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Connecting Rod Big End Bearing Diameter and Engine Stroke

At high mean piston speeds the diameter of the rod bearings in an engine is quite significant for how efficiently the engine can operate. The significant number is the ratio of stroke length to rod journal diameter, shorter stroke engines need smaller rod bearings. Multiple cylinder engines with rod bolts already have significant problems with high reciprocating losses at elevated mean piston speeds due to the heavy connecting rods. When the rod journal diameters are radically oversized these problems get much worse and engine efficiency ends up very low. As with any problems with high reciprocating losses it is the efficiency under reduced loads at higher engine speeds that suffers most dramatically, although peak output at maximum engine speed is of course also much lower than it would be with properly sized rod journals.

Rod Journal Dimensions
Origins of Oversized Rod Journals
Early Big Rod Engines
The Long Stroke Solution
Smaller Engines
Net Results
Big Rods in Motorcycles

Rod Journal Dimensions

Just as reciprocating losses for the weight of the piston and weight of the small end of the connecting rod are proportional to the square of the stroke of the engine reciprocating losses for the weight of the big end of the connecting rod are proportional to the square of the diameter of the big end of the rod. What this means is that the ratio of the stroke of the engine to the diameter of the rod bearing ends up being quite significant for how efficiently an engine can run at elevated mean piston speeds. Since automotive engines with rod bolts have such heavy connecting rod big ends this ratio of engine stroke to rod bearing size ends up being particularly significant.

The surface area of the bearing is also important, because frictional losses at low speeds are proportional to the surface area of the bearings. Frictional losses at low speeds are however also roughly proportional to the cube of the speed of the bearing, so oversized bearing diameters do end up being somewhat significant even at low engine speeds. Because low speed frictional loses are proportional to the surface area of the bearings it is often thought that simply reducing the width of the bearings will increase efficiency. Narrower bearings do sort of work for increasing efficiency at low mean piston speeds, but narrow bearings do essentially nothing to reduce reciprocating losses at elevated mean piston speeds. Main bearings can be made narrow and larger in diameter with only moderate penalties to efficiency, but narrower and larger diameter rod bearings do not work.

What this all means is that there is some ideal stroke length to rod journal diameter, and deviating much from this ideal ratio will necessarily reduce engine efficiency. Depending on the weight of the pistons, the bore to stroke ratio, the compression ratio of the engine, whether it is a forced induction or normally aspirated engine and what type of lubricating oil is used the width of the rod bearings might be made somewhat wider or narrower. The ideal stroke length to rod journal ratio however remains fairly constant with all types of engines. The ideal ratio remains fairly constant, but forced induction engines might still benefit from slightly smaller stroke length to rod journal ratios. The main exception to this is small diesel engines that normally operate at rather low mean piston speeds. Small diesel engines that always operate at lower mean piston speeds can get away with a smaller stroke length to rod journal ratio simply because reciprocating losses don't tend to be much of a problem. A gasoline engine with a sufficiently short stroke that usually operated at lower mean piston speeds could likewise get away with a smaller stroke length to rod journal ratio. This would be a gasoline engine with a stroke length less than about an inch and a half. It should also be noted that these small engines that always operate at lower mean piston speeds would never attain quite as high efficiency as engines operating up at the ideal mean piston speed for the fuel being used. This ideal mean piston speed is probably a two inch stroke gasoline engine operating at somewhere around 6,000 or 8,000RPM.

Origins of Oversized Rod Journals

As with most severe design problems with automotive engines the radically oversized rod journals can tend to look like just another example of the auto makers intentionally building non-functional engines. There are however some other perspectives on the oversized rod journals that have been prevalent in automotive engines in the past sixty years or so.

Back in the 1930's through the 1950's the common type of automotive and light truck engine was the long stroke flathead. These old engines typically had stroke lengths around four to four and a half inches, with four and three eighths being the standard stroke length for Chrysler, Studebaker and Willies. Fords and Chevys tended to have slightly shorter three and three quarter to four inch stroke lengths. The rod journal diameters of these engines was fairly universally right around two inches. The Studebakers were unusually small at 1.8 inches, and the Chevys were unusually large at 2.3 inches. Most other automotive engines had about 2.1 inch diameter rod journals.

