at lower engine speeds that 20 degree BTDC spark timing is a substantial amount of advance for light loads in full flame front travel mode. And that is how most of the 11.5:1 motorcycles ran in the 1990’s. They topped out with maximum power just slightly above the 8,000RPM point of maximum cylinder filling and down at 3,000 to 4,000RPM the rather long 260 degree at 1mm valve lift camshaft yielded a much reduced volumetric efficiency that allowed the same 20 degree BTDC spark timing to deliver crisp and clean low power output in full flame front travel mode. With such a long duration camshaft and something very close to fixed spark timing from 3,000 to 8,000RPM late compression ignition did not come on until somewhere up around 4,000 to 4,500RPM. In that in between engine speed range from 4,000 to 6,000RPM the 20 degree BTDC spark timing was able to sort of support light loads in full flame front travel mode but late compression ignition was also available at wider throttle openings for reasonably large torque generation. Then depending on the bore diameter of the engine it was just late compression ignition from 5,000 or 7,000RPM on up to the maximum engine speed of nine or ten thousand revolutions per minute. With a fixed advance curve the point where maximum cylinder filling occurred at around 8,000RPM tended to result in an on or off power delivery. Close the throttle to where late compression ignition ceases and the engine just won’t make power at all and might even backfire. Some CDI ignition systems even backed off on the spark timing through the range of engine speeds where peak cylinder filling occurred. A gradual ramp up from the 15 or 20 degree BTDC low idle timing setting to perhaps as much as 25 degrees BTDC around 5,000 or 6,000RPM and then a dip back down to 15 or 20 degrees at 7,000 or 8,000RPM followed by more gradual advance back up to 20 or 25 degrees BTDC up at 10,000 or 12,000RPM for some over rev. A pronounced dip in the advance curve at and slightly above the engine speed range where maximum cylinder filling occurred was characteristic of the cam and bucket valvetrains common on high performance street motorcycles throughout the 1990’s. Faster opening and closing valves on a roller cam are better able to work well with just a single fixed spark timing value or a more flat and gradual ramping up fixed advance curve on a CDI ignition system.
If an engine with a fixed advance curve gets right up very close to the maximum compression ratio then it will only run in the engine speed range where maximum cylinder filling occurs once it has pulled up from a slightly lower engine speed. A street bike or road race engine with a traditional CDI ignition and a compression ratio very close to the maximum for the fuel being run would always be up shifted so that the engine speed drops down bellow where maximum cylinder filling occurs. At engine speeds just below where maximum cylinder occurs some extra spark advance is available so the engine can at least sort of run at reduced loads in full flame front travel mode. If an engine with a fixed advance curve is extremely close to the maximum compression ratio then it will not run at all at reduced loads at the engine speed where maximum cylinder filling occurs. An engine with a throttle position sensor does not have these problems and can run extremely close to the maximum compression ratio with no difficultly.
It certainly was possible to get quite close to the maximum compression ratio with a fixed advance curve CDI ignition on high speed street bike engines if the gasoline remained the same week after week and year after year. The reality though was that very nearly the same maximum power output was attainable with a slightly lower compression ratio and a bit more spark advance. My 10.2:1 Husqvarna WMX 610 really worked great at a static timing setting of 23 degrees BTDC and a maximum advance of 26 or 27 degrees BTDC up at maximum engine speed with the crankshaft wiggle advance. That 27 degrees BTDC at 6,000RPM certainly was a bit too early for absolute maximum torque generation, but by 7,500RPM where the engine made maximum power it was pulling just about as hard as it possibly could have. The main problem with that stock engine was that the factory stock Mahle piston was just way too heavy at 406g. The heavy piston meant that the power leveled off after 7,000RPM. It would rev much higher, but the power was just flat up there. The combination of the mean piston speed approaching the high end of the range and the very heavy piston meant that there was just no reason to try to rev past 7,500RPM other than to get into the next gear up at high enough of an engine speed that big power was available. With the piston cut down to 368g on the same otherwise stock 10.2:1 engine it makes dramatically more and more power to 7,500RPM and only really levels off at 8,000RPM.
With an even lighter 332g cut down piston in my new hot rod 610 motor increasing power is available all the way up to 8,500RPM. The even lighter piston is of course a big part of it, but bigger valves, a bigger camshaft and the higher 11.3:1 compression ratio all play a roll also. What I find with the new hot rod engine on pump gas is that the engine is capable of making more and more power up to 8,500RPM, but then when it hits 8,500RPM it really is hitting a wall. On the best race gas it might rev higher, but on normal pump gas (regular or premium) the mean piston speed of a three inch stroke at 8,500RPM really is the absolute top end for big power production. The power builds dramatically from 6,000 to 7,000RPM like it always did and then it continues to build dramatically from 7,000 to 8,000RPM with the lighter piston. Then from 8,000 to 8,500RPM the power increases much more gradually and by 8,500RPM it is not increasing anymore. The engine will sometimes continue to rev past 8,500RPM but it is just totally flat. The Husqvarna 610 runs like this at static timing settings anywhere from 21 degrees BTDC up to 31 degrees BTDC depending on what sort of fuel happens to be available. Up at the higher end at 28 to 31 degrees BTDC on the static timing setting though the engine is usually much louder and harsher and both peak torque and peak power are noticeably lower than with less spark advance even if the engine will rev all the way out. The difference between a static timing setting of 21 degrees BTDC and a static timing setting of 26 degrees BTDC is sometimes just a matter of the engine being louder and harsher at low engine speeds bellow 6,000RPM. I have however often suspected that when the engine pulls really hard and is able to rev all the way out at a static timing setting of 26 degrees BTDC despite being loud and harsh that it is a case of race gas being substituted for normal premium pump gas. The engine does not run as well with 26 degrees BTDC on the static timing setting, which is about 29 or 30 degrees maximum advance with the crankshaft wiggle advance, but better fuel just powers through the problem and makes the same big power output despite more noise and more harshness. Up at static timing settings of 28 degrees BTDC or more which are maximum advance values of more than 31 or 32 degrees BTDC the engine not only gets extremely loud and harsh even at very high engine speeds, but it also always makes a whole lot less power despite the fuel probably being some really hot stuff that would work smashingly in 13:1 or 14:1 engines.
