I have been complaining for the past two years about how my 10.2:1 Husqvarna motors just won't run well on the new higher pressure gasoline available most of the time these days. With dirt bikes at 13.5:1 to 14.4:1 and street bikes at 13.0:1 the lower pressure premium gasoline just has not been available. How I actually ended up finally running a higher compression ratio which is more compatible with the new gasoline is a somewhat strange and entirely fascinating story.
Blown Head Gasket
Riding at 12.5:1
Racing on Regular
Sophisticated Engine Management
When I first put the new taller compression height Woessner brand piston in the rebuilt 1997 Husqvarna 610 motor I was thinking that the compression ratio was going to come out at 10.9:1. That was based on the thicknesses of the Woessner gaskets when they were new and not yet installed. When I actually assembled the new Woessner equipped engine and torqued the head bolts the gaskets squished right down to the same total thickness as the stock Husqvarna gaskets for an 11.3:1 compression ratio. So much for 10.9:1!
The reason that I had decided to go up to a 10.9:1 compression ratio was that the 10.2:1 engines just were not working anymore. Occasionally I managed to get some lower pressure gasoline that worked pretty well down at spark timing values of 21 to 26 degrees BTDC, but most of the time it was just oppressively high pressure gasoline that was extremely loud and harsh in the 10.2:1 engines and was just always requiring 28 to 31 degrees BTDC at 3,500 to 5,500RPM to make power. This higher pressure gasoline often did make substantial power, but the engine was extremely loud and harsh and low end torque suffered dramatically. The problem obviously was that the new gasoline was for much much higher compression ratio engines. Sometimes I somehow ended up with slower flame front travel speed gasoline that actually delivered better low end torque and smoother and quieter operation at 3,500 to 5,500RPM with the 28 to 31 degree BTDC spark timing, but the engine then would not rev out past about 5,500 or 6,000RPM and peak power production was very low compared to what my Husqvarna 610 was usually able to do at 6,500 to 8,000RPM. Most of the time the 91 (RON+MON)/2 octane rating premium gasoline was fairly fast flame front travel speed fuel that had no trouble up to 8,000RPM in the big Husqvarna 610 motor. One interesting thing that I noticed was that even with the fastest flame front travel speed fuel the engine did not rev quite as high with extremely excessive amounts of spark advance. With the static timing setting at 21 to 26 degrees BTDC the stock 10.2:1 610 motor was nearly always able to twist out to 8,000RPM. When the static timing setting climbed up to 28 to 31 degrees BTDC though even very fast flame front travel speed fuel just would not make power up at as high of engine speeds, often flattening out around 6,500 to 7,500RPM. The 10.2:1 engine just could not muster enough pressure at high engine speeds for that supper high pressure gasoline.
What it came down to was that the engine just would not work with more spark advance. Going up to a static timing setting of 26 degrees BTDC still delivered pretty good performance even if the engine got a lot louder and harsher. Going up to a static timing setting of more than 26 degrees BTDC, which is a spark timing of about 29 or 30 degrees BTDC with the three or four degrees of crankshaft wiggle advance, just did not work. On slower flame front travel speed fuel the engine would not rev out past 5,500 to 6,000RPM and on faster flame front travel speed fuel the engine lost torque bellow 5,000RPM and then often would not rev quite all the way out to the 8,000RPM normal maximum engine speed. The only way that the big 610 motor would run at it's best was with reasonably fast flame front travel speed gasoline and spark timing later than about 29 degrees BTDC (a static timing setting no earlier than 26 degrees BTDC). When the flame front travel speed of the gasoline was just a bit slower the engine favored the 24 to 26 degree BTDC range of static timing settings. On the fastest flame front travel speed gasoline the engine ran better and better the later the spark timing could be made, although I was never able to run a static timing setting later than 21 degrees BTDC on the stock 10.2:1 engine.
Because excessively early spark timing always resulted in much worse engine performance, and I was being forced to run spark timing values in the 30 to 35 degree range most of the time it was obvious that the gasoline was for much higher compression ratio engines. It was time to go up a bit on the compression ratio, and I had hoped that 10.9:1 would be a sufficient increase to get at least some of the lost performance back.
When I ended up with the 11.3:1 compression ratio after the new gaskets immediately compressed down I was not too disappointed as it had been obvious that the gasoline was for much much higher compression ratio engines. Sure enough the 11.3:1 compression ratio was nowhere close to excessively high on the new high pressure gasoline that was always coming out of the pumps. At first I ran spark timing values of 30 degrees BTDC and then 25 degrees BTDC on the new 11.3:1 engine and I had much less trouble with excessively high pressure gasoline. Sometimes there was hesitation even at 30 degree BTDC spark timing, but even down at 25 degree BTDC spark timing the 11.3:1 engine was much more consistently making big power without large amounts of hesitation or large amounts of excess harshness.
The good times at 11.3:1 did not however last. The first change was that for a period of time in June and July of 2015 the 91 (RON+MON)/2 octane rating premium gasoline was suddenly always much slower flame front travel speed fuel and the engine would not rev out well. At the same time the 91 (RON+MON)/2 octane rating premium gasoline also was a much lower pressure fuel and I was sometimes able to run as little as 16 degrees BTDC on the static timing setting. With this lower pressure fuel the 11.3:1 engine was running great and making substantial torque all the way down to 3,000RPM with spark timing values in the 16 to 21 degree range, but the engine just would not rev out well and advancing the spark timing did not help much at all.
Then as suddenly as it had come the low pressure gasoline was gone and it was back to supper high pressure and fast flame front travel speed gasoline that was loud and harsh at all engine speeds. For a short time in August and September of 2015 the 91 (RON+MON)/2 octane rating gasoline switched around quite a bit so that some days the 11.3:1 engine would make absolutely huge torque from 3,500 to 8,500RPM at a static timing setting of 24 degrees BTDC and then on other days I had to go up to 27 or even 29 degrees BTDC on the static timing setting and the engine became a whole lot louder and harsher with big torque coming only between 5,500 and 7,500RPM.
Then things started to get really weird. The gasoline was still changing around all the time, but it just was not making as much power at any static timing setting. I was having to run 28 to 31 degrees BTDC on the static timing setting (spark timing at high engine speed of about 31 to 34 degrees BTDC spark timing with the crankshaft wiggle advance) to get the hesitation down to manageable levels but the engine was often able to run and make a bit of power from 4,000 to 6,000RPM even with the static timing setting backed off to 24 to 26 degrees BTDC. With the static timing setting backed off though there were just huge amounts of hesitation at all engine speeds and fuel mileage plummeted. The gasoline was still rather fast flame front travel speed fuel, even with the static timing setting backed off it would make torque at 5,000RPM with small throttle openings once it got going. The combination of the gasoline being for higher compression ratio engines and just not making as much power meant that much larger amounts of spark advance were being required. The interesting thing about this fast flame front travel speed but less powerful fuel was that the engine was actually noticeably smoother and quieter at 5,000 to 6,000RPM with the excessive levels of spark advance than was usual on more typical gasoline. The excessive levels of spark advance however still resulted in extremely loud and harsh operation from 3,500 to 5,000RPM.
With these varied problems with frequently changing gasoline that was nearly always for much higher compression ratio engines I began to think about a 610 Husqvarna motor build that I called the "regular racer". The basic concept of the regular racer was to use the bigger 1992 camshaft and an even higher compression ratio to get higher engine speeds and more power out of poor quality gasoline. The big 1992 Husqvarna camshaft is not only a longer duration camshaft with 260 degrees at 1mm valve lift to the 242 and 250 degree at 1mm valve lift 1991 and 1994 camshafts; but the big 1992 camshaft is also just a much more aggressive stick. The maximum valve lift is a bit more on the 1992 camshaft, and the valves open and close much faster. Particularly the intake closing ramp on the 1992 camshaft is extremely steep, with the valves basically just dropping and crashing shut at the highest engine speeds. Increased duration always pushes the torque peak of an engine up higher, but particularly with a basic ignition system where more additional advance at higher engine speeds cannot be provided for a longer duration camshaft is absolutely essential for getting more engine speed out of slower flame front travel speed gasoline. The 242 degree at 1mm valve lift 1991 camshaft installed straight up with split overlap at TDC and the 250 degree at 1mm valve lift 1994 camshaft installed advanced 3.5 degrees of crankshaft rotation already both delivered peak cylinder filling up around 4,500 to 6,000RPM, but slower flame front travel speed fuel and fixed spark timing requires that maximum cylinder filling come up at maximum engine speed. The 260 degree at 1mm valve lift 1992 camshaft would work to push the peak cylinder filling up beyond 6,000RPM on the Husqvarna 610 motor. More lift and faster closing intake valves also just simply delivers better flow at the highest engine speeds to get every bit of power out of an engine.