When shorter three inch stroke engines were introduced in the late 1950's the rod journal diameters were however not reduced correspondingly. This made the new short stroke engines inefficient at elevated engine speeds where they otherwise would have run best. The big question is why this happened, why were the rod journals not made smaller to go along with the new shorter stroke lengths?

One explanation might be that the automakers were trying to avoid screaming gasoline engines that would run all the time up at 7,000 to 10,000RPM. It has been quite common over the past century for people who are ignorant of how gasoline engines run to try to demand slower spinning engines. Aside from the fact that gasoline engines cannot be made to work well at less than about 4,000 or 6,000RPM there are some very significant advantages to slower spinning engines. Slower turning engines are easier to build for one thing, and they also are quieter and generally more aesthetically pleasing. The simple widespread desire for slower turning engines is probably part of why the new three inch stroke automotive engines introduced in the late 1950's retained the large two inch rod journal diameters.

The other thing that was going on though was that the old long stroke engines tended to suffer premature rod bearing failure. The reason that the long four and three eights inch stroke gasoline engines suffered rod bearing failure was however not because the rod bearings were undersized. The problem was rather that the stroke length was simply too long for a gasoline engine. With the long four and three eighths inch stroke length the engines had to run at lower engine speeds, always bellow about 5,000RPM. The radically undersquare configurations with three inch bore diameters and poorly flowing flathead valvetrains further limited operational engine speeds, and these engines were in practice run between 2,000 and 3,500RPM under heavy loads with light load operation down to about 1200RPM. With low 6:1 compression ratios these engines mostly operated in full flame front travel mode, but they did make much more power and attain significantly higher efficiency if they were allowed to run in late compression ignition mode up at above 2,500RPM. The problem was that 2,500 to 3,500RPM was in fact far too slow for late compression ignition. Late compression ignition down at these low engine speeds resulted in high peak cylinder pressures that "pounded out" the rod bearings. Not only was 2,500RPM far too low of an engine speed for late compression ignition, but late compression ignition also ended up used all the way down to 2,000RPM where the problems were even more severe. With wide two to one jumps between transmission gears and an engine that absolutely would not rev above 4,000RPM it was inevitable that heavy load operation at 2,000RPM and bellow would occur.

It was the poorly running 4.375 inch stroke not the small 2:1 stroke to rod bearing diameter that destroyed the rod bearings, but this distinction was somehow lost. The new three inch stroke engines with 1.5:1 stroke to rod bearing ratios were better able to rev to 3,000 to 5,000RPM where gasoline engines run better, but the radically oversized rod bearings meant that efficiency up at these elevated engine speeds ended up being extremely poor.

Early Big Rod Engines

Radically oversized rod bearings in short stroke engines actually go all the way back to the 1951 Studebaker 232 V8 which had a three and a quarter inch stroke and two inch rod journals for a 1.625:1 stroke to rod journal ratio.

In 1955 both Chevy and Studebaker introduced even shorter stroke engines with extremely radically oversized rod journals. With the 224 V8 Studebaker reduced the stroke to 2.81 inches with the same two inch rod journals for a stroke to rod ratio of 1.4:1. The ubiquitous small block Chevy V8 was introduced in 1955 both as the three and three quarter inch bore 265 and the three and seven eighths inch bore 283. Both early versions of the small block Chevy had a three inch stroke and two inch rod journals for a 1.5:1 stroke to rod journal ratio.

Ford followed suit with the 221 overhead valve V8 in 1962 and the 260 and 289 in 1963. These small block ford engines all had the same two and seven eighths inch stroke and 2.1 inch rod journals for a staggeringly small 1.35:1 stroke to rod journal ratio.

The Long Stroke Solution

Instead of reducing the rod journal diameters to fix the problems with low efficiency and poor performance the automakers instead went back to longer strokes. In 1960 Chrysler introduced the OHV Slant Six both as a three and an eighth inch stroke 170 and as a four and an eighth inch stroke 225. Both had the same 2.2 inch rod journal diameter. The 170 Slant Six then had a 1.43:1 stroke to rod journal ratio and was a sluggish performer. The 225 Slant Six with it's 1.89:1 stroke to rod journal ratio on the other hand was a strong torquer that was very popular and stayed in production until 1987. The amount of torque that the 225 Slant Six was able to belt out at 3,000 to 3,500RPM was quite impressive, and the still rather low displacement delivered best in class fuel economy for many decades. For pushing a heavy torque converter equipped full size car or light truck around town and down smaller rural roads the 225 Slant Six produced enough torque for V8 like drivability, and the long stroke and small displacement used less fuel when idling along in traffic.