I have also often gotten the new hot rod 610 motor with the bigger valves and bigger camshaft to make power all the way up to 8,000RPM on regular 87 (RON+MON)/2 octane rating gasoline, although the slower flame front travel speed of regular gasoline usually means that the engine gets very harsh and unusable down at 3,000 to 3,500RPM with lots of spark advance and then it also hesitates up around 6,500 to 7,500RPM before taking off again and pulling hard the last bit up to 8,000RPM once it is finally well warmed up in sixth gear at 100mph. This is not how a dirt bike should run, it is more like a land speed record racer on the slower flame front travel speed fuel. What I dislike the most about the slower flame front travel speed fuel is just that it takes so long to top end the bike with all of the hesitation. Not only is this more difficult because it requires a much bigger stretch of straight and flat ground, but it also means that the engine is up at quite high engine speeds for much longer periods of time that are more likely to damage the rod bearing with the inadequate oil reed valve lubricating system. I have run the engine many times for quite long 10 second stumbling sessions up to 7,500RPM on regular or slower flame front travel speed gasoline being pawned off as 91 (RON+MON)/2 octane rating premium gasoline, and it has so far seemed to hold up to the abuse. When the engine is actually running well like a dirt bike though it just rips up to 8,000 or 8,500RPM very quickly even in lower gears and there is no need to keep it up at that high engine speed for more than a second or two to get into the next gear. Even in sixth gear with the 15/50 gearing it pulls up to 115mph at 8,500RPM so quickly that it can be done on rather short little stretches of straight and mostly flat ground.
Another problem I have with the slower flame front travel speed fuel hesitating and causing large delays in acceleration around 7,000RPM is that the engine stays at high engine speeds so long that the oil breather reservoir fills up and spills over. Just for a quick jaunt up to 8,000 or 8,500RPM without hesitation the engine speed is not up really high for so long that any oil escapes. When the engine hesitates and gets stuck at 6,500 to 7,000RPM for long seconds though the breather reservoir fills up and spills over causing an ugly oily mess all over the bottom of the bike and it even gets thrown up onto the inside of the rear fender. Not only does this make a mess of the bike, but it also means that I end up losing some oil and I worry about what the oil level is.
The slower flame front travel speed fuel is no problem at all on the old stock 10.2:1 motor with the smaller valves and smaller camshaft, it just won’t rev up at all. With the slower flame front travel speed fuel the stock 10.2:1 motor just ends at about 6,000 or 6,500RPM and that is it, nothing more. No problems at all other than the fact that I know it usually revs out to 8,000RPM with ease.
I suspect that the Husqvarna 610 would run well and make big power to 8,500RPM with even less spark advance on fast flame front travel speed fuel, but I have never managed to get low enough pressure fast flame front travel speed fuel to try this. The stock 10.2:1 engine has often revved all the way out to 8,000RPM with just 21 degrees BTDC on the static timing setting, but I have never been able to run this same fuel in the new hot rod 11.3:1 engine to try even less spark advance. The least spark advance that I have ever been able to run successfully in the new 11.3:1 motor is 24 degrees BTDC on the static timing setting, which did work well other than it being a bit louder and harsher than with less spark advance. When I have had the new 11.3:1 motor running well down at 18 degrees BTDC and even as low as 16 degrees BTDC on the static timing setting it was in fact somewhat slower flame front travel speed fuel that just would not rev out. On fuel that was running well from 3,000 to 6,000RPM and hesitating it’s way up to 7,000RPM at a static timing setting of 18 degrees BTDC I tried advancing the spark timing to get better high engine speed performance. This did not work though. I just kept bumping the static timing setting up to 21 degrees BTDC then finally 24 degrees BTDC on that same fuel and it just would not rev out. The hesitation from 6,000 to 7,000RPM did diminish and I was able to get it going a bit up at 7,500 to 8,000RPM, but by the time I had 24 degrees BTDC on the static timing setting the engine was just totally unusable down at 3,000 to 3,500RPM and even from 3,500 to 4,000RPM it was super harsh if the throttle was opened more than a very small bit. I also tried (temporarily) richening the main jet from 175 to 185 on this same fuel at 24 degrees BTDC on the static timing setting and this did help a very small amount, but pretty much all it did was waste lots of fuel and cover my exhaust system with black soot. The horrible hesitation around 7,000RPM did not go away, it just diminished a small amount.
The clear indicator of slower flame front travel speed fuel is the harsh and unusable lower engine speeds when the static timing setting is advanced so that the engine will rev out. On the fastest flame front travel speed fuel the Husqvarna 610 is often very crisp with instant torque from 4,000 to 5,000RPM, but hesitates some with lots of lag down at 3,500RPM and sometimes won’t make torque at all at 3,000RPM. Interestingly the new hot rod 610 motor does not even need the absolute fastest flame front travel speed fuel to rev all the way out. Many times I have had it running at around 24 or 26 degrees BTDC on the static timing setting where it is equally crisp with just small amounts of lag right from 3,000RPM on up to 5,000RPM and is able to rev out to at least 8,000RPM with no hesitation once well warmed up on a big pull. Even if it does cause some loss of crispness down at 3,000 to 3,500RPM I do however prefer the faster flame front travel speed fuel just because big power up at 5,000 to 8,000RPM is always instantly available even when the engine has cooled off coasting down a hill. For most purposes the big power at over 6,000RPM is only required for bigger and faster trails where the engine stays well heated up anyway, but still it is nice to always have all of the power and revs available in second or third gear to launch up a steep climb with good traction.
I know that richening the needle clip position can help with delivering instant crisp power, but richer settings cause so many other horrible problems that I always prefer the leanest needle clip position. For one thing the richer needle clip position can easily result in a lunging sort of power delivery at 3,500RPM where the bike just jumps forward ripping your arms out of their sockets without even putting much power to the ground. The leanest needle clip position and the slightly leaner number 60 pilot jet versus the stock number 62 pilot jet go a long way to taming the beast and keeping the 3,500RPM lunging down to a manageable level.
Why you might ask does an overly rich mixture at small throttle openings cause so much worse problems with lunging and choppy power delivery? Obviously leaning out to the point where the engine makes less power would reduce lunging, but so would less engine displacement. No, that is not the whole story. Basically a significantly larger amount of fuel can be made use of in late compression ignition mode than in full flame front travel mode; so richening the mixture increases the already substantial difference in power output between full flame front travel mode and late compression ignition mode even all the way down at 3,500RPM. Then there is also the fact that an overly rich mixture actually reduces the rate of flame front travel on the same fuel, further increasing the difference in power output between full flame front travel mode and late compression ignition mode even way down at 3,500RPM. In fact this unusually intense lunging caused by an overly rich mixture down low is noticeable down to much lower engine speeds. Certainly down to 3,000RPM and even down to 2,500RPM if an engine is able to get into late compression ignition mode then an overly rich mixture will cause much more choppy power delivery. This should not however be misinterpreted as meaning that late compression ignition works well down at 2,500RPM. Just because late compression ignition can still make more torque than full flame front travel mode operation down at 2,500RPM does not mean that 2,500RPM is a good engine speed for late compression ignition. As engine speeds are reduced bellow 4,000RPM late compression ignition gets increasingly harsh and inefficient. Down at 2,500RPM even with very small amounts of spark advance late compression ignition causes high spikes in cylinder pressure that result in harsh, inefficient operation and dramatically increase loads on pistons, rings, rods and bearings.