As far as an increased compression ratio for slower flame front travel speed and higher pressure gasoline the options were limited on the 610 Husqvarna. Going up to 11.3:1 with the 0.030" taller compression height Woessner piston had been easy, but further compression ratio increases looked to be trickier. Removing the 0.020" base gasket on the 386 stroker motor had worked well to get the compression ratio to come out closer to the stock 10.2:1 compression ratio despite the shorter connecting rod. On that engine there had been some range of adjustment on the timing chain tensioner though. On the 610 motors the stock 10.2:1 head gasket and base gasket had usually resulted in a bit of slack in the timing chain even with the tensioner all the way in. Reducing the base gasket thickness promised to result in excessive amounts of cam chain slack. I did however think that I could probably modify the cam chain tensioner in some way to take up a bit more slack, and a 12:1 or 12.5:1 compression ratio seemed potentially attainable. Going higher than this on the compression ratio would require extremely drastic engine modifications. One way to go even higher on the compression ratio would be to use the shorter 7:1 Vertex brand 98mm piston with the timing set and shorter cylinder height of the 350 motor. This would require cutting a 610 cylinder down, and then also would require some additional material removal from the Vertex piston and the cylinder head to get the compression ratio down bellow 14:1. There is however plenty of extra "meat" on the crown of the short little 7:1 vertex 98mm piston, and the 610 Husqvarna cylinder head could also benefit from some extra relieving around the valves to decrease valve shrouding for better flow.
This particular idea for the high compression ratio regular racer would also be something of a "sleeper", as the shortened cylinder would make the 610 motor externally indistinguishable from the Husqvarna 350 motor. Since I had been able to lighten the 11.3:1 Woessner 610 piston down to the same 332g as the 7:1 Vertex brand piston there did not seem to be much reason to go that route of a shortened engine. The shorter 7:1 Vertex piston could probably be cut down to even less than 332g, and moving the weight of the cylinder head, carburetor and header pipes down a quarter inch would of course be an improvement if a rather small improvement.
Instead of actually building the sleeper regular racer Husqvarna 610 motor I just continued to ride the 610 Husqvarna that I already had, unbeknownst to me though someone at Woessner had similar ideas and had already been subtly working toward the same end product.
When I adjusted the valves at 30 hours on the new 11.3:1 engine I had checked the torque of the head bolts, and they felt tight. Since the gaskets had crushed down so much upon first assembly I was a bit concerned, but the fact that the bolts were still tight at 30 hours reassured me that the gaskets were doing fine.
The first sign of trouble was that after the weather had already begun to turn cold the 610 motor began losing coolant. It was just a spurt now and again, but the engine had never done that all summer long, even when accelerating fast in 110 degree heat. I had never put the overflow tube on the bike when I put it together, so this little bit of coolant blow out was hard to miss as it soaked my face with sweet tasting but toxic ethylene glycol. After I got soaked a few times on rides I had to put the overflow tube back on. This made the bike rideable again, and I was amazed that I was losing only a few ounces on a three hour ride. I kept ridding the bike this way for a few rides, but the coolant lose appeared to be getting worse despite the weather turning even colder.
Then the engine became hard to start sometimes. For the first time I had to resort to rolling the points ignition equipped 610 down a hill sometimes to get it started. The hard starting rapidly got worse, and then one day when I pulled the spark plug out it was wet with water. Not wet with gasoline but wet with water and when I touched the tip of my tongue to the electrode it tasted like ethylene glycol.
I dried the spark plug and kicked the bike over a few times and I was able to get the engine started with some difficulty. Just to be absolutely sure what was going on I drained the coolant, removed the spark plug again, dried it and kicked the bike over to clear the cylinder. This time the bike fired up extremely easily on the first kick. I shut it off and kick started it again, instant start on the first kick. I even rode the bike up the driveway and back and it still restarted extremely easily after this. Then when I poured water into the cooling system immediately after starting the engine it started to stumble and died a few seconds later. Obviously it was a blown head gasket sucking water into the cylinder.
The first thing I checked was the head bolts, they were all very loose. Each one just tightened down extremely easily as soon as I turned them. I might have been able to stop the coolant leak just by torquing the head bolts, but that stunt of running the engine for short periods with no coolant obviously had totally blown out the head gasket so I tore the top end down. What I found was a head gasket burned out on two sides. Not too surprisingly the gasket burned out on the front and back of the engine near the smaller 8mm head bolts. The cylinder head on the Husqvarnas is held on by four 10mm head bolts, but they are not perfectly evenly distributed around the cylinder. On the front and the back where the distance between the 10mm head bolts is about a half inch longer an 8mm bolt fills in the gap. The 8mm bolts don't go all the way down to the case though, they just attach the cylinder head directly to the cylinder. Because the 8mm bolts are so much shorter they don't stretch as much, so they loosened up more as the Woessner gaskets squished down.
After 115 hours of operation the Woessner piston was still in perfect condition, the black low friction coating had all worn off, but the small ridges cut in the skirts were still intact. The skirt diameter was only 0.0006" smaller with all of the black coating worn off, so it is really a very thin coating. The U.S Chrome plated cylinder had not fared so well. I found some small steel particles embedded in the piston skirts, and these had scored the cylinder quite noticeably. The score marks and scratches were not very deep, but it was enough to make the cylinder look somewhat unappealing. The scoring started down about an inch from the top of the cylinder, so the critical ring seal at the very top was not even much affected.
Instead of sending the cylinder off to be re-plated again I just cleaned it up a bit with 400 grit wet/dry silicone carbide paper. I hardly took any material off and the score marks subsided considerably. None of them were more than a few ten thousandths deep. The cylinder was still not great after I cleaned it up, but certainly serviceable for a dirt bike. I also carefully scraped each little piece of embedded steel I could find out of the piston skirts to minimize further scoring of the cylinder walls.
I am not quite sure where these little pieces of steel came from, but the rod bearing is a likely culprit. The rod bearing was however still tight, with no noticeable radial play when still full of oil. These little bits of steel might also have been left over from the previous catastrophic failure of this 1997 engine which had required a new cylinder and a new cylinder head. When I got the engine back in early 2015 it was still full of rather large quantities of aluminum from the broken up original piston. I did not see much in the way of steel material coming out of the cases when I rinsed them, but there may have been a few bits tucked away here and there. In any case these little bits of steel certainly were wreaking havoc on the cylinder walls, but in 115 hours of operation the damage was still quite minimal.
Reusing the Woessner head gasket was absolutely out of the question as it was totally burned out on two sides, with the packing between the sides of the metal band nearly completely missing. I did however still have the old stock Husqvarna head gasket that came with the motor. This gasket was in worse condition than any Husqvarna head gasket I have ever seen, but the area around the metal band still looked just about as good as they ever do. It was just the rest of the gasket that was all dinged up with chunks out of it near some of the bolt holes. I have always had very good luck reusing Husqvarna head gaskets by treating them either with Copper Coat brand gasket spray or just RTV high temperature silicone gasket sealer. In fact in all the times I have disassembled Husqvarna engines I only once bought a new head gasket, $100 for just one gasket for a single cylinder engine seemed pretty steep. That's six times I have reused original Husqvarna head gaskets and I have never had a failure. The one new Husqvarna original head gasket I bought also worked great and never squished down even in 300 hours of operation. I did check the torque on the head bolts several times, and they never loosened up. Any small amount that the original Husqvarna head gaskets do squish down is easily taken up by the stretch of those long 10mm bolts that go all the way down into the cases.
The crappy Woessner gaskets on the other had were a full 0.030" thinner when I took them off than when I put them on. That was 0.010" of squish down when I first torqued the head bolts on the new gaskets, and then another 0.020" of squish down over 115 hours of operation. Not only did the Woessner gaskets get thinner, but they actually failed. On both the head gasket and the base gasket the perforations in the steel center shim pushed up through the gasket material on both sides and left rather substantial indentations in the cylinder head and cylinder gasket surfaces. Luckily these indentations were only superficial, and the surfaces were easily scraped flat again.
It is hard to say just what the squished down gaskets meant for the compression ratio of the engine since the bolts were so loose, but it certainly was running with more than an 11.3:1 compression ratio there at the end. Going just by the thickness of the gaskets it looks like the compression ratio had been 12.0:1, substantially more than the 11.3:1 that the engine had started out with. It seems that most of this squishing down had taken place towards the end of the 115 hours that I ran the engine. At 77 hours I had the clutch out to check the oil reed valve and the timing chain was still tight. When I took the engine apart at 115 hours the timing chain was substantially loose.
