Ever wonder just why a big four cylinder almost invariably has more punch down low, and less need to rev than a similar-sized six or V8? There’s a very simple but easily forgotten law of physics that explains it. Of course, details of their construction and tuning can affect or alter that somewhat too, but fundamentally, the law applies to all IC engines. To underscore the point, we’ll start by looking at three engines with similar features but vast differences in displacement per cylinder and corresponding torque and horsepower peaks.
These images represent three engines from the mid-’50s to the mid-’60s. Each of them was designed for maximum power output, with a hemi/pent roof combustion chamber, overhead cam(s), and large valves and ports, all the classic hallmarks of a high-output engine.
The one on the left generated 5.4 hp per cubic inch @21,500 rpm and 1.06 lb.ft. per cubic inch @17,000 rpm
The one on in the middle generated 1.48 hp per cubic inch @6500 rpm and 1.26 lb.ft. @5250 rpm
And the one on the right generated 0.27 hp per cubic inch @2000 rpm and 0.87 lb.ft. @ 1350 rpm
Now for the big difference: their displacement per cylinder. From left to right, in cubic inches: 1.5, 67, and 181.6
Like so many other things in nature, engines don’t scale without significant impacts.
Before we explain the physics, let’s take a quick look at these three engines, as they’re interesting in their own right.
First: 1966 Honda RC116 50 cc racing motorcycle (note: the cross section at the top is actually from a different Honda engine; close enough). This masterpiece was Honda’s last 50cc racing bike, the culmination of several generations of 50cc racing bikes going back to 1962.
Here’s how the tiny 49.8 cc (3.0 cubic inch) twin cylinder engine looked. From its 50cc (3.0 ci), it generated 16.5 hp @21,500 rpm, and 3.25 lb.ft. of torque @ 17,000 rpm. Each 25 cc cylinder was fed by a four-valve head.
And here’s the crankshaft, pistons and connecting rods.
Given its very narrow power band, it took a nine speed transmission to put that power to the ground. Max speed: 175 kmh, or 110 mph. Brakes? Calipers working on the rims, just like on a bicycle; lighter than drum brakes. Weight: 50 kg, or 110 lbs. Here’s a video of it in a bit of mild action:
Two: 1955 Meyer-Drake Offenhauser 270 racing engine:
The M-D Offy four cylinder racing engine is an American legend, dominating oval track racing from midgets to Indy 500, for decades. An evolution of Miller racing engines of the 1920s, which started out as an improved copy of the first DOHC Peugeot racing engine of 1913, the Offy 270 was one of the larger ones.
Its 4.374″ bore and 4.5″ stroke yielded 270 ci (4.4 L) from four cylinders. That made for phenomenal torque; 340 lb.ft. at a reasonably low 5250 rpm for a racing engine. That was key to its ability to accelerate out of corners at its torque peak without shifting, and then reach its top speed and full power peak of some 400 hp (depending on fuel, etc.) on the straights, also at a fairly modest 6500 rpm.
Its torque output of 1.26 lb.ft. per cubic inch is exceptional for a naturally aspirated engine, and one of the keys to its long career.
Its distinctive bark and roar were familiar sounds to the generations of Americans that watched it on tracks of all sizes and kinds.
Three: Hall-Scott 400:
The Hall-Scott 400 six cylinder gas engine was the culmination of a long line of legendary H-S engines designed to for maximum performance in trucks and buses, using the same basic principles (OHC hemi head) that Hall-Scott initially used in racing and aero engines. Since diesel engines inherently generate less torque than gas engines (unless boosted), it offered a way to cover more distance per day, if at the price of higher fuel costs.
With a bore of 5.75″ and a massive stroke of 7″, it displaced 1090 ci (17.9 L). Its torque curve was fairly flat, and peaked at 1350 rpm. And hp peaked at a very low 2000 rpm. But note that even though it was clearly designed for maximum torque, given its application, its torque output per cubic inch is the lowest of the three engines at 0.87 lb.ft. per ci. We’ll explain that shortly, as it’s all part of the same issue.
So on to the physics: Scaling up creates issues, all across nature. If you double (square) the dimensions of a sphere, cube or cylinder, the resulting volume is then 8x (cubed) larger. That creates two issues with engines that we’ll deal with here.
The first one has to do with mass, as the mass (weight) of an object also is 8x greater when its dimensions are doubled. If you were to scale up a mouse to the size of an elephant, it would collapse under its own weight. The elephant’s skeleton is much more than just proportionately stronger. That increases weight very disproportionately, hence the elephant needs very thick bones. And it’s forced to move relatively much more slowly compared the mouse.
This is of course also very much true in engines, in terms of the increased strength that would be required for the components of a larger engine to turn at the same speed as a smaller one. It’s obvious and intuitive: larger engines need to rotate at lower speeds than smaller ones, otherwise the huge masses of the reciprocating parts could never be contained; there’s just no materials strong enough, up to a point.