Another huge disadvantage of going richer on the pilot jet or the needle clip position is that the engine then does not run over as wide of a range of elevations. If the engine is overly rich down low to where it blackens the exhaust at sea level with only small throttle openings then climbing up to even slightly higher elevations is going to make dramatic changes in the way the engine runs. By staying reasonably lean down low all of the fuel is still able to burn up at higher elevations and the change in the way the engine runs is due only to slightly lower compression pressures with the thinner air. Even with the pilot jet and needle clip position on the lean side I still use a big 175 main jet on the 40mm DellOrto which delivers pretty much absolute maximum power at wider throttle openings and even slightly blackens the exhaust system if I run a lot with the throttle wide open. I always thought that the 180 main jet that was stock on the 1991 WMX 610 was a bit too fat, but really it is only the 185 size that is extremely excessive. Both the 175 and 180 main jets work great and deliver huge power, the 175 just saves a bit of fuel when going very fast and delivers a bit better high elevation performance up at the top of the engine speed range.
They even went all the way down to factory stock 170 main jets on some of the later 610 models (kick start not the heavy left hand output 610e), but this would be more like street jetting than dirt bike jetting. The Husqvarna 610 running on the main jet on the street is kind of a ridiculous notion anyway. I don’t even know if you could get high enough gearing on the thing to run down the freeway on the main jet, and if you did it certainly would be up at well over 100mph. With the 15/50 gearing the 610 is only on the main jet for heavy acceleration, and any constant speed cruising is firmly way down at small throttle openings where the main jet is mostly totally insignificant.
It might be argued that a richer pilot jet and a richer needle clip position would be required for crisper throttle response when highway cruising, but this is total non-sense. The only thing that overly rich mixtures down at low throttle openings are good for is hiding other problems. Particularly if the fuel is for much higher compression ratio engines then dumping more fuel in can get the engine into late compression ignition mode more easily with less spark advance. With a bunch of extra fuel dumped in though the engine does not actually run any more efficiently, quite the contrary it runs less efficiently with dirtier exhaust emissions. The richer mixture down low just hides the problem of the fuel being for a much higher compression ratio engine.
With the leaner number 60 pilot jet and the needle clip in the leanest position the 40mm DellOrto is not too lean. In fact the mixture is probably still on the rich side if anything. When the 610 is running crisply it pulls very hard with the throttle open just 1/4, really every bit as hard as with the needle clip one groove richer. Sure there might be significantly more power at the smallest throttle openings with the richer mixture, but opening up to just ¼ throttle at 4,000 to 5,000RPM results in really very nearly just as much power as the engine is ever capable of making at 4,000 to 5,000RPM. There is a tiny bit more power to be had with more fuel, twisting up to get onto the richer main jet does provide a very slight boost in power output. It is however an extremely small difference in output for quite a large increase in fuel consumption. I notice that when the engine is not running crisply enough and I have to twist up past half throttle all the time to get some extra fuel from the main jet in order to get late compression ignition the fuel consumption increases a whole lot. It is the difference between 0.6GPH and 0.9GPH in the same type of ride mostly on small dirt roads and trails. The 0.6GPH is very low fuel consumption for the big 577cc carbureted engine, but amazingly it will do this with quite a bit of big power being laid down at 4,000 to 6,000RPM for heavy acceleration out of turns and up hills as long as the engine is running crisply enough that it will do it on the lean mixture at small throttle openings. If the throttle has to be opened up wide all the time for the richer mixture on the main jet then the average fuel consumption spikes up dramatically.
To be fair to all those dirt bike tuners who have run extremely rich mixtures over the years I think I know how this can come about. If the fuel being used just won’t pop off easily enough at the compression ratio of the engine and it is either not possible to advance the spark timing or if the spark timing is already too far advanced then richer mixtures certainly can help deliver the desired crispness. I am also aware that lower energy density fuels are sometimes encountered. If someone is trying to tune an engine while using lower energy density gasoline then they are of course going to tend to arrive at richer settings. Even gasoline with 10% ethanol would tend to require about 4% higher fuel flow rates than normal gasoline, and if the ethanol content is even higher then richer settings yet can be required. Running just ethanol, E100, would tend to require about 35 or 40% higher fuel flow rates than for gasoline, a dramatic difference. I see though that no alcohol is allowed by the Best In The Desert sanctioning body, so extremely rich jetting would not be required.
Even without any problems with changing energy densities of gasoline it is very easy to get confused about how to get an engine to run better and make more power. Power output and crispness are results of how an engine is running, what causes various levels of crispness and power output are the ten parameters of gasoline engine operation. These ten parameters are the flow capabilities of the engine, pumping losses, the weight of the rod and piston, the compression ratio, the temperature and pressure requirements of the fuel for late compression ignition, the flame front travel speed of the fuel, the air/fuel mixture ratio, the spark timing, the shape of the combustion chamber and the bore and stoke dimensions of the engine.
The main determinants of flow capabilities are the size of the valves and the camshaft profiles. The duration and timing of the camshaft determine what engine speed the engine will work best at and the rate of opening and closing of the valves determines how wide of a range of engine speeds will deliver high volumetric efficiency. Bigger valves of course also contribute to wider ranges of engine speeds. How much room there is around the valves once they are open also maters. On the intake valves it is mostly the total circumference length that is significant, where on the exhaust valves the total valve area is more significant. The circumference of the intake valves is so important because at the top of the engine speed range much of the intake charge is flowing into the cylinder late once the intake valves have already begun to close. This also means that shrouding of the intake valves by part of the cylinder head is generally more of a problem than one side of an exhaust valve being partially blocked. It is mostly the much larger total circumference of two intake valves versus just one larger diameter intake valve that allows four valve per cylinder engines to work so much better than two valve per cylinder engines. The size and shape of the intake ports as well as the size and shape of the exhaust ports and the entire exhaust system of course also play a roll. Both exhaust scavenging and the length and diameter of the intake stack can also change the volumetric efficiency of an engine. Higher compression ratios also generally result in better flow.
Pumping losses on the intake are not all that significant for maximum power output, but it should be kept in mind that sucking against a vacuum at low power output is a big reason that gasoline engines never deliver particularly good efficiency at low power output. A two liter automotive engine for example might be wasting as much as 7 or 8hp just to suck against an intake vacuum at 2,000RPM, more power than is being used to drive the car along at 40mph. Pumping losses on the exhaust stroke are often much more significant for maximum power output, undersize exhaust valves don’t necessarily significantly reduce the volumetric efficiency of an engine, but it can take quite a bit of power for the piston to push the hot exhaust gasses out of undersized exhaust valves.