Since the engine had still been requiring rather large amounts of spark advance on fast flame front travel speed fuel it seemed like sticking with a compression ratio over 12:1 was a good idea. Getting the thinner total gasket thickness was a simple matter of using the 0.020" base gasket off of the Husqvarna 350 instead of the 0.050" base gasket for the 610. This brings the compression ratio up to 12.5:1, a very substantial increase over the stock 10.2:1 compression ratio.
Once I got the top end buttoned up and the head bolts fully torqued down the timing chain was substantially loose. What I found though was that if I pushed in on the ratcheting tensioner without the spring and cap installed I could rather easily get it to click a few more clicks down and the timing chain tightened up nearly fully. The plastic tensioner bottoms out on the ends, but by pushing in harder on the ratchet mechanism the middle of the long tensioner bows out and takes up some of the slack. With just a very small amount of residual slack in the timing chain the motor seemed like it would do just fine with the new lower cylinder head position.
Another consequence of the lower cylinder head position is that the cam timing changes slightly. Lowering the cylinder head by 0.030" introduces 0.060" of slack in the chain, and on the 19 tooth 5/16" pitch crankshaft sprocket this represents a three and a half degree of crankshaft rotation change in the cam timing. Sure enough when I carefully checked the cam timing it looked like it was in there straight up with split overlap right at top dead center to within a half a degree. Previously I had estimated that the cam was in there advanced three and a half degrees of crankshaft rotation after the Woessner gaskets had squished down to the same thickness as original Husqvarna gaskets. In terms of the all critical intake valve closing time the 1994 250 degree at 1mm valve lift camshaft installed three and a half degrees advanced was very close to equivalent to the 1991 242 degree at 1mm valve lift camshaft installed straight up with split overlap at top dead center. Now that the cylinder head has been moved down 0.030" to increase the compression ratio to 12.5:1 that 250 degree at 1mm valve lift camshaft is installed straight up, so it now is finally really like a bigger camshaft with a later intake valve closing time. Just three and a half degrees of crankshaft rotation is not a lot, but it does make some small but significant difference.
At some slower engine speed where less than peak cylinder filling occurs because some of the intake charge flows back out the open intake valves that three and a half degree difference would be equivalent to the difference between 12.5:1 and 12.2:1. The later intake valve closing time does not necessarily reduce peak cylinder filling if the engine is capable of flowing up to the new slightly higher engine speed, but at reduced engine speeds there is certainly some small difference. A more interesting number might be what the actual difference in engine speed would be for a 250 degree camshaft installed straight up or advanced three and a half degrees. That is much more difficult to estimate theoretically not only because the actual flow modeling is rather difficult but also because there are just so many different things going on that play a role in how well an engine will flow at any particular engine speed. Suffice it to say that the three degree change is small but significant. Based on general camshaft sizing guidelines for pushrod engines that 51 degree BTDC intake valve closing time versus a 47.5 degree BTDC intake valve closing time would be approximately a 300RPM shift in the location of the power band.
These general guidelines for camshaft sizing would indicate that a stock 195 degree at 0.05" valve lift automotive camshaft would deliver peak cylinder filling down to about 2,200RPM, a 220 degree at 0.05" valve lift mild performance camshaft would deliver peak cylinder filling down to about 3,300RPM, a 250 degree at 0.05" valve lift oval track racing camshaft would deliver peak cylinder filling down to about 4,800RPM and a 270 degree at 0.05" valve lift drag racing camshaft would deliver peak cylinder filling down to about 6,000RPM. In reality camshafts work very nearly as well down to even lower engine speeds. A really big street performance camshaft with 260 degrees duration at 0.05" installed two degrees advanced has been known to actually deliver quite high torque generation down to 3,500RPM. The 242 degree at 1mm valve lift 1991 Husqvarna 610 camshaft always seemed to be able to deliver pretty impressive torque generation all the way down to 3,000RPM and the 250 degree at 1mm valve lift 1994 Husqvarna camshaft installed 3.5 degrees advanced also torqued pretty well all the way down to 3,000RPM.
How wide of a power band a camshaft will deliver near peak cylinder filling over depends entirely on the type of valvetrain and how well the engine will flow. The stock 1991 Husqvarna WMX 610 motor with the 242 degree at 1mm valve lift camshaft was nearly always able to pull hard out to 8,000RPM or higher. Parallel valve pushrod engines don't rev nearly as high on short duration camshafts because they just won't flow up at higher engine speeds. The mild performance 220 degree at 0.05" valve lift camshaft in small block Chevy will often pull fairly well to 6,000RPM but even a giant 270 or 275 degree at 0.05" valve lift drag racing camshaft in a totally worked out normally aspirated pushrod V8 is usually only good for about 8,000RPM. That giant camshaft is giving up power everywhere bellow about 6,000 or 6,300RPM and peak torque production is actually considerably less than for a shorter duration camshaft but the little bit of extra punch up to 8,000RPM certainly does win races.
An interesting side note is that at 115 hours the valve lash on the rebuilt 1997 610 motor was essentially exactly where I had set it at the 77 hour service. In the first 30 hours of operation the lash had opened up a bit, and again at 77 hours the lash had opened up some but not as much as it had in the first 30 hours. From 77 hours to 115 hours though the change was negligible.
When I first assembled the rebuilt 1997 motor back in May I had used some old 1991 valve adjusters that were in excellent condition on the also nearly new valves in the 1997 cylinder head. Even though the parts were in good condition they were not yet worn in to match each other. In the first 50 hours or so the adjusters and valve stem ends wore in a bit and a few thousandths of play developed. Once the parts were "meshing" just right though the rate of wear dropped off dramatically.
On my old stock 1991 WMX 610 motor I used to let the valves go for quite a long time between adjustments, and the valve lash always opened up only very small amounts. When I first got the bike I checked the valve adjustment even more frequently than the 30 hour intervals recommended in the 1991 Owner's Service and Tuning Manual, but when I saw such small amounts of change even at high engine speeds I decided they could go longer and sometimes I let the valve adjustment go for more than 100 hours. The adjusters and valve stem ends on that engine were worn down considerably, but they had always been run as a matched set.
With the bike all back together I set the static timing setting at 23 degrees BTDC and the 12.5:1 engine fired right up. When I shut the motor off after a brief period at idle it would however not restart. It had been blowing some blue smoke, so I figured I had just lubricated the rings with too much oil on reassembly. Cleaning the spark plug got it to fire right back up, and the blue smoke was quickly gone. When I rode off I was pleased to find instant torque at all engine speeds and not much in the way of hesitation. The motor was running great and making quite a bit of power and it was just a total joy to have the 610 Husqvarna back down to a static timing setting of 23 degrees BTDC again. It was immensely more smooth and quiet than it had been up at 28 and 29 degrees BTDC on the static timing setting, and torque at 3,500 to 5,000RPM was dramatically improved. In fact all the way down to 3,000RPM the engine was running really quite well. It was also able to rev out pretty good, although it was seeming like the flame front travel speed of the gasoline was a bit low as it was requiring a rather larger throttle opening to run up at 5,000 to 6,000RPM. When I really went after it I easily got about 7,000RPM with only slight amounts of hesitation, but the tachometer was not working well. Later I changed the tachometer setting from 1 per 1 revolution to 2 per 1 revolution and it began to work well again.
Another thing I noticed right away was that I could not use the 7.4V battery to power the ignition system anymore. On the 10.2:1 engines the two cell 7.4V model airplane batteries work great, and even on the 11.3:1 engine I had mostly been able to use the 7.4V batter although sometimes I had had a bit of trouble with the ignition system cutting out at that low voltage. On the new 12.5:1 engine the 7.4V battery just flat out did not work. As soon as the engine got going above about 5,000RPM with the throttle wide open the ignition would just cut out every time. At higher battery voltages this cutting out was not present at all. Even just a little 1.3Ahr 12V AGM lead acid battery was perfectly sufficient to power the ignition system all the way up without the ignition system cutting out at all.
Higher compression ratios increase the density of the air/fuel charge and require more spark energy to fire the spark plug. This cutting out was only seen at higher engine speeds for two reasons. One thing going on here is that at low engine speed the long duration camshaft does not allow the cylinder to fill as fully, but there is also the fact that the spark energy of a points ignition system drops off at higher engine speeds. With the massive 200 degree dwell angle this particular points ignition system has little in the way of difficulty operating at high engine speeds, but the spark energy is still going to tend to be a bit lower at very high engine speeds.