One could increase the bore and stroke of an engine by a factor of say x2, which would increase its displacement by x8, if we used super strong components. In fact, modern F1 engines (before 2014) did just that, and can rev as high as 20,000 rpm with a displacement per cylinder of 300 cc, or a bit more than x8 of the Honda’s tiny engine (190 cc). This is the result of huge progress in material strengths like titanium, as well as better breathing, but we’ll leave that aside for now, and also acknowledge that this was impossible back then. And that it would still be impossible to build an engine of the Hall-Scott 400’s size and have it be able to rev that high. Maybe someday.
For now, we can put aside these issues of mass and component strength, because there’s a more fundamental one yet: volumetric efficiency. Even if we had infinitely strong materials, this issue still governs the operating speed and power peaks of a naturally-aspirated gas engine.
Volumetric efficiency (“VE”) is the actual amount of air flowing (“breathing”) through an engine, compared to its theoretical maximum. Basically, it is a measure (percentage) of how full the cylinders are filled during the intake stroke. That is primarily dependent on airflow through the valves and ports. It was discovered quite early (as in this 1901 Truscott marine engine) in the engine’s development history that maximizing valve and port size by using canted valves in a hemispherical combustion chamber maximized VE, with a corresponding increase in torque and hp output. The hemi head was quickly adopted by racers starting in 1905, and became almost universally used where maximum VE was desired. Today essentially all gas IC engines use a variation of the hemi/pent head.
Here’s the big(ness) problem:
This is the formula for determining the volume of a cylinder. If its main dimensions (radius & height, corresponding to 1/2 of bore & stroke) are doubled (squared), the volume increases by x8 (cubed). In terms of volumetric efficiency, it’s easy to predict that in order to fill that cylinder as full as possible, the area of the valves and ports should also increase proportionately (x8).
But when the radius or diameter of a circle (valve) is doubled (x2), the area is only increased by x4, or at half the rate of increase as the cylinder volume.
As an example: let’s say we have a single cylinder engine with bore and stroke of 2″ each = 6.283 cubic inches
And there’s room for two 1″ diameter (0.5″ radius) valves in the head = 0.79 sq. in. of intake valve area.
Resulting in an intake valve area to displacement ratio of about 1:8.
Now we double the engine’s dimensions:
Bore and stroke of 4″ each = 50.265 cubic inches
The valves now have 2″ diameters (1″ radius) = 3.14 sq.in. of intake valve area.
Resulting in an intake valve area to displacement ratio of about 1:16
The problem is now very obvious. As displacement per cylinder increases, the valve area to displacement ratio becomes worse, thus limiting the engine’s volumetric efficiency, at least at higher speeds. Thus the rpm of max VE decreases, resulting in ever lower maximum engine speeds, as the engine is increasingly limited by its breathing ability the faster it tries to run.
This means that completely apart from the issues of reciprocating masses, the larger the engine, the lower it inherently revs, as its valves will become increasingly unable to adequately pass enough air/fuel. Correspondingly, its torque peak (at max. VE) will be at ever lower rpm, and thus as well its hp peak.
Meanwhile, a very small engine has such an abundance of valve area, that its max. VE (and torque peak) will tend to be very high, as well as its hp peak.
Of course there are many variables in engine design and tuning that will offset this basic principle to some degree or another, but the principle prevails.
The most obvious way to negate the issue of small valve area was to increase bore relative to stroke. The Ford Kent 1.0 L ohv four from 1959 was one of the first of very significantly oversquare (bore greater than stroke) engines, with a bore of 3.19″ and a stroke of 1.91″, resulting in a bore-stroke ratio of 1.67:1, one of the highest ever. This direction certainly improved volumetric efficiency, but also resulted in a relatively high-revving (6000 rpm) engine in an otherwise very mild state of tune. And it also meant a relatively higher torque peak than the typical British long-stroke engines of the time, which made it feel rather weak-chested.
The oversquare era did not last long, as it was found to be inherently “dirtier” in terms of smog-forming emissions, undoubtedly due to its shorter combustion cycle. The trend has been to longer strokes and more undersquare engines, but offset by ever-better breathing heads with four valves and improved valve timing due to variable valve timing technologies.
In the racing world, massively oversquare engines rule, as it of course allows for larger valves as well as reduced piston speeds and reciprocating masses through lighter and stronger components. This is a Ferrari from 2000; further advances have been made since then.
The downside to shortening stroke is that intake velocity is proportional to piston velocity. So the point of optimum volumetric efficiency, due to intake inertia, will occur at an ever higher rpm. That is why long stroke, long rod engines tend to produce peak torque at a lower rpm than short stroke, short rod engines. Peak engine torque will usually occur at the point of peak volumetric efficiency.