The weight of the reciprocating assembly is significant because it ends up taking quite a bit of power to throw the piston and rod up and down at high engine speed. These reciprocating losses tend to be proportional to the weight of the piston and rod, cut the weight of the piston and rod by 20% and the reciprocating losses are reduced by 20%. This is very significant considering that some engines end up losing a third or half of their power output at maximum engine speed just to throwing the pistons and rods up and down. Usually it is automotive engines with rod bolts and extremely over weight pistons and rods that end up with such extremely large reciprocating losses. Dirt bikes without rod bolts tend to have rather light weight rods and most modern dirt bikes have very light minimalistic pistons.
It is the effective compression ratio that is actually significant for how much spark advance will be required on a particular fuel, and long duration camshafts result in significantly reduced effective compression ratios at low engine speeds. Most high performance engines do however attain close to 100% volumetric efficiency over some range of engine speeds, and within this range of engine speeds the effective compression ratio is very nearly as high as the static compression ratio. All else being equal higher compression ratios also yield slightly higher thermodynamic efficiency.
The temperature and pressure requirements of the fuel for late compression ignition determine what the maximum compression ratio can be, and also play a significant role in how much spark advance will be required. It is the difference between the compression ratio of the engine and the maximum compression ratio that will work with the fuel being used that determines the amount of fuel that will have to be burned in flame front travel mode before late compression ignition can take place. The farther the compression ratio is reduced bellow the maximum compression ratio the more fuel will have to be burned in flame front travel mode which will require more spark advance.
The flame front travel speed of the fuel determines how high the engine can efficiently rev in full flame front travel mode, and the flame front travel speed of the fuel is also very significant for late compression ignition operation as well. To get the amount of fuel burned that is required to bring the temperature and pressure up to the point where late compression ignition will take place some spark lead is required. To get that same amount of fuel burned with slower flame front travel speed fuel will require more spark advance. And slower flame front travel speed fuel ends up requiring even more additional spark advance than might have been expected because the earlier spark timing and longer duration of flame front travel combustion causes more heat to be transferred to the cooling jacket which in turn requires that a larger amount of fuel be burned in flame front travel mode before the temperature and pressure in the combustion chamber have come up to where late compression ignition will take place.
The combustion chamber shape is significant for both full flame front travel mode operation and for late compression ignition mode operation. In full flame front travel mode what is most important is that the spark plug be located directly in the middle of the combustion chamber. If the spark plug is located off to the side of the combustion chamber then it is going to be difficult to get all of the fuel to burn in full flame front travel mode and the engine is going to tend to be very dirty with high emissions of unburned hydrocarbons when it runs in full flame front travel mode. For late compression ignition mode it is the shape of the combustion chamber in the immediate vicinity of the spark plug that is significant. If the spark plug is recessed into a corner in the combustion chamber then much more spark advance is going to be required to get the amount of fuel burned to enter late compression ignition mode. Very small spark plug gaps also introduce a delay in the flame front getting going, but this only changes the shape of the required advance curve and has little to do with how well the engine will run provided that the correct advance curve can be provided.
The mixture ratio is obviously significant for how much power will be produced, up to rather rich mixtures increasing the amount of fuel delivered will result in at least small increases in power output. Extremely overly rich mixtures actually reduce power output by displacing some of the intake air, but this does not normally come into play because those extremely rich mixtures are quite a bit richer than the ideal mixture even for maximum power output. Overly rich mixtures also reduce the actual flame front travel speed, which means an engine won’t be able to make as much power in full flame front travel mode when it is running rich. Richer mixtures do get more fuel burned by the flame front in a shorter time though, so richer mixtures certainly can allow less spark advance all else being equal.
The bore of the engine is of course significant for power production, all else being equal power output is proportional to the area across the top of the piston. Thermal limits before pistons melt also tend to be proportional to the area across the top of the piston, and this is even true for engines of different stroke lengths. The stroke length mostly determines what engine speed the engine can run at, the maximum engine speed tends to be proportional to the stroke length. Increasing the stroke length of an engine increases the displacement but it does not necessarily increase the maximum power output since the engine then can’t rev as high. Longer stroke engines do however tend to make more power simply because engines have an easier time flowing well at lower engine speeds. This is true with stroke increases up to a certain point, but excessively long strokes are inappropriate for gasoline engines simply because gasoline engines down work well at less than about 6,000RPM. Hotter burning fuel of course can support higher mean piston speeds and allow longer stroke engines to rev higher. Longer strokes tend to result in lower minimum fuel consumption levels for the same displacement, but again this only works up to a certain stroke length with a gasoline engine since engine speeds bellow about 4,000 or 6,000RPM just can’t deliver as high efficiency. In full flame front travel mode smaller bore diameters allow higher engine speeds. In late compression ignition mode though large bores work fine up to quite high engine speeds, although on mechanically controlled engines and engines with fixed advance curve CDI ignitions larger bores do benefit considerably from flame front travel speed fuel.
The reason that the 610 needs the big 175 main jet is just so that it will rev out to maximum engine speed with the fixed spark advance. Even at wide open throttle with the very light 332g piston the crankshaft wiggle advance comes down at much lower engine speeds certainly less than 7,500RPM so the last pull up to 8,500RPM is just on a fixed spark timing where the volumetric efficiency of the engine is not increasing either. With the 250 degree at 1mm valve lift and 106 degree lobe center camshaft installed four degrees advanced the engine really begins to get on the cam at 4,000RPM and seems to deliver maximum volumetric efficiency at perhaps about 6,000RPM. This really is about the same as the stock 1991 camshaft with 243 degrees of duration at 1mm valve lift and the same 106 degree lobe center installed straight up with split overlap at top dead center. With the big aggressive roller cam and large valves the volumetric efficiency also remains rather high all the way up past 8,000RPM, but it certainly is not increasing any more up there either. Without richening up for the last bit of the pull it just would not rev so high. It is like some other kind of ignition system that provides more spark advance on up to the highest engine speeds to provide some overrev.