With the new engine running pretty well I headed off to town for some fresh 91 (RON+MON)/2 octane rating premium gasoline. The combination of a slightly slower flame front travel speed and just 23 degrees BTDC on the static timing setting meant that even though the engine was running really amazingly well fuel mileage had not correspondingly increased. I had been counting on 50MPG for the circuitous casual cruise to town, but I ran out of gas one mile up the road from the nearest gas station. About 46MPG I believe it was. Luckily it was all flat and slightly downhill for that mile, so it was not too difficult of a walk pushing the bike. At least when I got to the gas station I knew I was filling up only with the new gas not mixed with anything else in the tank.
Somewhat confusingly the 2.40 gallon tank took 2.57 gallons. I have never actually measured the volume of the tank, but I thought it was much closer to the 2.40 gallon specification than that. At least the price was very low at $2.55 per gallon of 91 (RON+MON)/2 octane rating premium gasoline, the lowest gas price I have seen in quite some time.
Premium in name only as it turns out. The engine again ran great, but there seemed to be a bit more hesitation with the same 23 degree BTDC static timing setting. The engine was also still revving out pretty well but it certainly was not hitting that last big drive up past 8,000RPM. When I climbed up to 3,000 feet of elevation the hesitation got a whole lot worse and the bike was not fun to ride because power just was not there when the throttle was twisted. I then bumped the static timing setting all the way up to 27 degrees BTDC, and this really got the motor to come alive. I then went all the way up to over 5,000 feet of elevation and the engine was pulling supper strong with only small amounts of hesitation up at higher engine speeds.
When I descended back to 1,000 feet of elevation the engine was still working really quite well although it was excessively crisp over a wide range of engine speeds. Pretty big instant torque was still available from 3,500 to 7,000RPM, but the engine was loud and harsh and actually ran better with the throttle only cracked open just enough to enter late compression ignition mode. With 27 degrees BTDC on the static timing setting the engine was able to rev all the way out, but it was hesitating some all the way up at the top around 7,500 to 8,000RPM. Certainly a slower flame front travel speed fuel. Normally the big valve 610 engine always gives a solid pull to 8,500RPM if it is able to hit 7,600RPM at all.
The next day the engine would not run at 23 degrees BTDC, it just would not pop off on late compression ignition at all. It still fired up easily and idled along fine, but it just would not make any power. It would rev up some to about 5,000RPM quite easily, but there was just no power anywhere. I again bumped the static timing setting up to 27 degrees BTDC and the big power came back, but this time it was just hesitating horribly over a wide range of engine speeds even down at 1,000 feet of elevation. Wow that is some really very high pressure fuel that won't even run in a 12.5:1 engine! There was only a small bit of fuel in the tank so I poured in a couple of quarts of old 91 (RON+MON)/2 octane rating premium gasoline that I had lying around and I set the static timing setting down at 21 degrees BTDC.
When I first rode off there was still no late compression ignition and not much power. As I cruised along at 35 to 45mph in full flame front travel mode the engine was working fine, but it obviously did not want to go fast with no late compression ignition and spark timing of just 21 degrees BTDC. After about 10 miles I was amazed to find that when I opened the throttle I got late compression ignition again. Wow! As I rode along the engine got crisper and crisper and pretty soon it was actually working again even though there was some substantial hesitation. It was even able to rev out to 7,000RPM on a big pull, but there was lots of hesitation from 6,000 to 7,000RPM.
When I got to a gas station I filled up with 2.35 gallons of 91 (RON+MON)/2 octane rating premium gasoline, but the pump was so slow that a few ounces spilled on the ground. Of course the Husqvarna gas tanks don't work quite like the gas fill on a car or light truck, it requires care and proper technique to fully fill the tank without spilling gas. The procedure normally is to just stick the nozzle into the tank a bit sideways so that it goes fully down into the side of the tank and the vapor recovery gasket seals against the opening in the tank. The pump can then just be run rather fast to fill the tank and it shuts off automatically when the liquid reaches the end of the nozzle. This is however far from a full fill as the nozzle is quite far down into the small tank. To get the last bit of the tank full it is necessary to remove the nozzle far enough that the level in the tank can be seen past the now disengaged vapor recovery boot. Sometimes it has also been necessary to manually pull the vapor recovery boot back to engage a limit switch that allows the pump to dispense fuel. The tank is then topped up visually as motorcycles have always been fueled.
On that particular day the pump was just going really slow, it took a long time to fill the tank and then the pump did not shut off at the top even though the nozzle was fully inserted into the tank and the vapor recovery gasket was fully seated around the opening in the tank. Instead of shutting off when the liquid level reached the end of the nozzle as is normal the pump just kept going until I saw gas squirt out from under the vapor recovery gasket and I released the leaver. A very annoying mess, and one totally due to a malfunction of the pump.
When I rode off the engine was running about the same, but the flame font travel speed of the gasoline was seeming even lower. There was big torque reliably available over a wide range of lower engine speeds from 3,000 to 5,000RPM and the amount of torque generated at these engine speeds was quite high. The higher engine speeds were however requiring very large throttle openings and the engine would not rev out past 6,000RPM. The engine was still usable, but it is always disappointing to lose the higher revs even on a bike that makes gobs of power way down at 4,000 to 6,000RPM. When I got home I checked the static timing setting again, and it was still right at 21 degrees BTDC.
A few days later I took the 12.5:1 engine out for a short one hour ride down at 1,000 feet of elevation ostensibly on the same gasoline that was still left in the tank. It was however not the same gasoline. With the static timing setting up at 22 degrees BTDC this time the engine was hesitating even worse. It would make some power, but very reluctantly and the amount of torque generation was considerably less than the last time I rode the bike. Amazingly the flame front travel speed of the fuel was seeming fairly fast as the hesitation was somewhat worse down at 3,000RPM than up at 4,000RPM but it was just hesitating way to much everywhere and would only rev out to 6,800RPM on a big pull. I stopped and bumped the static timing setting up to 24 degrees BTDC, and this did help get crisper operation down at 3,000 to 4,000RPM but it was still hesitating quite a bit at higher engine speeds. With 24 degrees BTDC on the static timing setting the engine was able to rev all the way out to 7,300RPM, but it was hesitating horribly everywhere above 6,000RPM and power output seemed low at all engine speeds.
When I got back I took the last bit of gasoline out of the tank and put it in the empty tank on my 1992 TE with the stock 10.2:1 1991 WMX 610 motor in it. With a static timing setting of 30 degrees BTDC the 10.2:1 engine was able to make quite a bit of power and rev all the way out. With that much spark advance the engine was very loud and harsh down at 3,500 to 5,000RPM, but amazingly not quite as loud and harsh as is usually the case with that much spark advance. There was some considerable lag before the 10.2:1 engine would get going, but once it lit off on late compression ignition it was able to pull pretty darn hard from 5,000 to 7,000RPM and it also was able to rev much higher. There was not much more power up above 7,500RPM but the engine did pull all the way out to 9,000RPM, the highest I have ever seen on a tachometer on any of the 610 motors. Certainly a somewhat slower flame front travel speed gasoline, but with a huge 33 or 34 degree BTDC spark timing value the stock 1991 WMX 610 motor was able to make power all the way up to 9,000RPM.
Another strange thing I had noticed was that the 12.5:1 engine smelled bad at low idle even with 24 degrees BTDC on the static timing setting. The 10.2:1 engine with 30 degrees BTDC on the static timing setting and low idling at 2,400RPM did not have this stinky exhaust smell on the same gasoline. Normally with any static timing setting from 15 degrees to 20 degrees BTDC the 610 motor idles crisply and cleanly at around 1800 to 2000RPM. With anything more than about 23 degrees BTDC on the static timing setting the idle normally gets rather loud and harsh and tends to be up around 2,200RPM. It was very surprising that the engine was idling down so low with 24 degrees BTDC on the static timing setting, and somewhat disappointing that it smelled so bad. I had noticed this same smelly exhaust and low idle speed on a previous tank of gasoline as well, but when I rode to town and filled up with a full tank of fresh gasoline the engine then idled more crisply and did not stink even though the flame front travel speed of the fuel was rather low and large throttle openings were required to run at 5,000RPM.
I then tried the two motor gasoline swap with some other old 91 (RON+MON)/2 octane rating premium gasoline I had lying around in a different container. This time the 10.2:1 stock 610 motor hesitated really pretty badly with 29 degrees BTDC on the static timing setting, but it was able to make power and rev out once warmed up. I drained the gasoline and put it in the empty tank on the 1991 WMX 610 with the 12.5:1 motor. With the static timing setting still at 24 degrees BTDC the engine made more power and revved out better on this other 91 (RON+MON)/2 octane rating premium gasoline, but it was hesitating just horribly. At all engine speeds there was just a huge amount of hesitation before the engine finally got going and made power. Once well warmed up the engine was able to rev out pretty well, but power delivery was still somewhat week as it was just hesitating so much at higher engine speeds. I then bumped the static timing setting up to 27 degrees BTDC and this got more power with less hesitation. There was still some hesitation across a wide range of engine speeds, but once well warmed up the motor pulled strong to 8,000RPM. Now that is some very strange specialty fuel that will run at 29 degrees BTDC in a 10.2:1 engine but still hesitates badly at 27 degrees BTDC in a 12.5:1 motor. No doubt a lower energy density fuel that benefits from the richer jetting on the stock 10.2:1 motor.