The basic principle, that increasing an engine’s displacement by making its dimensions ever larger would not increase its power proportionately, was understood quite early. William Maybach, the brilliant pioneer in the field, thus created the first two cylinder engine with Gottlieb Daimler in 1889. Increasing the number of cylinders was/is the most expedient way to increase power, for a given displacement. This one had 34 cubic inches and made all of 1.5 hp @700 rpm. By 1899, Maybach built the first four cylinder, and so it went, to six, eight, 12 and 16 cylinders.
Adding more cylinders increased power and of course smoothness, but depending on each cylinder’s displacement, the torque curve did not benefit. Which explains why small multi cylinder engines have all disappeared. 2.0 L sixes were once common, and Ferraris mostly had 2 to 3.0 L V12s. Good maximum hp output per displacement, but terrible torque curves.
We’ve seen rather extreme examples for illustration purposes. How about some common examples, from this period, comparing two engines with similar displacement overall, but different number of cylinders?
Here’s the stats and dyno charts of the Chevy 292 six (left) and 283 V8 (right):
292 six:
170 gross hp @4000
153 net hp @3600
275 gross lb.ft @1600
255 net lb.ft @1600
283 V8
185 gross hp @4600
150 net hp @4200
275 gross lb.ft. @2600
245 net lb.ft @2600
Similar displacement, similar head architecture, but different cylinder count. The 292 six had the same 3.875″ bore as the 283, but with its long 4.125″ stroke, it made its torque at a full 1000 rpm lower than the short stroke V8. One can debate minor differences in details, but fundamentally, these were quite similar except for the cylinder count, and a 1000 rpm difference in torque peak is very noticeable in their driving characteristics.
Truck operators strongly prefer a low rpm and flat torque curve, as it means that the engine can typically be run in the area of peak torque, which is also the area of peak volumetric efficiency and therefore the most fuel-efficient range.
I don’t have dyno charts, but here’s the ratings for 1969 Ford 300 six and 302 V8 (in gross numbers):
300 six: 170 hp @3600 rpm 283 lb.ft. @1400-2400 rpm
302 V8: 210 hp @4400 rpm, 295 lb.ft.@ 2600 rpm.
Note that the long stroke 300 six has a peak torque band that extends from 1400 to 2400 rpm. It doesn’t get better than that, and makes it a much more suitable engines for trucks than the higher-revving 302 V8.
This was a very real issue back when Henry Ford brought out his V8 in 1932. It was a well known fact that although it made 5 more peak hp than the Chevy six and had the same torque (130 lb.ft), but that torque peak arrived at a 50% higher rpm than the Chevy six, making it better choice for most normal driving, Bonnie and Clyde excepted.
The 1932 Ford Model B four had 200 cubic inches and made only 50 hp compared to 221 ci and 65 hp for the V8, but its torque peak was at a much lower engine speed, making it feel quite brisk at low speeds. I found this at a forum on the subject:
There was a real comparison in Rod&Custom Magazine in the ’70’s. Harrahs let them road test two ’32’s, a B roadster and a V8 Cabriolet, and they included some gentle acceleration tests and a drag race. The B came off the line best and then lost ground to the V8, and the B also showed a spurt forward gaining on the V8 at each shift, with the V8 then gaining at the higher revs in each gear.
That perfectly describes the different characteristics of two similar-sized engines with different displacement per cylinder.
One more data set, comparing the smaller 136 ci Ford flathead V8-60 and the 134 ci Willys Go-Devil flathead four:
V8-60:
60 hp @4000 rpm; 96 lb.ft. @ 2600 rpm
Go-Devil four:
60 hp @3600 rpm; 105 lb.ft. @2000 rpm
The Ford V8-60 was universally panned in the US as being weak-chested. 2600 rpm was unusually high for a torque peak back then. The Willys four was known for its lusty power at low rpm.
There will likely be exceptions between two particular engines, but undoubtedly because of certain design factors, and the differences in displacement per cylinder not being very great. If you can find some, I’d be glad to hear of them. But generally speaking, this principle is readily experienced in the characteristics of engines with relatively greater or smaller displacement per cylinder.
I am not an engineer or knowledgeable in physics. There are undoubtedly other factors that can affect torque and hp peak. But this (reduction in VE rpm due to valve area reduction) is the most significant one. The way to offset it most effectively is with forced induction (super/turbocharging). That of course overcomes the limitations of the valve area, depending on the boost level. And that explains the great attraction to forced induction in modern engines, allowing them to generate much higher power without the corresponding increases in weight and friction losses.
With today’s performance levels, rim brakes are fraught with issues like rim collapse and bead melting…on bicycles. I imagine they only worked on a racing motorcycle because braking was the last thing a rider wanted to do with a tiny engine that needed 17,000 RPM to climb out of a corner.
Ferrari in the ’50s was very involved in exploring the advantages of different engine configurations for a given displacement. While their road cars were all powered by their signature V12s; they had Mondial and Monza four cylinder racing cars as well as both Lampredi inline and Dino V-configuration six cylinder racing cars. It was all about the power characteristics needed for particular tracks. They also experimented with a big displacement two cylinder racing engine, but I don’t know that it was every installed in a car.