The only thing that might provide some additional boost in volumetric efficiency up at the top of the engine speed range is the really very short 7” total intake runner length on the 610 Husqvarna. That seven inches from the intake valves to the back of the carburetor where the intake boot opens up to a much larger diameter is really extremely short for a three inch stroke engine. I have seen 250F dirt bikes with longer total intake stack lengths. What I would guess, although it really is just a wild guess is that the Husqvarna 610 camshaft really does begin to work towards maximum volumetric efficiency down at around 4,500 to 6,000RPM, then the crankshaft wiggle advance comes on at perhaps 7,200RPM and finally the short seven inch intake stack probably hits at about 7,600RPM to help out with the last pull up to 8,500RPM. The reason that I think the crankshaft wiggle at wide open throttle comes at 7,200RPM is that when the engine is hesitating it will sometimes rev all the way to 7,000RPM more easily with the throttle partially closed. From 7,200RPM up though it always pulls best with the throttle wide open. Even with the needle clip in the leanest position and the big 175 main jet there were times when the engine was hesitating quite badly but I could still get it to rev to 8,000RPM by first leaving the throttle substantially closed up to 7,200RPM before opening wide for the last pull up to 8,000RPM. The reason I think the velocity stack is tuned for 7,600RPM and higher is that even with the stock Swedish SEM CDI ignition with absolutely fixed spark timing from 2,000RPM on up the engine still was sometimes able to rev to 8,500RPM. With the stock CDI ignition with no advance curve the engine often seemed reluctant to go past 7,000RPM, but if it managed to make it to 7,600RPM then it was nearly always able to give a good hard pull on up to 8,500RPM.
Essentially what it comes down to is that the Husqvarna 610 motor is probably very carefully designed to work great with a points ignition system and a lightened piston. I came about these modifications entirely by accident. The points ignition was an act of desperation because I could not get the stock CDI ignition to work at all. The lightened piston was just a little side project that I started years before by cutting down a Wiseco XR 400 piston to go in my toasted WXE 350 motor. When I found that the piston I was going to use for rebuilding the WXE 350 motor was a Wiseco I remembered having read about cutting Wiseco pistons for XRs down in the pages of Dirt Bike Magazine back in the 1990’s. Then when I saw the Wiseco XR 400 piston in person it was obvious that it was designed to be cut down to much lower weights. The strategy I used to cut the Wiseco XR 400 piston down was however unique in that I undercut the pin bosses instead of just shortening the piston pin as was apparently the standard procedure for modifying the Wiseco pistons. I also rounded off the extraneous outer parts of the pin bosses and cut down the connecting webs out to the skirts. Without shortening the substantial XR skirts or compromising the strength of the pin bosses I got the 85mm Wiseco XR 400 piston down to 245g, a nice weight reduction from the 270g stock 84mm Mahle piston. Cutting the Wiseco piston down to be lighter than the stock Husqvarna piston had worked so well that when I took the stock 10.2:1 610 motor apart to fix the exhaust valves I decided to do some cutting on the stock Mahle piston. And that worked so well cutting the stock Mahle piston down that when I got the new Woessner 610 piston, which was already as light as my cut down Mahle piston, I decided to cut that one down even further. It looks like I have probably landed on the ideal piston weight for the 610 motor at 332g. Any lighter than that and the engine would have to rev too high to make use of the crankshaft wiggle advance.
An even lighter piston with some other type of ignition system might yield even more improvements in efficiency and performance, but from a racing perspective the 332g piston is light enough. Lightening from 406g to 368g freed up much more power from 7,000 to 8,000RPM. Then going down to 332g freed up another chunk of power from 7,500 to 8,500RPM, but the 8,500RPM limit is not due to the weight of the piston and rod. The 8,500RPM limit quite clearly is the maximum mean piston speed for normal pump gas in a three inch stroke engine. Even with the 406g piston and the smaller valves the stock 10.2:1 engine was able to rev to 8,500RPM, it just did not make more power because the heavy piston was sucking down so much power above 7,500RPM. The fact that the 610 motor will rev to 8,000RPM or even 8,500RPM with the smaller 30mm and 35mm 1986 to mid 1990’s (1994?) valves and still usually won’t rev past 8,500RPM with the larger 32mm and 36mm 1997 valves indicates that it is not a flow problem that stops the engine at 8,500RPM. The bigger valves and longer duration camshaft do certainly contribute to a bit more power at 7,500 to 8,500RPM, but it is not a dramatic difference. The dramatic difference from 7,500 to 8,500RPM is the weight of the piston, and the wall that the engine runs into at 8,500RPM is the fact that the pump gas just does not burn hot enough for higher mean piston speed operation. This same mean piston speed would be 12,100RPM on a 2.11 inch stroke Yamaha 250F or 10,300RPM on a 2.5 inch stroke KTM 450.
How then is it that 250F dirt bikes can rev to 14,000RPM and 450F dirt bikes can rev to 12,000RPM? For one thing specialty race fuels may sometimes have higher maximum temperatures of combustion than pump gas. It would only make sense that race gas would be made up of the hottest combustion fuels that could be found since higher mean piston speeds would allow higher output from the same displacement and stroke length engine. The other thing going on is that a computer controlled race engine can dial the time of late compression ignition around earlier than the 15 or 20 degree ATDC latest time of late compression ignition. Mostly what dialing the time of late compression ignition around earlier is useful for is just getting higher engine speeds. When KTM 250SX-F race bikes twist to 15,000RPM this probably requires an earlier time of late compression ignition than the latest possible time of late compression ignition around 15 or 20 degrees ATDC.
Even though dialing the time of late compression ignition around from 15 degrees ATDC closer to top dead center may help gasoline engines run at higher mean pistons speeds up above 10,000RPM this should not be confused with a full compression ignition engine. If full compression ignition occurs without the spark plug firing this is going to happen not at top dead center, but rather around no later than about 20 degrees BTDC. This is extremely early for compression ignition to occur in a gasoline engine and would not work at all at any kind of reasonable engine speeds. That is a very big difference between compression ignition at 20 degrees BTDC and compression ignition at 15 degrees ATDC. Very small full compression ignition engines have been built, the most common and well known being the 0.4 inch stroke 049 model airplane engine. These small carbureted cylinder port engines without sparkplugs operate up at mind bogglingly high engine speeds in the 15,000 to 25,000RPM range.
I really don’t know just what engine speed an earlier time of late compression ignition begins to be an advantage, but I am pretty sure it is above 10,000RPM. The Husqvarna WXE 350 with the points ignition system would sometimes hesitate a bit around 7,000RPM but then would give a big strong pull up to 8,500RPM. If I advanced the spark timing just two degrees to get rid of the hesitation around 7,000RPM then it would pull all the way on up to over 10,000RPM with that same fixed spark timing value. Even though the WXE 350 would rev past 10,000RPM it really did not make more power up there. Maximum power with the 2.48 inch stroke seemed to come down at around 8,500 to 9,500RPM. Above about 9,000RPM it was just totally flat, and more revs did not yield more power. Of course the main reason that the stock WXE 350 would not make more power up at 10,000RPM was just that the piston and rod were so radically over weight. I mean it is the same rod as in the 610 for crying out loud! And the stock 3.31 inch Mahle piston was very heavy at 270g also. Proportionally the 270g piston in the 3.31 inch bore WXE 350 would be like a 428g piston in the 3.86 inch bore Husqvarna 610.