Perhaps unsurprisingly the leaner jetted 12.5:1 motor was considerably smoother and quieter than the richer jetted 10.2:1 engine running nearly the same amount of spark advance on the same fuel. Torque generation from both engines was rather similar, although there was more lag and less hesitation on the 10.2:1 engine and more hesitation and less lag on the 12.5:1 engine. The 10.2:1 engine took a second to get going, but once up and running in late compression ignition mode it gave a strong pull all the way up. The 12.5:1 motor delivered more instant torque, but there were dead spots at various engine speeds where the engine would hesitate unless well warmed up on a big pull. Once well warmed up on a big pull the 12.5:1 engine actually seemed to be making more torque and more power on the same fuel even with leaner jetting. This then was a somewhat slower flame front travel speed fuel so that a big four inch bore engine tended to require a rather large amount of spark advance to do much of anything up at above 7,000RPM and also a lower energy density fuel so that a lower compression ratio with very rich jetting appeared to run better than expected. Lower energy density and a slower flame front travel speed: Sounds like some ethanol in there to me.
Some of the difference is also in the exhaust system. As well as the dual inline muffler system with the 14 disk Supertrapp works on the 1991 exhaust system the stock 1992 exhaust system with dual parallel J&R Answer mufflers probably does flow at least a small amount better. The header pipes and main "Y" are a bit bigger on the 1992 exhaust system, and the 1992 exhaust system is in fact loud beyond all reason. Compared to the ear splitting bark of the 1992 exhaust system the Supertrapp equipped 1991 exhaust system is the low purring of a kitten.
It is mostly in the low to midrange engine speeds from 3,000 to 6,500RPM where the 1992 exhaust system is dramatically louder. Up at the top end screaming at 7,500RPM and above the dual inline mufflers on the 1991 exhaust system still are rather loud. I often get the impression when ridding the 1992 bike that the exhaust system was designed to produce as much sound way down low at 3,000 to 4,500RPM as possible so that the higher engine speed operation does not sound as loud in comparison. The 1992 J&R Answer mufflers certainly do work to quiet the scream of the engine up at the highest engine speeds. Even at that though the 1992 exhaust system is still noticeably louder at 7,500 to 8,500RPM than the Supertrapp equipped 1991 exhaust system, the difference just is not nearly as large up at those high engine speeds.
The fact that the 610 motor has always seemed to make just as much power and torque on the quiet 1991 exhaust system as on the loud 1992 exhaust system indicates that the difference in flow, if any, is actually very small. Even with leaner jetting the rebuilt 1997 motor has always seemed to make noticeably more torque across the entire engine speed range than the stock 1991 610 motor in the 1992 chassis when the fuel is swapped back and forth between the two bikes. Of course that difference in torque is in the compression ratio. On the same gasoline the higher compression ratio engine is unavoidably going to make more torque if it can be made to run at all.
A few days later I fueled up the 12.5:1 Husky again with some 91 (RON+MON)/2 octane rating gasoline that I had lying around. With the static timing setting down at just 21 degrees BTDC this gasoline was again just barely lighting off on late compression ignition. At first with the engine cold I was not getting any late compression ignition at all, then after about a mile it began to pop off a little bit around 3,000 to 3,500RPM. A few miles later the engine actually began to sort of work with late compression ignition reliably available from 3,000 to 4,500RPM. It was making some torque, but there was a lot of hesitation and the engine had to be well warmed up from a few good little pulls before it would start to light off. Going up through the gears though I was able to lay down a bit of power and scoot along. I rode up from 1,000 feet of elevation to 3,500 feet of elevation and amazingly I was still getting some late compression ignition and substantial torque around 3,500RPM. I went all the way up to 5,500 feet in the snow, and up there I was neither able to get any late compression ignition or cared much about whether I did or not. The amount of power the engine was making just in full flame front travel mode from 2,500 to 4,500RPM was plenty to spin the rear tire through the 10 inch deep slightly wet snow. When I descended back down to 2,000 feet of elevation the engine was again laying down some substantial torque, and climbing up a winding dirt road I was able to lay down quite a bit of power up to about 5,000RPM once the engine was well heated up. Coming out of the turns it was taking a few seconds though and sometimes an upshift before it would get going and make torque.
I stopped and bumped the static timing setting up to 23 degrees BTDC, and this got the engine going much better. The 12.5:1 engine was then quite crisp at 3,000 to 3,500RPM with instant torque always available with a small twist of the throttle. It was also pulling pretty well up to 6,000RPM, but up at the higher engine speeds there was just way too much hesitation. Even from 4,500 to 5,000RPM the power was not there sometimes once the engine had cooled off. It was able to rev out to 7,000RPM on a big high speed pull, but there was tons of hesitation from 6,000RPM up. This sounds very much like slow flame front travel speed fuel, but at high elevation I had also been noticing rather crisp and loud full flame front travel mode operation around 3,000 to 3,500RPM which with just 21 degrees BTDC on the static timing setting would indicate that the flame front travel speed of the gasoline was reasonably fast. All I could come up with was that the energy density of the gasoline was just extremely low.
When I got back from the two hour ride I drained one gallon of gasoline out of the tank, so it had been just 0.7GPH for a mixed ride with lots of sixth gear cruising. Less than I would have expected with all of that hesitating and only 21 degrees BTDC on the static timing setting for most of the ride.
I then put that one gallon of gasoline into the empty tank on my 1992 TE with the stock 10.2:1 610 motor. Thinking that this was a lower pressure gasoline I set the static timing setting down at about 26.5 or 27 degrees BTDC and amazingly the rich jetted 10.2:1 motor ran and made power without much hesitation. Even with the big 180 main jet and the needle clip one position richer though the 10.2:1 engine felt extremely lean. When I first tried to accelerate hard the engine cut out for a split second at about 1/4 throttle and 5,000RPM before recovering and continuing to pull. Once the engine was fully warmed up this cutting out subsided to where it was hardly a problem, but the richer jetted engine was still just feeling very lean. It did however make power and rev all the way out to 8,500RPM on a big pull. Again though just as on the richer jetted 12.5:1 motor the engine was crispest way down at 3,000 to 3,500RPM. Even at that the 10.2:1 engine had quite a bit of lag and it took a second to get going after the throttle was opened. That is some very low energy density gasoline that makes the richer jetted stock 10.2:1 motor feel too lean. And again the lean jetted 12.5:1 running with some hesitation at 23 degrees BTDC while the rich jetted 10.2:1 engine runs mostly without hesitaon at 27 degrees BTDC clearly indicates very low energy density gasoline.
The big clue that the 91 (RON+MON)/2 octane rating premium gasoline has been a much slower flame front travel speed fuel is that the engine has been reliably pulling hard all the way down to 3,000RPM even when hesitating badly on obviously rather high energy density gasoline. On faster flame front travel speed fuel these big cam Husqvarna motors loose torque down at 3,000RPM first even if they will still light off and make power up at 4,000 and 5,000RPM once well warmed up. The even bigger 250 degree at 1mm valve lift 1994 camshaft now installed straight up with split overlap right at top dead center should even more dramatically demonstrate this tendency for the motor to lose torque down at 3,000RPM first. It is only slower flame front travel speed fuel that will still light off on late compression ignition down to 3,000RPM when the engine is hesitating badly up at 4,000 to 5,000RPM with a long duration camshaft.
That tank of slower flame front travel 91 (RON+MON)/2 octane rating premium gasoline I rode the bike to town for also seemed to have a rather high energy density. It was making lots of torque with an authoritative heavy sound at 3,500 to 4,500RPM, and despite the rather lean jetting the exhaust ended up blackened after a two hour ride. The blackening of the exhaust also has to do with the slow flame front travel speed though for two different reasons. When the engine hesitates at higher engine speed on slow flame front travel speed fuel the last of the fuel burned in the far corners of the combustion chamber in full flame front travel mode burns cold and slow and tends to blacken the exhaust system. The other reason that slower flame front travel speed fuel blackens the exhaust is that the slower flame front travel speed gasoline tends to require large twists of the throttle up onto the richer main jet for the engine to run up at 5,000RPM and above. Twisting up onto the main jet all the time always uses more fuel and would tend to blacken the exhaust more.