I am not a physicist or an engineer either, and therefore have a question: are there any engines of similar displacement and different cylinder counts that vary mostly in bore rather than in stroke length? As opposed to the Chevy 292/283 that share a bore diameter and use longer stroke to increase per-cylinder displacement.
Intuitively, it would seem that longer stroke designs offer a leverage advantage in low rpm situations, much like pedaling a bike with a twelve inch pedal crank will start from zero much faster than one with a three inch pedal crank. And, of course, that leverage advantage turns into a disadvantage at higher rpms. Automotive engine stroke differences are certainly not that wide, but the principle makes some sense to me. So I wonder if we would see the same patterns in peak hp and torque if the engines shared similar stroke length and varied only in bore diameter.
The 1932 Ford tests you note also make sense. I read once that the Model A with its 200 cid 40 horsepower four was one of the fastest accelerating cars on the road in the 0-40 mph range, regardless of price. Its torque peak was at 1000 rpm.
This is oversimplifying it a bit, but the bike crank analogy doesn’t apply here. On the bike, you’re pushing down on the short or long crank with the same force from your legs. But in two engines of same displacement, the one with the longer stroke will have a smaller bore. So the cylinder pressure from combustion is acting over a smaller surface area, supplying less force even though its acting on a longer lever arm. Add to that the fact that the small bore engine has to have smaller valves, so the cylinder filling is worse, and that brake mean effective pressure (BMEP) may be even less than it could be with a larger bore, compounding the issue.
You explained it better than I did.
My response was not directed at the issues you brought up here, but limited to the invariably great difference in valve sizes.
As I said, there are many other factors too: valve lift and duration, manifolding, etc. And the one you pointed out is a factor too. Undoubtedly if we found a very oversquare four and an undersquare six of equal displacement, the torque characteristics would tend to be much more similar, or possibly be even reversed.
Making direct comparisons is somewhat fraught. My goal was to illuminate the primary factor at work.
But it might be fun to try to find some like that to compare. The problem is that they may well differ in other respects more than the examples I gave, which are relatively similar in terms of valve size and cylinder head configuration.
Paul and Dman, thanks for the explanation. I noodled around for a bit looking for, say, a pair of inline flatheads where a six and eight were of similar displacement but where the difference was in the bore and not in the stroke. I ran out of available time before I got anywhere with it. It seems that stroke was the easiest thing to adjust when moving up and down in displacement.
Ford As were very light so they went quite well for the era.
This is great, Paul. Honestly, I’d never considered how valve size mathematically can’t keep up with greater displacement, nor had I considered the greater piston speed of a longer stroke configuration in terms of airflow. Very well done – I appreciate the lesson!
Dear Paul, great explanation. I worked with these principles for years when I sold motor trucks. However, I can never explain the matter as you do!
Great read Paul. But the 32 Ford V8 had 65HP, not 85 unless I mis-read something.
Thanks. Fixed now.
Apropos of the mouse-elephant comparison, there’s a famous essay by J. B. S. Haldane, “On Being the Rith Size.”
http://www.phys.ufl.edu/courses/phy3221/spring10/HaldaneRightSize.pdf
Wonderful essay. Thanks so much for recommending it.
You’re a really fine teacher-explainer, Paul—on top of all the fact-gathering. You’d have made a great Professor-of-Something if that’d been your choice . An excellent read for us today!
Great article Paul, as an engineer I found it highly readable and enjoyable.
Brought to mind the terrible 267 small block Chevy in my fathers Impala, which had the same 3.48 inch stroke as the 305 and 350 but ran out of breath at revs.
And how much I used to enjoy the instant torque in my 258 six powered AMC 🙂
Thanks for the great explanation and overview. The Fords and Chevy engine comparisons are very interesting and explain some design decisions. The Chevy 292 vs 283 issue was resolved by switching to the longer stroke 327 crank to produce the 307 V8 while around the same time Chevy also put the short stroke 283 crank in a 327 block to make the high revving 302 for the Z28. Ford seems to have addressed the 300 vs 302 issue by using the the 330 ci baby FE in light and medium trucks and reserved the 302 for 1/2 tons and Broncos.
Switching topics, a bit of quick math indicates that Honda may have gone modular with their tiny Moto GP engines since the 125cc 5 cylinder engine had similar dimensions to the 50cc twin and could share internals.
The valve area discussion gets into the rationale for four and occasionally five valve layouts since several smaller valves make more efficient use of combustion chamber space while also keeping valvetrain weight down. This does generally require switching from a hemi to a pent roof design, although Honda did make a radial valve design for the XL500/XL650 family of singles for many years.
My wife thought I was watching fart videos when the Honda was getting started.
Much as I enjoyed this physics lesson – and I never realised Honda had used desmodromic valves on their motorcycle engines – can I raise an issue.
A cylinder of double the size, having 4 x the valve area and 8 x the volume to fill, will be sucking the air in at a higher velocity, which will make up some ( not all) of the shortfall.