The absolute maximum engine speeds seemed to come at the same mean piston speed for both the 350 and the 610. The 8,500RPM maximum on the three inch stroke 610 is the same mean piston speed as 10,300RPM on the 2.48 inch stroke 350. The difference is that the 610 has a much lighter reciprocating assembly, where that big 313g rod and 270g piston on the little 350 is just much much heavier. The result is that even the stock 610 at least continued to make small amounts of additional power up to 8,500RPM where the stock WXE 350 just totally fell flat way down at 9,000RPM and even seemed to experience a bit of a fall off in power as it revved past 10,000RPM.
Dialing the time of late compression ignition around earlier would mostly allow an engine to rev to a higher engine speed, and very short stroke race engines have been used at up to about 19,000RPM. Dialing the time of late compression ignition around at more moderate 10,000 to 15,000RPM engine speeds may however also help to squeeze out higher mean piston speeds in all out race engines. Efficiency always drops off as the mean piston speed gets so high that the hot gasses will not expand fast enough, but dialing the time of late compression ignition around to top dead center instead of the 15 to 20 degree past top dead center latest possible time of late compression ignition would help to build pressure earlier so that the engine could still make power at higher mean piston speeds. This is related to the reason that gasoline engines always appear to work better up to somewhat higher mean piston speeds than diesel engines.
The ideal mean piston speed for a diesel engine appears to be about 2,000 or 2,500RPM for a six inch stroke engine. That would be only 6,000 to 7,000RPM for a 2.1 inch stroke 250F dirt bike. Since gasoline engines operating in late compression ignition mode burn all of the fuel all at once right at the time of late compression ignition no later than about 15 or 20 degrees ATDC though considerably higher mean piston speeds appear to work quite well. Even a four inch stroke gasoline engine revs to 6,000 or 7,000RPM if it will flow well enough, double the mean piston speed of a diesel engine. Part of the reason that four inch stroke gasoline engines rev to such high mean piston speeds is that they simply won’t run well at much lower engine speeds. No gasoline engine, no matter how long the stroke, will run quite as well at engine speeds less than about 6,000RPM. So the big four inch stroke gasoline engine twists out to 6,000 and 7,000RPM to do it’s best work even though that mean piston speed is quite excessive. The other thing going on though is that when a long four inch stroke gasoline engine runs at 4,000 or even 6,000RPM the fuel is still burning all at once so fast no later than 15 or 20 degrees ATDC that the pressure in the cylinder spikes up and peaks quite early before the piston has had a chance to move down. Because the cylinder pressure unavoidably spikes up excessively early at these low engine speeds there is already that stored up extra pressure required to run at mean piston speeds higher than would normally be ideal.
Essentially it ends up being a case of an excessively long stroke gasoline engine also causing higher mean piston speeds to appear to work better than they otherwise would. A two inch stroke 250F revving to 14,000RPM really is an excessive mean piston speed, and the only way that this is made to work well and win races is probably by dialing the time of late compression ignition around to be earlier than the latest possible time of late compression ignition at 15 or 20 degrees ATDC.
So what is the ideal engine speed for a gasoline engine? Probably around 8,000RPM. At this engine speed longer three inch stroke engines run really great even if the torque has sometimes dropped off a bit due to excessively heavy pistons and rods, inadequate valvetrains or because the mean piston speed is just getting too high for the fuel being used. At 8,000RPM a two inch stroke engine has also begun to run as well as it is going to run even if it can be made to continue to make quite big torque all the way up past 11,000RPM. If 8,000RPM is the ideal engine speed then the range of engine speeds where gasoline engines run really well would be about 6,000 to 9,000RPM. The reason that dropping off more to lower engine speeds is better than going to higher engine speeds is that lighter loads favor lower mean piston speeds. A gasoline engine might not be able to do quite as well at 4,000 to 6,000RPM under a full load as it can at 8,000RPM, but somewhat reduced load operation still in late compression ignition mode does work well all the way down to about 4,000RPM. Even a two inch stroke engine will zip along quite nicely at 4,000RPM under a somewhat reduced load so long as it is still able to get into late compression ignition mode without huge amounts of spark advance.
Part of what is going on with the WR250F sucking down so much fuel is probably just that the engine speeds are excessive. This is sort of the holy grail of race engines. The idea that a bigger gasoline engine would use less fuel. People have been crowing about this for decades, but it just was never true when three inch stroke engines were being compared to four inch stroke engines. The three inch stroke was too long and the four inch stroke was extremely excessive. The two inch stroke is probably just right for a gasoline engine, but a two inch stroke gasoline engine can still be made to look excessively small for best efficiency if it is run very hard at extremely high engine speeds. It might be possible to use less fuel in a high speed desert race with a 450F that has the same 1.5:1 bore to stroke ratio as the 250F, but this would be a bit misleading. It is not the longer 2.5 inch stroke that uses less fuel, it is the larger displacement that uses less fuel. Both the two inch stroke and the two and a half inch stroke do great at 8,000RPM, it is just that the 250F does not make enough power to be competitive against larger displacement machines if it is run most of the time way down at 8,000RPM far below where it can make maximum power. An even more radically oversquare 350cc two inch stroke engine could probably be made to do even better on fuel than the two and a half inch stroke 450F for a high speed desert race. Or as the article in the November issue of Dirt Bike Magazine pointed out the 300cc class limit leaves considerable room for increased performance. What was not exactly mentioned in the November issue of Dirt Bike Magazine was that for a high speed desert race that higher performance (and lower fuel consumption) would be attained with a big bore kit to bring the 250F out to 300cc instead of stroking out to 300cc.
Still though the less than 25mpg average fuel mileage from a 250F is just really hard to believe. That has got to be a result of programming the thing to dump as much extra fuel in as possible under all conditions without immediately flooding and stalling being a problem.
I see that Troy Vanscourt and Austin Miller got a DNF in the Vegas to Reno, I am guessing they ran out of gas and did not make the next fuel stop. Or perhaps the plug fouled so many times on the overly rich mixture that they lost so much time they had no chance of finishing in daylight and decided to drop out.