It has to be said that as a dirt bike rider I generally much prefer fast flame front travel speed fuel that is capable of delivering good power and smooth engine operation over a wide range of engine speeds and engine loads. Slower flame front travel speed fuel may however be an increasing reality with so many sophisticated computer controlled engines on the road which can take good advantage of the lower cost and higher refining efficiency of heavier, denser and hotter burning slow flame front travel speed gasoline.
Many hard core racers have of course long known a thing or two about slow flame front travel speed gasoline. One of the big things is just that it tends to be cheaper than the fastest flame font travel speed fuel if it can be made to run in popular engines. And that has been the difficulty, slower flame front travel speed gasoline is just harder to run in mechanically controlled engines. For racing there is no way around large amounts of spark advance to get a big bore engine to rev all the way out on slower flame front travel speed fuel. Twisting a four inch bore engine out to 8,000RPM on slower flame front travel speed fuel is inevitably going to require in the neighborhood of 30 to 33 degree BTDC spark timing.
There is just not enough time for pressure to build at 8,000RPM without sufficient spark lead. The slower the flame front travel speed the more spark advance is required, the faster the engine speed the more spark advance is required and the bigger the bore diameter the more spark advance is required. This is an unfortunate reality because engines just don't run well with large amounts of spark advance. With slow flame front travel speed gasoline though there is just no easy way around high engine speeds requiring loads of spark advance.
If it is assumed that some faster flame front travel speed gasoline has a 30% faster linear flame front travel speed than some slower flame front travel speed gasoline then high engine speed operation on that slower flame front travel speed gasoline is going to require at least a 30% larger numer of degrees of crankshaft rotation from the time the spark plug fires to the time of late compression ignition. If the time of late compression ignition is assumed to be 15 degrees ATDC then an engine running spark timing of 20 degrees BTDC on fast flame front travel speed fuel would tend to require spark timing of 30 degrees BTDC or a bit earlier on the slower flame front travel speed fuel. This of course assumes that a large four inch bore engine running at 8,000RPM with a spark timing of 20 degrees BTDC was already running as close to the maximum compression ratio for that fuel as was desirable for the application. An even better match between the compression ratio of the engine and the temperature and pressure requirments of the gasoline might allow the four inch bore engine to rev to 8,000RPM with a bit less spark advance, but this is where an engine becomes finiky and difficlut to tune with simple mechanical controlls.
Sophisticated computer controlled engines can of course do better even on slow flame front travel speed fuel, but substantially reducing the full load spark advance at 8,000RPM on a four inch bore engine requires a very good match between the compression ratio of the engine and the temperature and pressure requirements of the fuel being used. For traditional racing done with locked down spark timing or simple mechanical advance mechanisms slow flame front travel speed gasoline is always going to equate to rather large amounts of spark advance at high engine speeds. The inevitable result of large amounts of spark advance is harsher and less efficient operation at all reduced engine speeds.
The worst case scenario for slow flame front travel speed gasoline is just a single locked down spark timing value. With around 30 to 33 degrees of advance locked down the engine will be very loud and harsh at all engine speeds, and it will just be impossible to get good torque production down at 3,000 to 5,000RPM. This is true even when the camshaft, intake stacks and exhaust system are all perfectly tuned so that the engine has just the right level of crispness over the entire engine speed range. As hard as it is to attain this perfect level of tune where the engine easily enters late compression ignition mode at each engine speed at the latest possible spark timing value that is still not enough to overcome the inevitable problems associated with large amounts of spark advance at lower engine speeds.
Basically what it comes down to is that the latest possible time of late compression ignition of approximately 15 or 20 degrees ATDC is only attainable with smaller amounts of spark advance. With 30 degrees of spark advance that latest possible time of late compression ignition is such a small target to hit that it just does not happen. No matter how carefully an engine is tuned earlier than about 27 degree BTDC spark timing just won't deliver the latest possible time of late compression ignition. This is true on any bore diameter and with gasoline of any flame front travel speed. More spark advance just can't hit the latest possible time of late compression ignition. At 15 to 23 degree BTDC spark timing that latest possible time of late compression ignition is rather easily attained even with slightly sloppy tuning. Up at 26 and 28 degrees BTDC absolutely perfect engine tune can still deliver close to that latest possible time of late compression ignition, but it is not easy and ultimately not very likely. A big problem up at that large amount of spark advance is just that the engine has to have large amounts of lag and hesitation to reliably hit the latest possible time of late compression ignition. Dial in enough crispness that the lag and hesitation goes away and the time of late compression ignition falls over to an earlier time.
It is this falling over to an earlier time of late compression ignition at too low of an engine speed that is the problem with large amounts of spark advance on slow flame front travel speed fuel. Getting the time of late compression ignition substantially after top dead center is always a bit of a challenge, but it is much easier with more moderate amounts of spark advance. When the spark plug is firing at 30 degrees BTDC the time of late compression ignition tends always to fall over to an earlier time. That earlier time of late compression ignition is probably about 5 degrees ATDC, although it can easily be even earlier if the engine is running with extremely excessive levels of crispness.
An advance mechanism can go a long way to getting slower flame front travel speed gasoline to work well over a wide range of engine speeds. That advance mechanism however needs to continue advancing all the way up to maximum engine speed. An advance mechanism that tops out at 3,000 or 3,500RPM on an engine that revs to 6,000 or 8,000RPM is of use only for delivering quieter and smoother idling and does very little to improve torque generation at 3,000 to 5,000RPM. With the advance topping out at 3,000 or 3,500RPM the spark timing is going to have to be set for maximum engine speed, and on slow flame front travel speed fuel that is going to be way too darn early for 3,000RPM operation if the engine is to rev out to 7,000 and 8,000RPM.
Some traditional CDI motorcycles and dirt bikes were build with digital advance curves that kept advancing up to higher engine speeds. For these to work on slow flame front travel speed fuel though they have to be perfectly matched to the fuel that is actually used and it almost goes without saying that an excellent match between the compression ratio of the engine and the temperature and pressure requirements of the gasoline is a mandatory pre-requisite for good results in running slow flame front travel speed gasoline. Since motorcycles and dirt bikes have nearly always been run on faster flame front travel speed premium gasoline these advance curves have rarely been perfectly correct for much longer than one tank of gas.
If a good match between the compression ratio of the engine and the temperature and pressure requirements of the gasoline can be provided for then an advance curve could potentially deliver greatly improved performance over a range of engine speeds on slower flame front travel speed gasoline. The big thing that an advance curve allows for is a smaller amount of spark advance at reduced engine speeds. This really only works over the range of engine speeds where good cylinder filling is attainable, so a competent valvetrain is also a pre-requisite for good results on slow flame front travel speed gasoline. If fairly high cylinder filling can be attained over a range of engine speeds from say 4,000 to 7,000RPM then a well matched advance curve can deliver the big 30 or 33 degree BTDC spark timing to rev out to 7,000RPM and then back off to 23 or 25 degrees BTDC down at 4,000RPM so that the latest possible time of late compression ignition can be hit without punishing levels of lag and hesitation. If such an advance curve was perfectly tuned to the fuel and engine being used and a good match between the compression ratio of the engine and the temperature and pressure requirements of the fuel existed then the range of engine speeds could be stretched out even farther for some overrev to 8,000RPM or good torque generation down to 3,000RPM.
The wider the range of desired engine speeds the more difficult it would be to get the advance curve to work well on slow flame front travel speed fuel, and of course slow flame front travel speed gasoline is just never going to do as well as faster flame front travel speed gasoline over a wide range of engine speeds and engine loads. Even a very good functioning advance curve for slower flame front travel speed fuel is still going to result in an on or off power delivery up at the top of the engine speed range where the engine just won't run under reduced loads.
For racing an inability of the engine to run under reduced loads up at the top of the engine speed range is acceptable, up to a certain point. If the engine cuts out and loads up so severely when backing off at high engine speed that the spark plugs foul and the engine then has trouble getting going again this is neither going to win races or provide much fun for anyone other than perhaps spectators who are enthralled by the simple fact that an engine can be made to run so poorly. Some sort of load dependant advance mechanism can go a long way to preventing cutting out and loading up when running slow flame front travel speed gasoline up at high engine speeds. Even if all that the load dependent advance mechanism is able to do is dial in some fixed amount of additional spark advance when the throttle is closed it can be of some use over a certain range of engine speeds. If the amount of load dependant advance is fixed and does not vary with engine speed then it is inevitably going to be too much additional advance at low engine speed and not enough additional advance up at high engine speed. Still though an additional five or ten degrees of spark advance coming on when the throttle is closed can deliver much better light load performance at least across the middle of the engine speed range.