That Offy engine really was pretty amazing for a four-pot of such enormous size.
Uncle, The Honda engine in the illustration was a CB450cc parallel twin of the black bomber of 1965. It had torsion bar valve springs. I don’t know of any desmodromic Honda engines. I agree with your second sentence, as would Paul. The volumetric efficiency would decrease some percentage.
Even with computer controlled port fuel injection, the 302 (5.0) in my 1994 F-150 was a miserable engine, especially with 3.55 final drive. By comparison, our 1993 1500 Chevy with the 4.3L throttle body injected six and 3.42 gearing (both same tire size) climbed grades more easily with less downshifting. Torque peaks were almost 1000rpm different. The Chevy did run out of breath sooner when real pushed hard, 160 vs 185 peak hp.
Paul has talked the drivability aspect of low rpm torque. As someone who has owned a few, and ridden a few more, high-rpm Japanese 4 cylinder motorcycles, I’ll add another perspective to that. These engines may seem very smooth and pleasant at rpm’s that seem quite high to folks who have never ridden one, say 5000 or 7000 rpm. But not peppy. A modern 600 cc fuel injected four cylinder sport bike makes very little power at those rpm, and needs to get up closer to 9000 or 10000 rpm to provide any decent acceleration. A Yamaha R6, for example, has a torque peak at 10500 rpm and power peak at 14500. At 9000, let alone 14000, these motors feel difficult to control and spinning very fast unless you’re used to it. Not vibration like a big single or twin, but a busy-ness that is indescribable until you experience it. And riding these bikes and shifting at 6-8000 rpm is fine, but more work than most 500-600 cc twins or even singles with 1/3 the peak power.
Excellent article. One change/correction I would make is the sentence, “Peak engine torque will usually occur at the point of peak volumetric efficiency.” Peak engine torque ALWAYS occurs at the point of peak volumetric efficiency. Peak torque is a function of peak volumetric efficiency, they are one in the same.
Really excellent stuff.
I know you claimed in your packaging that you aren’t an engineer or physicist (“if questions persist, consult your physicist”), but I’ll still venture one question. Do I understand correctly that peak v/e is the point at which the cylinder is filling and emptying at peak efficiency, and if that’s right, what is peak power?
The simple way to think of VE is as the percentage of the cylinder that gets filled before the valve closes. So for a engine running at low rpm with part throttle might be running at 30% VE while running at higher rpm with full throttle might be running at 90% VE. Throw a turbo or or supercharger on it and when it is under sufficient boost be running at more than 100% VE.
Power is the rate at which work can be done. Horsepower is an agreed upon made up unit, that started it’s life as a marketing term. IE this engine can do the work of X horses. Of course every horse is going to be different even within the specific breed originally used. So the agreed upon definition of 1 HP is 550 ft/lb per second. That means that HP is actually a calculation or Torque x RPM/5262. That 5252 is a constant that accounts for the conversion from minutes to seconds and makes it all jive with the agreed upon definition of a Horsepower.
What that means is that torque = HP at 5252 rpm, assuming the engine is capable of turning that fast. It also means the torque curve and HP curve will cross at that point, if the graph is using the same scale for both.
This of course is a turbo engine but it is one that uses the same scale for both units and is capable of turning more than 5252 rpm. What it shows is that the effects of higher rpm make up for the loss in torque at higher rpm, which puts power peak at a higher rpm than torque peak. That is how that little Honda engine Paul used as an example can post such high hp/l numbers.
Thanks for that, scoutdude. I’m very slow on these things, so will ponder it for a while.
Btw, I’ve never before heard an explanation for horsepower coming to have a defined, agreed meaning before, but ofcourse it must have, and presumably pretty early on in engineering! After all, what on earth is one horse’s power, other than some unreliable variable?!
From Wikipedia, “Engineering in History recounts that John Smeaton initially estimated that a horse could produce 22,916 foot-pounds (31,070 N⋅m) per minute.[10] John Desaguliers had previously suggested 44,000 foot-pounds (59,656 N⋅m) per minute, and Tredgold suggested 27,500 foot-pounds (37,285 N⋅m) per minute. “Watt found by experiment in 1782 that a ‘brewery horse’ could produce 32,400 foot-pounds [43,929 N⋅m] per minute.”[11] James Watt and Matthew Boulton standardized that figure at 33,000 foot-pounds (44,742 N⋅m) per minute the next year.[11]
So yeah there was some significant differences between original estimates of the early years and the limited experimentation that was rounded to the definition used today.
The metric system is cleaner since it was done from the other end, so to speak. The unit of metric power is Watt which was given to the product of the math (lifting 1 kg 1 m in 1 sec) to resolve all the units used in the calculation.