Just jokes about Troy and Austin, of course I have no idea how their race went. I was reading the BITD rule book though, and that is full of some pretty funny jokes also. It seems that there is no displacement class for my 400 WR two stroke. The lightweights class is open to two and four stroke bikes up to 300cc and the “unlimited” class is open only to four strokes. What this means is that no one under the age of 30 would be allowed to race the 400 WR since it would have to be raced in an “age” class. The other strange displacement thing about the displacement divisions is actually mostly insignificant since I am fairly sure no one would race a 100cc to 125cc four stroke in a desert race, but it is kind of funny none the less. The lightweights class is open to bikes displacing from 126cc to 300cc, but the “unlimited” class is open to four strokes displacing more than 100cc. This means that a CRF150 four stroke would be in the lower displacement class but an XR100 bored out to be over 100cc would be in the unlimited displacement class. That is decidedly backwards! Again though who would want to try to do 500 miles of high speed ridding on an XR100, the time for that team would be measured in days not hours. Another consequence of the way these rules are written is that 125cc two strokes would not be allowed to race in the displacement divisions either, but would be able to be raced in the age divisions by a rider over 30. Anyone under the age of 30 that wants to race a two stroke is limited to 126cc to 300cc. A big bore kit on a 125 two stroke though or one of the 144cc factory two stroke bikes would be allowed to race against 300 two strokes in the lower displacement class. Boy, that is some lower displacement class racing against a KTM 300! One of the most powerful dirt bikes available is in the lower displacement class! In a long high speed desert race though I am pretty sure that as the hours ticked by the 300 two stroke would be feeling slow compared to a 450 four stroke. The 450F just goes whenever you twist the throttle, where the 300 two stroke has to be willed up into the narrow power band to make big power. And actually the KTM 300 probably does not even make more peak power than a KTM 450. When it is considered that cylinder port two strokes don’t attain more than about 85% cylinder filling the 300cc two stroke looks like a 510cc four stroke. And further when it is considered that the KTM 300 has a longer 2.83 inch stroke versus a KTM 450 with a 2.5 inch stroke it works out that both engines would make approximately the same power at the same mean piston speed.
My 1991 WMX 610 would be fairly competitive in desert racing, but surprisingly not because of the motor. With the oiling system problems the 610 would be at a significant handicap against the 450F bikes in sustained high speed cruising. The displacement advantage disappears quickly when the 610 has to be kept bellow about 7,000RPM for sustained cruising where the 450F can scream at 9,000 or even 10,000RPM all day long. What the 1991 WMX 610 does however do is handle rough terrain at high speeds really extremely well. For one thing it is actually a bigger bike than the modern 450F, the swing arm is longer, the wheel base is longer and the steering head angle is more relaxed. The lower seating position on the WMX 610 is also sometimes an advantage for high speed stability. Over rough terrain it is still a stand up bike leg pump bike, but sitting down on the WMX 610 at high speed usually feels much more secure than sitting down on a modern dirt bike. The suspension on the 1991 WMX 610 is also oriented to extreme high speeds, it works better and better absorbing bumps and staying in control up to very high speeds. Other bikes get dicey at higher speeds and just bounce all around.
What the big power of the 610 was good for was passing. It did not even need an opportunity, it would just blast past the 250 two strokes out of just about any turn. At a desert race without a mass start this is no longer so significant though. The 450F bikes also take much of the passing advantage that the WMX 610 had away. The reason that the 610 was able to pass a 250 two stroke so easily was just that all kinds of big power was available right away. The two stroke required clutch slipping and this introduced some delays before it was able to get going. Before the two stroke could get going with it’s modest 45hp the 610 was already past and pulling hard up to 65 or 70hp at 8,000RPM. It used to be said that the 250 two strokes could be tuned up to do as much as 60 or even 70hp peak output on race gas, but then the power band was even narrower and more difficult to reliably get to the ground. Clutch slipping also wastes considerable power dissipated as heat, so a narrower power band really is slower. The 450F dirt bikes are a lot faster than the 250 two strokes ever were, and the KTM 450F is obviously well able to be tuned to pull massive amounts of power over a very wide range of engine speeds. The stock 1991 WMX 610 might be able to deliver more maximum power than a stock 2016 KTM 450SX-F, but I doubt if it would really seem faster from gear to gear. The power band where the 610 makes maximum power is quite narrow from 7,500 to 8,500RPM. The 610 pulls darn good all the way down to 4,000RPM, but anywhere bellow 7,000RPM it is giving up quite a bit to a fast 450F. One might argue that this is because the computer controlled EFI systems on the 450F bikes can do a better job, but the reality is simply that the three inch stroke is excessively long. Computer controlled EFI systems certainly can do amazing things, but those amazing things are largely wasted on an engine with an excessively long stroke length. The shorter two and a half inch stroke simply delivers a wider range of engine speeds over which big power can be produced. The only good thing about the extremely narrow power band on the Husqvarna 610 is that after shifting at 8,500RPM the power down at 6,300RPM is low enough that scary things don’t happen even in slightly compromised traction situations. It is just never possible to shift down into the full 70hp tire churning output. The big output always only comes after the engine is revved up after the shift. The extra control that is gained by shifting down into only about 50hp makes the 610 much more manageable. Any time that it looks like the situation is going to get a bit dicey it can just be shifted sooner to drop down into even lower power. This works all the way down to a still sizeable torque output at 3,000RPM when the engine is running well without excessive amounts of spark advance being required because the fuel is for much higher compression ratio engines. When the bike is in control and ready for takeoff then the shifts can be stretched out to 8,500RPM for really big acceleration. Instead of a touchy throttle to modulate full power delivery it is just a matter of short shifting in preparation for upcoming terrain features. And of course the modulateable lower power output levels in full flame front travel mode at 2,500 to 4,000RPM are also very useful for slippery and difficult terrain.
The KTM 450SX-F with big power available anywhere from 8,000 to 12,000RPM is better though. It is just ready to go all the time, twist the throttle and it pulls big power. And with the computer controlled engine management system tuned just right all of that power is fully modulateable with the twist grip control as well. Having fully modulateable near maximum power available all the time is a big advantage for racing. The only disadvantage of the 450F bikes is that their power bands tend to be flat on top. Maximum power comes in the middle of the operable engine speed range, and then they continue to rev quite a ways with the power level or slightly dropping off. That dropping off of power as the engine is revved farther takes a lot of the fun out of ridding a dirt bike. It might still be fast in the hands of a skilled racer, but going into the overrev all the time makes for a boring and uninspiring feeling engine. Really the only reason that the 450F bikes have to be revved so high that the power flattens out in the overrev is to keep up with the Husqvarna 610. If a bit less power to the ground is acceptable then the KX450F and YZ450F can work great when used just from 6,500 to 9,500RPM. In this lower speed range there is a nice gradual ramping up of power over a wide range of engine speeds. Forty horsepower down at 6,000 or 6,500RPM is a bit disappointing compared to 60hp at 10,500RPM from a KTM 450SX-F, but that 40hp to shift down into is still a whole heck of a lot faster than what the 250 two strokes were typically able to do. Of course the new electronic power valve and throttle position sensor equipped two strokes are able to be programmed to work essentially as well as if they had EFI, so they also are a lot better than they used to be with mechanically controlled power valves and fixed advance curve CDI ignition systems. The 300cc big bore kits also allow for wider and shorter ports, which goes a long way towards mitigating the worst tuning problems that cropped up on the under square 250 two strokes from the last three decades. Oh and those Moto Tasnari V-Force reed valves, they basically have twice the capacity of traditional reed valves which looks like quite an advantage as well. I have to figure out how to cram one of those into my 400 WR one of these days.