Vacuum advance is difficult to apply and has many severe limitations, but it is in fact tunable. Getting vacuum advance to do something useful for a race engine running slow flame front travel speed gasoline would require moving the vacuum port higher up the barrel away from the closed throttle plate. The higher the vacuum port the higher the engine speed would have to be in order to pull that vacuum advance in. With the vacuum advance coming only up above say 4,000RPM then a larger amount of additional advance could be used that actually would do some good on slow flame front travel speed fuel up to 6,000 and even 8,000RPM. Taken to an extreme the vacuum advance might not pull in until 6,000RPM and would be a sizeable perhaps 15 or even 20 degrees of additional advance to allow slow flame front travel speed fuel to actually run under a reduced load up at 8,000 or even 10,000RPM. Without an advance curve that continued to advance all the way up to maximum engine speed a high speed only vacuum advance would be quite tricky to use as there would be a pronounced tendency for the engine then to only be able to run with the throttle closed enough to pull in that vacuum advance. The level of complexity possible has seemingly no end, multiple vacuum advances might be used on separate vacuum ports to deliver an ignition map better matched to the requirements of the engine over a wider range of engine speeds and loads.
Once multiple vacuum advance mechanisms are being thrown around though some sort of transistorized ignition system starts to make a lot more sense. A little bit of electronic control can go a long way to delivering reliable, flexible and adjustable ignition performance. The main problem with electronic ignition systems from the 1970's through the first decade of the 21st century was simply that they were not adjustable and were compromise designs intended to sort of work over a wide range of levels of engine tune and fuel properties. The result was that they very rarely worked well for any purpose. Just a little bit more adjustability and the addition of a load dependant advance mechanism could make basic electronic ignition systems work quite well if the properties of the fuel remained the same for a while. With fuel properties dramatically changing from week to week nothing short of a sophisticated feedback type computer controlled engine management system works well, and even those often have not been able to keep up so to speak.
When feedback type computer controlled engine management systems are used to fine tune the use of slower flame front travel speed gasoline the problem of attaining a good match between the compression ratio of the engine and the temperature and pressure requirements of the fuel takes on new meaning. What a computer controlled engine really needs to run well on different types of gasoline is some means of varying the level of maximum cylinder filling. A variable vane geometry turbocharger works for this purpose, as can a fully variable valve timing system. In this situation where the level of maximum cylinder filling automatically adjusts to gasoline with changing temperature and pressure requirements then the higher pressure fuel that allows the highest maximum cylinder filling is of course going to make the most power. Any lower pressure fuel that requires a reduced level of maximum cylinder filling is just not going to offer the same level of maximum torque production. The exception might be some really weak high pressure specialty fuel that just had a much lower energy content or a much lower maximum temperature of combustion potential which still would not be able to make as much power as a slightly lower pressure fuel that burned with an abundance of high temperature heat.
All of this about computer controlled engine management systems is very relevant to the dirt bike industry in general since mechanically controlled engines have been hard to come by on dealer show room floors for quite a few years now. For someone just trying to have some fun and scoot around on a pile of obsolete Husqvarna parts though everything from vacuum advance to oxygen sensors is pretty much irrelevant. There is not much chance of a 40mm DellOrto slide type carburetor being drilled for a vacuum advance port, and less chance that a vacuum advance mechanism is going to find it's way onto a points mounting plate. If it was never made to work well on automotive engines in neigh on 100 years of trying why would it be expected to work on a finicky high strung race engine?
Transistorized ignition systems are actually easier, and somewhat ironically the higher the level of sophistication in the electronics industry the easier custom retrofit systems tend to be. The ultimate level of sophistication and ease of application is starting with a computer system that is already up and running on a standard operating system. Then running the engine management system is a simple task of installing sensors and actuators and writing the control program. Anyone who has ever used a computer system that will run a C++ program however would easily be able to see how much easier and more reliable a set of points really is.
An existing computer system may be easy to apply to a difficult application, but stepping back a bit in electronics complexity does have it's merits. The ultimate in functionality and reliability in engine management is an excruciatingly simple computer system designed specifically for the task at hand. The need for high speed calculations requires large amounts of processing power only when a generic processor architecture and board architecture are used. When the computer system is designed to do just the few simple iterative tasks of arriving at the correct spark timing and fuel flow rate then fast response is possible with ridiculously small levels of complexity.
In the past when good functioning and simple computer systems were implemented for engine management other problems always cropped up to distract from the competence of the engine management system itself. A large butterfly throttle valve is too twitchy at reduced engine speeds. A vee shaped slide type throttle valve could be just as cheap and be shaped to deliver any sort of throttle response. Instead drive by wire systems have become nearly universal. Maintaining optimal catalytic converter temperature is difficult with rich fuel mixtures. Engines run just fine with rather lean mixtures though, and taking this to an extreme even mixtures so lean that maximum power output suffers significantly are hardly a problem for performance or engine compactness if the engine is designed from the ground up for those lean mixtures. Instead multiple oxygen sensors and multiple catalytic converters have been requiring increasingly sophisticated computer systems to manage successfully. The whole catalytic converter concept is sort of a mistake anyway. When an engine is actually running well there is not much for a catalytic converter to do other than slowly collect deposits and eventually plug up and fail. If a gasoline engine has to low idle as quietly as possible in a confined space a catalytic converter can be useful. Like a heavy lift forklift slowly driving through an office while desk workers talk on the phone, a catalytic converter would be great for that application. If an engine is allowed to growl a bit at low idle fuel consumption is often actually substantially lower and the engine runs so cleanly that a catalytic converter really has hardly anything to chew on. And of course the biggie has been trying to apply computer controlled engine management systems to pathetic poorly flowing, heavy, long stroke gasoline engines that just can't be made to run well over a sufficiently wide range of engine speeds and engine loads. Shorter two inch stroke gasoline engines with roller followers or sufficiently large diameter dual overhead camshafts are no more difficult or expensive to produce, and getting the weight of the pistons and rods down to acceptable levels just requires reasonably competent designs in the same materials as have widely been used.
There are still large design problems that get in the way, like the fact that high cylinder counts are required for good transmission efficiency but higher cylinder counts tend to push total displacement up beyond required levels. This is a severe problem, but it is more like that six cylinder gasoline engine just stubbornly keeps coming out at 600cc which tends to produce somewhat more power than a full size automobile really needs to accelerate from turn to turn on small twisty paved roads at 30 to 45mph. If the fact that an automobile is going to accelerate with big 40 to 50hp output at peak drive train efficiency can be accepted then building a gasoline engine to do this is rather straight forward. It is not that even smaller gasoline engines can't be produced, it is just that there is a lower limit bellow which either transmission efficiency or engine efficiency does begin to drop off. The problem is however not on quite the same scale as might have been expected. The old myth that reducing stroke lengths bellow four inches resulted in less efficient gasoline engines was totally bogus. It is diesel engines that benefit from long three and four inch strokes even for small power output applications. The reality is that it is increasing stroke lengths beyond about two inches that results in reduced maximum attainable thermodynamic efficiency in gasoline engines, so gasoline engines can be made rather small. The limit to how small gasoline engines can be made while retaining high thermodynamic efficiency relates to the fact that rather high engine speeds are involved where oversquare configurations for large valve sizes are favored. Competent valvetrains do however allow square or slightly undersquare gasoline engines to flow well enough up to reasonably high mean piston speeds for normal applications. For an automotive application there really is no need to twist out to 14,000RPM. A gasoline engine is already doing as well as it is going to way down at 8,000RPM and even all the way down to about 4,000RPM surprisingly good operation is possible at least under somewhat reduced loads.
There are some concrete reasons why these ideas have been so hard for so many people to grasp for so many decades. The simple fact that the entire concept of late compression ignition has long remained a seldom understood black art is part of this. One common misconception was that there was only one type of compression ignition in gasoline engines, and that was mistakenly taken to be full compression ignition which only workes up at extremely high engine speeds. A somewhat more enlightened, but still stubbornly useless, misconception has been that the approximately 5 degree ATDC earlier time of late compression ignition was the latest possible time of late compression ignition. It is easy to see why this particular misconception has been so common; huge numbers of production gasoline engines routinely ran so much spark advance that the later time of late compression ignition actually was quite rare. When the only time of late compression ignition that is observed is the earlier 5 degree ATDC time of late compression ignition then there is a tendency to think that gasoline engines can't be made to run even sort of well below about 8,000 or 9,000RPM. There is actually a world of difference between this earlier time of late compression ignition at about 5 degrees ATDC and the latest possible time of late compression ignition somewhere around 15 or 20 degrees ATDC. Gasoline engines still don't do well at less than about 4,000 or 6,000RPM, but they can in fact make some medium large amounts of reasonably efficient torque all the way down to 3,000RPM. The big difference here is between thinking that gasoline engines just can't run at all bellow 8,000RPM and the fact that although efficiency does drop off somewhat minimum engine speeds in the 3,000 to 4,000RPM range are reasonable for most applications.