Good article Paul. For me, I have never had a hard time grasping the differences in power bands when comparing a large displacement engine to a small displacement engine. It all is fairly logical to me. The concept that was always more difficult was in comparing power bands of engines of the displacement but with different number of cylinders. This one is a little more tricky to understand, mostly because there is that caveat, “when all else is equal.” There is almost no instances when all else is equal. The volumetric efficiency is key is key to understanding the difference in the power band of the 6 cylinder vs the 8 cylinder of the same displacement, but there are so many other components in the engine that affect VE beyond the valve size to cylinder bore ratio, such as cylinder head design, intake manifold design, exhaust design, camshaft specs, etc.
The examples of the 292 vs the 283 or the 300 vs 302 demonstrates the differences okay, but there are big differences between those engines in cylinder head port design, camshaft specs, exhaust and intake manifold designs etc. Ultimately the I-6 engines have most of their components designed for low RPM power bands in comparison to the V8s, which helps to improve their VE at lower RPMs compared to those V8s. I have found a better pair of engines to demonstrate the differences of a V8 to a 6-cylinder of the same displacement, perhaps the closest in existence.
The Chevrolet 4.3L V6 and the L99 4.3L V8 are nearly identical in size and both measured in the more precise SAE net measurements. The L99 is actually 263 cubic inches, but is often incorrectly called a 265 cid (an L99 uses the 3.736″ bore of a 305, not the 3.75″ bore of a 265). So the L99 is within 1 cubic inch of the 4.3L V6 which is 262 CID. Both are SBC designs, and both share a fairly similar head design, with similar ports, and combustion chambers and both have a form of MPFI. The only major differences are the intake manifold design, the V8 has smaller valves, the compression is slightly higher on the V8, and it’s likely the cam specs aren’t identical (I don’t have the cam specs for them handy). Let’s compare:
1994 Chevrolet 4.3L V8 L99 (3.736” x 3.00”)
200 hp @ 5200 RPM
245 lb-ft @ 2400 RPM
1994 Chevrolet 4.3L V6 L35 (4.00” x 3.48”)
200 hp @ 4500 RPM
260 lb-ft @ 3400 RPM
Now in this case when things are more similar the differences aren’t as great. The V8 has the lower peak torque but also at a lower RPM, but look at the numbers more closely. The V8 doesn’t make its peak horsepower until far higher RPM than the V6. So at 4500 RPM, the V6 is also making significantly more torque than the V8. After that point the V6 would start to drop off in power while the V8 is still climbing to that 200 hp max.
The 4.3L V6 makes considerably more peak torque, albeit at a higher RPM than the V8. If we had the dyno graphs for these two engines, I bet at 2400 RPM, the V6 was making more torque than the V8. But even without the dyno graphs, the V6 clearly has a lower power band than the V8 which is primarily due to the differences in volumetric efficiency.
Intake and camshaft design can influence assumptions. The LB4 (TBI) version of the 4.3 V6 used in 1994 C/K trucks:
165hp @ 4000rpm
235 lb-ft @ 2000rpm
Of course, we don’t have hp-torque graphs here for these engines to get a better picture.
I am aware of the LB4 version but it’s less suitable for comparison as their are more differences. The camshaft and cylinder heads on the TBI engines were significantly different. In comparisons the L03 305 TBI in the truck made 175 hp and 265 lb-ft. Mind you, it also has a significant displacement advantage over the 4.3L V6. If the L03 were destroked to a 263, the numbers would be a fair bit lower.
“Ultimately the I-6 engines have most of their components designed for low RPM power bands in comparison to the V8s which helps to improve their VE at lower RPMs compared to those V8s. I have found a better pair of engines to demonstrate the differences of a V8 to a 6-cylinder of the same displacement, perhaps the closest in existence.”
I do agree that is probably the best demonstration with the head and exhaust manifold designs being almost identical. Unfortunately the different intake and possibly cam timing does still prevent a true “all else the same” situation which is clouded further by the first part of your quote.
The old saying is HP sells but you drive torque, so if you aren’t selling HP you tend to choose cam timing and intake design to maximize torque down low where you “feel” it. The intended application also tells a big part of the story.
One of the better examples of how intakes and cam timing can dramatically change the power curve is the Ford 5.0. The differences in the intakes between the truck and Mustang applications is the visible differences. The truck unit favoring low end torque at the expense of high rpm power and the Mustang unit doing the opposite. Ditto for the cam timing in the two applications.
That and the other differences mean the Torque and HP peaks are significantly different between the two. The final iterations of the Mustang HO 5.0 makes peak power at 4200 rpm. In truck guise, from the same era, it peaks at 3800 rpm. For torque the Mustang peaks at 3200 rpm while the truck version peaks at 2400 rpm. Both share the same bore, stroke and valve sizes which means the cam and manifolds are the main reason that the truck torque peak is 800 rpm lower.
I didn’t ever say it was a dead on perfect comparison, and actually highlighted the differences. However, this was by far the best V6 vs V8 engine comparison I could find and IMO better than the straight-6 vs V8s used in the article.