The solution from ford was the 300 cubic inch inline six, also produced for a very long time from 1965 into the 1990's. The 3.98 inch stroke and 2.1 inch rod journal diameter yielded a stroke to rod journal ratio of 1.87:1, right in line with the 225 Slant Six. The big four inch stroke ford also was good for pushing heavy vehicles at low speeds through torque converters, but the one third larger displacement resulted in one third more fuel consumption at slow speeds.

At General Motors the trend of longer strokes and even higher fuel consumption continued. In 1970 the 454 big block Chevy made waves with it's four inch stroke and only very slightly larger 2.25 inch rod journal diameter. The 454 had essentially the same stroke and rod journal configuration of much earlier Chevy engines, but with huge four and a quarter inch bores stuffed full of monstrously large intake valves a lack of power was a thing of the past. The long stroke pushrod 454 might have only revved to about 5,000RPM in mild stock trim, but lots of displacement meant lots of torque.

Smaller Engines

In the 1960's and 1970's European and Japanese automakers began introducing smaller motors to the U.S. market, but extremely radically oversize rod journals persisted. The 1963-69 Mitsubishi 977cc inline four takes the title of the most radically oversize rod journals. With a short 2.36 inch stroke on two inch rod journals the stroke to rod journal ratio was way down at 1.15:1. The later Mitsubishi 2351cc inline four used smaller 1.77 inch rod journals and a long 3.94 inch stroke for a very large 2.2:1 stroke to rod journal ratio.

Another early importer of radically oversize rod journals was Fiat. The 1969-73 1116cc inline four had an even shorter 2.18 inch stroke and still very large 1.8 inch rod journals for a 1.22:1 stroke to rod journal ratio. This same Fiat engine was later used with much infamy in the 1986-88 Yugo. In 1970 Fiat introduced a more reasonable 903cc inline four with a longer 2.68 inch stroke and 1.6 inch rod journals for a 1.7:1 stroke to rod journal ratio.

Also in the fray in the '60s was Isuzu. Their 1584cc inline four introduced in 1969 had a still small (by automotive standards) 2.95 inch stroke but large 1.9 inch rod journals for a 1.5:1 stroke to rod journal ratio. In 1986 Isuzu introduced a 2254cc inline four with a 3.5 inch stroke on the same 1.9 inch rod journals for a 1.8:1 stroke to rod journal ratio.

Toyota and Mazda were a bit later on the scene, but they also continued the trend of longer stroke engines having larger stroke to rod journal ratios. From 1982 to 1988 Mazda produce the 1789cc inline four with a 3.03 inch stroke and two inch rod journals for a 1.51:1 stroke to rod journal ratio. Then from 1987 to 1990 the 2184cc Mazda put a 3.7 inch stroke on the same two inch rod journals for a 1.84:1 stroke to rod journal ratio. From 1983 to 1990 Toyota produced the very popular 1587cc inline four with a 3.03 inch stroke and small 1.57 inch rod journals for a stroke to rod journal ratio of 1.92:1. Then in 1988 Toyota introduced a 2508cc V6 with a rather short 2.74 inch stroke on 1.88 inch rod journals for a 1.45:1 stroke to rod journal ratio.

BMW had done something similar with their inline six automotive engines in the 1980s as well. The 1977 to 1986 1990cc engine had a short 2.6 inch stroke on 1.77 inch rod journals for a 1.47:1 stroke to rod journal ratio. Then the 1982 to 1989 2693cc engine went up to a 3.19 inch stroke on the same 1.77 inch rod journals for a 1.8:1 stroke to rod journal ratio.

Net Results

Most automakers engaged in similar practices where the shortest stroke engines had big rod journals. Along with generally poorly designed and extremely heavy rods and pistons and less than stellar valve trains the oversize rod journals resulted in poor high engine speed performance. Shorter stroke engines that would not rev up appeared to work even worse than longer stroke engines, and fuel consumption remained high.