The last bit of this that is hardest to understand is that these numbers are not concrete, they swing up and down somewhat with different fuel properties. Hotter burning fuel attains higher efficiency at higher engine speeds. And combustion fuels are all pretty hot. Many people would like gasoline to burn colder so that peak efficiency would come down at the 2,500 to 3,500RPM engine speeds where four inch stroke engines are appropriate. Combustion fuels however remain stubbornly hot burning and those low engine speeds just can't be made to work well.
The actual engine speed where peak efficiency would occur remains a bit elusive. Again a big part of the problem is that what most people are thinking of is three and a half to four inch stroke automotive gasoline engines. Quite ironically this causes not only a tendency for casual users of cars to grossly under estimate the engine speed where maximum efficiency occurs in gasoline engines, but it also in an indirect sort of way very commonly causes more experienced racers and engineers to grossly over estimate the engine speed at which maximum efficiency occurs. Yep, that sounds confusing. Too long of a stroke causes opposite sorts of misconceptions depending on the individual. It does happen though.
The first one is pretty easy to understand. The stroke of the engine is so long that the engine just always uses less fuel at lower engine speeds regardless of how it is tuned or how it is run. Under light loads the four inch stroke is going to like 2,000 to 3,000RPM engine speeds. The fact that this is too slow for a gasoline engine is irrelevant because the heavy four inch stroke engine just don't want to rev much more.
How the long four inch stroke causes racers and engineers to mistakenly come to the conclusion that gasoline engines need to spin up above 9,000 or 10,000RPM to work well is much more involved and harder to follow logically. The first part of this is that longer three and four inch stroke gasoline engines are always operating at excessive mean piston speeds when they are running well and making big torque. With such a long stroke and the necessity of high engine speeds for late compression ignition to work well mean piston speeds get pushed very high. Even 4,000RPM is already getting up there in the territory of excessive mean piston speed for a four inch stroke engine. Despite this though four inch stroke gasoline engines still just run better and better up to 6,000 and even 7,000RPM. The really funny thing is that the four inch stroke gasoline engine actually needs an earlier time of late compression ignition to make maximum power at 6,000 and 7,000RPM. More torque is generated at the 5 degree ATDC earlier time of late compression ignition than at the 15 or 20 degree ATDC latest possible time of late compression ignition. What is going on here? If the problem is that the gasoline burns too fast to work at lower engine speeds then why does the four inch stroke engine need an earlier time of late compression ignition to make maximum power at 6,000 and 7,000RPM?
The answer starts out pretty simple. The excessive mean piston speed means that the piston moves down too rapidly for the expanding hot gasses to most efficiently push. Driving this excessive mean piston speed requires higher pressure and a higher temperature. Moving the time of late compression ignition around to somewhere close to top dead center instead of the 15 or 20 degree ATDC latest possible time of late compression builds pressure early so that stored pressure and heat are ready to drive the piston down as the long stroke rapidly accelerates away from the expanding combustion gasses. Earlier compression ignition builds pressure early so that excessive mean piston speeds can still make power.
People used to running three and a half and four inch stroke gasoline engines become familiar with the easily attainable earlier 5 degree ATDC time of late compression ignition, and they learn that it is only at this earlier time of late compression ignition that the engine runs best and makes the biggest power. This familiarity with and general feeling of desirability of the earlier 5 degree ATDC time of late compression ignition then results in these same people seeking to attain the same 5 degree ATDC time of late compression ignition on two and two and a half inch stroke gasoline engines. When a two inch stroke gasoline engine runs at this easy to attain earlier time of late compression ignition it really does have to spin extremely fast to run well and make good power. Like up at 9,000 and 10,000RPM. Because the two inch stroke engine gets very harsh and loses torque as the engine speed is reduced bellow about 7,000RPM at this earlier 5 degree ATDC time of late compression ignition it is understandably concluded that gasoline engines must spin up at more than 9,000RPM to run at their best.
So that's how an excessively long three and a half or four inch stroke results in misconceptions about ideal gasoline engine speeds falling in both directions away from reality. It is not however the end of the story. The really confusing thing is that two inch stroke gasoline engines actually do run extremely well at 9,000RPM at the earlier 5 degree ATDC time of late compression ignition. Not just sort of well, but smashingly spectacularly well with tons of power from the same boring old gasoline that has been sputtering along in four inch stroke gasoline engines at 3,000RPM for most of the history of internal combustion engines. Explaining this is tough.
At this point an analogy to a diesel engine is not only useful, but arguably indispensable in understanding what is going on. The time of late compression ignition in a gasoline engine is analogous to the injection start timing on a diesel engine. This is not the static timing setting on an inline pump, but rather the actual time that fuel begins to flow out of the injector and into the combustion chamber. This is pretty easy to see; when the fuel begins to be injected in a diesel engine is analogous to the time when the bulk of the intake charge begins to combust in a gasoline engine. The next one is a harder analogy to grasp: The temperature of combustion potential of gasoline is analogous to the injection flow rate in a diesel engine. This is a difficult one because in a diesel engine it is an injection system setting where in a gasoline engine it is a property of the combustion fuel itself. Gasoline engines only have one injection flow rate, much like a metering collar equipped diesel injection system. On a metering collar equipped diesel injection system the plungers and barrels or fuel camshaft can be swapped out to deliver a different injection flow rate for higher or lower engine speed operation or for heavier or lighter load operation. In a gasoline engine fuels of slightly different maximum temperature of combustion potential may sometimes be encountered, but for the most part this is a fixed and immovable parameter. It just is what it is.
If 3,000RPM is too slow for a gasoline engine but still works and 4,000 to 5,000RPM engine speeds start to work really very well then how is it that the two inch stroke gasoline engine appears to do so darn well up at 8,000 and 10,000RPM? What is probably going on is that the rate of compression ignition combustion is so fast and so hot on normal combustion fuels that peak efficiency actually requires an earlier time of late compression ignition. Huh? So hot and so fast that it has to happen earlier to be most efficient? That just sounds totally backwards. It is in fact backwards, but it also conforms to reality under certain circumstances. The analogy to a diesel engine clears up the logic and removes the totally backwards seeming nature of this.
The temperature of combustion potential of the gasoline (analogous to the injection flow rate in a diesel engine) is so high that it works best up at very high engine speeds. Those very high engine speeds are in fact so high that the combustion has to start earlier to work as well as it can. Yup, still sounds sort of backwards. The diesel engine analogy really does clear it up though. On a diesel engine with a fixed injection flow rate the engine will only work as well as it can over a certain rather narrow range of engine speeds. If that injection flow rate is high for a high engine speed then really good operation just is not going to be attainable at lower engine speeds. To rev the engine up to the higher engine speeds where the high injection flow rate works best also requires earlier injection start timing.
The rate of compression ignition combustion is so fast that it works best up at really extremely high engine speeds. At those extremely high engine speeds pushing 10,000RPM an earlier time of compression ignition just also happens to be required. The longer the stroke of the engine the lower the engine speed where an earlier time of late compression ignition is desirable. By the time the stroke is out to four inches an earlier time of late compression ignition works better all the way down to perhaps as little as 6,000RPM.
So there then is a completely worked out system of logic to explain observed behavior in gasoline engines. The only thing missing are actual hard numbers. All of the numbers used in these examples are just intuitive sort of guesses based on how various engines sound and where they make power. It is possible that petroleum companies are habitually supplying gasoline with somewhat higher or lower maximum temperature of combustion potentials than would tend to result in the highest refining efficiency. In either case, hotter than natural gasoline or colder than natural gasoline, a longstanding and widespread push in either direction throws all of the numbers off because all engines then appear to do better at say 20% higher or 20% lower speeds. There would be a tendency for all combustion fuels to have rather similar, but not identical, maximum temperature of combustion potentials. This can easily be seen in gases for welding and soldering. Butane just burns hotter in the same torch than propane does, and adjusting mixture or flame size does not change this fact. With internal combustion engines the maximum temperature of combustion potential is more difficult to quantify because combustion takes place at considerably elevated pressures. Higher pressures result in higher temperatures of combustion, but only to a certain point. Regardless of pressure there is a maximum temperature of combustion potential for any particular fuel. Matching mean piston speed and engine speed in gasoline engines to that maximum temperature results in peak efficiency and usually peak torque production comes at a similar engine speed in a good running high performance engine. Maximum power output might come at even higher mean piston speeds and even higher engine speeds, but that is just because more revs burns more fuel and makes more power even if both torque and efficiency are dropping off up at that very high speed.