I am well aware that there is a big affect from the camshaft and intake on the powerband. However, these engines are tuned fairly similarily. Had I compared the L99 to the LB4 V6, that wouldn’t have been as fair of a comparison. However, the L35 was tuned for more high RPM power like the L99. The fact that it has peak torque at 3500 RPM suggests a fairly aggressive camshaft.
I did agree with you that your choice was probably the closest 6 cyl to 8cyl design out there, at least of common engines.
I was just pointing out how much cam and intake design can effect where the power band happens with bore, stroke, valve sizes and head design the same.
If I understand this correctly, something analogous happens in cells, and it offers an explanation for the fact that the cells of single-celled organisms tend to be small rather than large, and that multicellular organisms are made of a large number of small cells rather than a smaller number of larger cells.
Though they are far more complex than engines, cells also take in carbon-based fuel molecules, oxidize them, and extract energy from their chemical bonds, which creates water and carbon dioxide as waste products.
Though I have not heard the term used in a biological context, the concept of volumetric efficiency seems to apply to cells.
If we make a simplifying assumption that a cell is shaped like a sphere, doubling the radius will increase the volume by 8 times, and the surface area by 4 times. As a cell grows, its volume, which is related to its metabolic needs, increases more rapidly than its surface area, which represents the membrane that the carbon dioxide and oxygen must diffuse across to enter and exit the cell. Furthermore, the ratio of surface area to volume diminishes as cell size increases. Therefore, a larger cell will be less metabolically efficient than a smaller cell. Bacteria, which often live in cutthroat competition with others of their kind, tend to be very small cells.
Returning to Paul’s point, the membrane area of a cell, like the valve area of a cylinder, grows by a factor of radius squared, whereas the volume of the cell, like the volume of the cylinder, grows by a factor of radius cubed. The ability of a cell or a cylinder to take in and expel gases is impaired as the volume grows (and the ratio of area to volume decreases), and these mathematical limitations have strongly influenced the evolution of organisms and engines.
I see not much mention of turbos here. I am not an engineer at all, but from what I have learned here is that a turbo is in effect making volumetric better by forcing air past the valves. I assume, and I may be completely wrong, that forced induction has the effect of making the power stroke longer, hence more torque.
The 1.8 litre EA888 in my Golf has a rated torque of 184 ft/lb at 1400 and it stays at the peak to 4600 RPM. It makes 170 HP but there are numerous YouTube dyno videos that show it making 190 HP at the front wheels. I am not sure how true this is but the car does not feel slow. The motor is so flexible that it will easily pull from 800 RPM, but only if I used premium fuel. Since life is short, it only gets Chevron 94, sans dethanol.
A stage 1 chip takes it to 232 hp and 236 ft/lbs, also at 1400 RPM.
The installation date of said chip is September 19, 2022.
You are on the right track but a boosted engine act more like it has a longer intake stroke.
Take your typical 2.0 4cyl which has 500cc cylinders. The reason an engine makes vacuum is because the cylinder is not being supplied with it’s full volume. So if you are running at part throttle and 40% VE then your cylinder is only getting 200cc worth of air. Open the throttle all the way and you’ll max out at less than 100% VE on a NA engine because the restriction from the intake valve still results in a cylinder that isn’t full. Now put pressurized air in that intake via a turbo and when boost is made the engine will be operating at more than 100% VE. So at 120% VE the cylinder is getting filled with 600cc of air.
So, when people say things like a Turbo 4 drinks as much fuel as a NA V6 they are not wrong. A 2.0T running at 135% VE is ingesting a similar amount of air, thus requiring the same amount of fuel, as a 3.0 V6 running at 85% VE which is in the neighborhood of the max a typical modern NA engine can achieve. Of course you don’t have to use “all the boost all the time”.
Excellent piece, Paul! To throw another factor into the equation, compare the slightly larger but much more oversquare GMC 305 V-6 to the 292. The ’67 305 produced 170 gross H.P. @ 4000 r.p.m., and 277 gross ft. lbs. of torque @ 1600 r.p.m.. Essentially the same numbers as the 292 but with a slightly lower compression ratio (7.75 vs. 8.0) and a 2bbl. carburetor. In this case it appears that the undersquare 292’s 1 bbl. carburetor, valve sizing, and cam profile are adequate to match the oversquare 305’s ostensibly better breathing. The lower piston speed of the 305 does not seem to be a performance factor, though it certainly would enhance engine service life. In any event, both 6 cylinder engines produce maximum H.P. and torque at lower r.p.m.’s than a comparable V-8, supporting your case.
Unfortunately automotive horsepower/torque ratings are more or less bunk.
An industrial rating is a better picture of an engines’s capability, preferably in KW.
Because… If an engine is sold as being be able to pump water at some gal@psi, or generate xxKW @#rpm, it’d better be up to it. Thus industrial engine customers are provided with known accurate data. A plus is that such charts would be available for this sort of academic study; rather than just a single advertised peak point.