Gasoline engines do in fact need rather short strokes to be able to attain high efficiency, but this fact was obscured by persistent use of oversize rod journals on all shorter stroke gasoline automotive engines. The reality is that a three inch stroke is about the maximum for gasoline engines, and a two inch stroke delivers a wider range of engine speeds with dramatically improved light load efficiency.

The 2.2 inch stroke Yugo should have worked very well, but it did not. The extremely radically oversized rod journals with heavy rods and heavy pistons on a mediocre valvetrain just did not deliver much in the way of performance. Had the 3.15 inch bore by 2.185 inch stroke 1116cc Yugo actually run well it would have made somewhere in the neighborhood of 150 to 170hp at 8,000 to 10,000RPM. Even more significantly, if the 2.2 inch stroke 1116cc Yugo actually ran well it would be able to deliver fairly good reduced load efficiency between 4,000 and 6,000RPM for high speed highway cruising at 50 to 80hp output. The thing is though that even 50hp is like 90mph in the Yugo, and of course 150hp is just way more power than the little front wheel drive car can make use of anywhere but out on a big high speed race track. It is just way too big of a gasoline motor for a small economy car. A car like the Yugo would do much better with a square two inch bore and two inch stroke four cylinder engine which would displace just 412cc. A little four cylinder like that could attain just as high operational efficiency under reduced loads at 4,000 to 6,000RPM, it is just that those efficient reduced loads would be around the 15 to 25hp range. Much more realistic for normal highway cruising at speeds around 50mph. The 412cc engine could make much more power up to 8,000RPM or higher, just how much power it could make would depend on how good of a valvetrain it happened to have. The thermal limit for a two inch bore four cylinder engine would be around 60hp or 70hp, which is more than a two inch square engine would be likely to make. With the use of a variable valve timing system though it might be possible to get fairly close to this thermal limit.

Big Rods in Motorcycles

The classic example of oversize rod bearings in a motorcycle engine is the 1960's Aermacchi derived Harley-Davidson Sprint, the Spaghetti Hoglet. Introduced in 1960 the Sprint 250 was an air cooled horizontal four stroke single made in Italy by Aermacchi which was at the time half owned by Harley-Davidson. The first version of the 250 was known as the "long stroke" with a stroke length of 72mm (2.83 inches) and a rather large (approx. 42mm) connecting rod big end bore. Sometime around 1963 a short stroke version of the Sprint 250 was introduced with a 61mm (2.40 inch) stroke length and the same large connecting rod big end bore. The really bazaar thing about the new short stroke motor was that it used a longer connecting rod so that the cylinder head remained in the same position. The short stroke Sprint 250 did rev higher and make more power, but it was held back by an inappropriately huge connecting rod. A larger displacement Sprint 350 was also introduced at the same time which used the 72m stroke and shorter connecting rod of the original long stroke 250 with the larger 72mm piston and cylinder of the short stroke 250 yielding a 293cc displacement. This worked so much better that an even larger displacement Sprint 350 with a 74mm bore and a 78mm (3.07 inch) stroke was introduced in 1964. Finally with the 78mm stroke the same 42mm connecting rod big end bore was not so extremely oversized, but that 78mm stroke was in fact getting so long that the engine would no longer twist up to 10,000RPM as the earlier motors had. The Aermacchi Sprint was just a two valve per cylinder air cooled pushrod engine, but when fitted with a 39mm intake valve, a 34mm exhaust valve, stiff valve springs and a big aggressive camshaft it was able to run competitively with other 1960's race engines.

Fast forward three decades to the 1990 introduction of the first Italian Husqvarnas and the same scenario was played out over again. The original Swedish 510 Husqvarna motor had a rather svelte connecting rod with a 38mm big end bore ridding on a 30mm crank pin. This was a slightly larger rod bearing than the 500cc two strokes had used, but was still rather small. When Cagiva overhauled the Husqvarna four stroke they introduced two new models, the 577cc WMX/WXC 610 and the 349cc WXC 350. Both of these new models used a 127mm long connecting rod with the same 38mm big end bore and 30mm crank pin of the Swedish 510. The big 610 also retained the same 3.01 inch stroke of the Swedish 510, and looked like a very big engine on that svelte connecting rod. The 350 with it's much shorter 2.48 inch stroke on the other hand ended up with a massively oversized connecting rod and a rod bearing nearly as oversized as the old short stroke Aermacchi 250. Just like the Aermacchi 250 had the Husqvarna 350 will make power to 10,000RPM, but it is held back by a ridiculously oversize connecting rod. The heavy oversized connecting rods in these short stroke motors result in the power output flattening out from 8,000 to 10,000RPM and reduced load efficiency at lower engine speeds also suffers immensely.