Basically what it comes down to is that the Husqvarna 610 pulls really hard from 3,000 to 8,000RPM, but it needs premium gasoline to do it. Race gas for much higher compression ratios that requires more than about 25 degrees BTDC on the static timing setting results in both loud and harsh operation and dramatically reduced torque production. Regular gasoline with slower flame front travel speeds does not do it either, even if it is well matched to the compression ratio, because the slower flame front travel speed just does not allow the engine to rev all the way out with the fixed spark timing. On premium gasoline with spark timing values of about 21 to 24 degrees BTDC the 610 rips over a very wide range of engine speeds, and it would probably work very well down to spark timing values of about 16 or 18 degrees BTDC also as long as the flame front travel speed of the premium gasoline is indeed reasonably high.
Whatever compression ratio was required to attain crisp operation with a static timing setting of 18 to 24 degrees BTDC on fast flame front travel speed premium gasoline would probably be possible on the Husqvarna 610. I would bet that the stock 10.2:1 compression ratio is pretty close to it, I was after all able to run 23 and sometimes even 21 degrees BTDC on the static timing setting with good results for many years from 2002 to about 2005. This 10.2:1 compression ratio could easily be attained with the new Woessner replacement pistons just by milling the top down, it only has to go 0.03” and there is plenty of meat on the piston to go this far. If further reductions in compression ratio were required there is room to add 0.02” to the base gasket thickness as these engines always seem to run with the timing chain tensioner all the way in. The Woessner piston of course provides an easy way up to the 11.3:1 compression ratio, but going higher could be difficult. Just using a thinner base gasket to go up to 12.0:1 might be possible, but it would require some modification of the chain tensioner to take up the additional slack. I seriously doubt that the cheapest fast flame front travel speed fuel would require more than a 10.5:1 or 11:1 compression ratio to attain the desired 18 to 24 degree BTDC static timing setting, but this is all very muddled and confused now with all of these 13:1 engines running around demanding extremely high pressure fuel.
Certainly computer controlled engines would be able to run closer to the maximum compression ratio than mechanically controlled engines ever were able to, but how big is the difference? I don’t know. All I do know is that many times in the history of gasoline engine use compression ratios on mechanically controlled engines have been increased up to around 10:1 or 11:1. The first time was in the late 1950’s when all of the big block automotive engines went up to 10:1 to 10.5:1 and some went all the way up to 11:1 or even 11.25:1. That did not last long, but then again in the 1980’s turbo charged automotive engines of 7:1 to 8:1 compression ratios were running five to seven pounds of boost which would have been effective compression ratios in the 10.5:1 to 11.5:1 range. Then in the 1990’s some automotive engines went up to 10:1 and now recently nearly all of the port injected automotive engines are at 10.5:1 with a few at 11:1. If the cheapest fast flame front travel speed fuel were for compression ratios higher than 11:1 it seems that each of these three times that compression ratios were increased they would have continued to increase up to close to the maximum compression ratio for that cheapest fast flame front travel speed gasoline. Since the compression ratios have always topped out at around 11:1 it seems that this is close to the maximum compression ratio for the cheapest fast flame front travel speed gasoline. Everyone could have been wrong all three of these times that the compression ratios were increased, but that is all that I have to go on. The fact that the gasoline is changing so dramatically every week now that so many engines are up at 13:1 seems like it probably indicates that 13:1 is in fact too high. The big question though is whether 13:1 is just too high for any mechanically controlled engine, or is 13:1 also too high for a computer controlled port injected engine running on the cheapest fast flame front travel speed fuel.
This also gets all muddled and confused because computer controlled engines don’t have such a need for fast flame front travel speed fuel. Fast flame front travel speed fuel still delivers a wider range of engine speeds and a wider range of engine loads where good performance and efficiency are possible even on computer controlled engines, but computer controlled engines also could potentially do fairly well on slower flame front travel speed fuels. This is significant because slower flame front travel speed fuels would tend to be somewhat cheaper, and those cheaper slow flame front travel speed fuels would also be able to be run in somewhat higher compression ratio engines than the cheapest fast flame front travel speed fuel could be run in. The cheaper slow flame front travel speed fuels can be blended with 10 or 20% of the cheapest fast flame front travel speed fuel so that they can be run in the same lower compression ratio engines, but this then somewhat detracts from the performance and efficiency of the engines running the slow flame front travel speed fuel. A slow flame front speed fuel is already handicapped compared to faster flame front travel speed fuel in terms of having to run over a narrower range of speeds and loads. Having to run that slower flame front travel speed fuel in a lower compression ratio engine makes it seem even worse. On top of the inherently lower efficiencies of lower compression ratio engines a blended fuel also tends to result in some problems with the fuel not being homogeneous. Blended fuels don’t appear to cause any problems down at low engine speeds around 3,000 to 5,000RPM where the cylinder pressure is spiking up higher than it needs to just after late compression ignition. At these lower engine speeds the fuel just all burns. Up at higher engine speeds though where gasoline engines run at their best the cylinder pressure remains more constant as the piston begins to move down and a non-homogeneous fuel can contribute to somewhat reduced performance and efficiency.
The solution is for fast flame front travel speed fuel to be run in lower compression ratio engines and slower flame front travel speed fuel to be run in higher compression ratio engines. For this to work though a meaningful rating system for the combustion properties of gasoline would be required. At a bare minimum both the flame front travel speed of the fuel and the temperature and pressure requirements of the fuel for late compression ignition would have to be separately rated. The current rating system is totally non-functional because nobody can agree on whether it is supposed to rate the temperature and pressure capabilities of the fuel or the flame front travel speed of the fuel. The single number rating system has long been considered to rate the temperature and pressure capabilities of the fuel, and this could work if the standard grades of gasoline were in fact what they are supposed to be. Specialty fuels being pawned off as standard gasoline though totally throws this system off. In a single number rating system where the temperature and pressure capabilities of the fuel is what is rated race gas and regular fall under the same classification, and that is just plain stupid. Specialty fast flame front travel speed fuel for very high compression ratio engines has always been much much more expensive than gasoline. Likewise both the cheapest premium gasoline with a fast flame front travel speed and a fuel with large amounts of slower flame front travel speed regular gasoline blended in would fall under the same classification. Again that is just plain stupid. Someone buying premium gasoline wants a fast flame front travel speed for high performance over a wide range of speeds and loads, if what they get instead is 80% slower flame front travel speed regular gasoline it is just not going to cut the mustard.
-Michael Traum