Cagiva addressed the oversize rod problem by boring the short stroke motor out to 410cc and going back to the biger valves of the 610 motor. The rod bearing diameter was still excessive for the short stroke motor, but piling more displacement on the same connecting rod alleviated the problems. With a higher revving short stroke motor the lack of a pressure oiling system was however much more of a problem, and the short stroke Husqvarnas were never popular.

KTM fixed the oiling problems by putting a pressure oiling system on their version of the Husqvarna motor, but they also did even stranger things with the rod bearing size. KTM had two basic versions of the single overhead cam four stroke, the KTM 520/525 and the KTM 620/625. The smaller KTM 520 was also offered with smaller 450cc, 400cc and 250cc displacements and the big KTM 620 was also offered with a smaller 400cc displacement. The defining feature of all of these 1990's and early 21st century KTM motors were very large rod bearings and very long rods. The 72mm stroke KTM 520 already had a substantially oversize connecting rod with a 43mm big end bore and a 35mm crank pin. This same connecting rod was also used on the smaller 450, 400 and 250 engines and was just out of this world ridiculously oversized for these shorter stroke motors. On the 56.5mm (2.22 inch) stroke of the KTM 250 the 43mm big end bore was oversized beyond comprehension. The 129mm center to center length of the KTM 520 rod was on the long side for the 72mm stroke 520 motor, but reasonable for all around good performance. On the shorter stroke 250 and 400 motors though the 129mm rod length was just absolutely ridiculously too long.

The one thing that the 1990's KTM 250 did do though was showcase just how fast a well oiled roller rod bearing can be spun. With the little 2.2 inch stroke 250 twisting out to 10,000RPM the rollers in the large 43mm big end bore were actually rolling 15% faster than the rollers in the extremely failure prone Husqvarna 350 rod bearing at the same 10,000RPM engine speed. Forcing oil into the rod bearing certainly does prevent failure up to even extremely high speeds.

The bigger KTM 620/625 motor (Known as the LC4) had an even more radically oversized 141.5mm rod with a 50mm big end bore and a 40mm crank pin. For the 78mm (3.07 inch) stroke of the 625cc motor the huge rod was dramatically oversized, both in center to center length and in big end bore diameter. For the smaller 64mm (2.52 inch) stroke 400cc version of the LC4 that huge 50mm big end bore is even more ridiculously oversized than the 43mm big end bore of the KTM 520 rod was in the small 250cc engine.

As severe as these oversized connecting rods and oversized rod bearings seem on dirt bike engines the problems caused are slight in comparison to the problems caused by radically oversized rod journals in automotive engines. Dirt bike engines don't have rod bolts, and this keeps the weight of the connecting rods down. Even the KTM 400 with it's huge 141.5mm center to center rod length and gargantuan 50mm big end bore still revs fairly well and makes lots of power. Compared to automotive engines the KTM 400, or any dirt bike really, tends to seem like it runs amazingly well and can rev to the moon. The 2.5 inch stroke LC4 KTM 400 ends up having the same connecting rod big end bore diameter as the Yugo/Fiat 1116cc 2.2 inch stroke automotive engine. Both of these engines have extremely radically oversized rod bearings, but it is the engine with the rod bolts that suffers much more severely.

It is possible to get automotive engines with rod bolts to work fairly well, but it requires that the weight of the pistons and rods be kept low and it also requires that the rod journal diameter be sized correctly for the stroke of the engine. The old four to four and three eighths inch stroke automotive engines actually did work with their 1.8 to 2.3 inch rod journals. The reason they were so inefficient and did not last long was simply that the four inch stroke was far too long for a gasoline engine. A two and a half inch stroke automotive engine with inch and a quarter rod journals would also work, but it would tend to be much more efficient and last longer than the old long stroke automotive gasoline engines ever did.

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