With an automobile engine if the forward thrust generated by the exhaust pipe and the pull of the cooling fan are factored in to boost output ratings a bit, nobody is going to argue. The other way too, if some concern would be better satisfied with a bit less power, no problem, we can hold back a bit on this run, or just reset the type.
Fuel consumption ratings are about the same way.
Unfortunately automotive horsepower/torque ratings are more or less bunk.
And what other system of rating an engine’s output do you suggest? KW rating is not really different than hp, just a different way of expressing the same thing: work. One can readily convert between them, by just dividing the hp number by 1.314.
Of course the gross ratings are not directly applicable to what an engine will generate as installed. Which is precisely why truck and industrial engines always came with net ratings, as installed.
Truck and industrial engine buyers needed accurate net power ratings, for obvious reasons, in the an engine’s ability to pump x amount of water or generate x amount of KWs.
There is no other method that I’m aware of that measures an engine’s ability to do work. Would you like to create a new one? Measuring them in KW is really no different than hp, as there’s a direct conversion factor: divide hp by 1.314.
Here’s the dyno charts for the engines that that was in the Dodge medium truck brochure back in the day. This was the case with other manufacturers. if it wasn’t printed in the brochure, it was available, certainly so to industrial customers.
Yes, SAE J816a was very specific and adequate test, I’m not saying otherwise. What I’m saying is that automobile ratings are prone to fudging far more than an engine which, for example, is destined to be coupled to a generator. When an engine is being packaged for industrial application it’s a given that it MUST be able to perform as rated.
With passenger cars it’s not that big of a deal. IE: “Oops, our new six is more powerful than the V8…” “No problem, let’s de-rate the six for a year until the new more powerful V8 is out, then we’ll bump the six up”
There are plenty of examples of under and over rating. Nothing changed except the rating.
Again, all I’m saying is that ratings for industrial applications are more reliable than ratings that mostly just have to satisfy some adverting need.
Sorry I mentioned it. Hope this clears it up.
In the past yes automotive HP rates were often bunk, but both over stating and under stating the true capability. A lot of that was due to the custom of listing gross ratings, which was compounded by marketing. For example all those Chevy engines that produced 1hp per cu in, that almost certainly were driven by the marketing dept. So good chance the engineering guys fudged things a bit to make the numbers the marketing guys wanted for the advertisement.
At the other end of it once insurance companies decided that HP ratings were a good determination of risk and started charging accordingly the industry started fudging the numbers the other way.
Then everyone switched to advertising the net HP measured in standardized conditions and everyone was selling the same vanilla ice cream, not their own version of a hot fudge sundae.
For average every day consumer cars the vast majority of buyers wouldn’t know what to make of a torque and HP curve and understand that the area under the curve matters more than the peak.
Now buyers of vocational trucks and stationary engines frequently do understand how to compare the curves In the case of stationary engines have done the engineering to determine the exact need and how much if any headroom is desired.
In the 60’s many IH ads touted how the SV’s SAE net HP was a greater percentage of the advertised gross than brand F or brand C results which isn’t too surprising since IH just had trucks so they were less concerned with giving the marketing guys the number that sounded good.
For example all those Chevy engines that produced 1hp per cu in, that almost certainly were driven by the marketing dept.
One engine, for one year only. And given its torrid performance, the ’57 FI 283 undoubtedly was capable of making all of those 283 hp.
An engine that would be an excellent subject for this academic study is the F Continental.
The same family was built in various bores and cylinder count as an L head. In one iteration it was supercharged. Finally it was topped off with a high-tech OHC head as the Jeep Tornado. And, somewhere no doubt, charts would be available.
By the way, that engine has more low low end torque than any engine in its class, and, a longer stroke length too. So ultimately it looks like it boils down to what was suspected all along, longer stroke = more torque.
I just remembered, there was an F-head version too.
I’d bet a hunch that none of ov the variations had a measurable affect on low RPM torque.
*of
*effect
I’d bet a hunch that none of ov the variations had a measurable affect on low RPM torque.
You lost your bet. And since you’re just making wild ass guesses instead of spending three minutes with Google looking them up, I had to do it for you.
Here’s the specs for the 1959-1962 L head Super Hurricane 226:
105 hp @3600 rpm;
190 lb.ft. @1400 rpm
And for the Tornado 230 OHC hemi:
140 hp @4000 rpm
210 lb.ft. @1750 rpm
And at 1400 rpm, it was making considerably more torque than the 226.
Precisely as I would predict: The improved valve opening area of the Tornado improved the engine’s breathing, resulting in its max. torque being both greater as well as occurring at a higher rpm. And thus hp is also increased, and also happens at higher rpm.
Which of course was the whole exercise of designing and building abetter-breathing head for it, right?
The Chrysler 225 slant six had a shorter stroke than the 230 Tornado, and made more max torque (215 lb.ft @1600rpm) than the Tornado. And significantly more torque than the 226 made at 1400 rpm.
PS: there was no F-head version of this engine. You’re thinking of the Willys four and the small Kaiser six.
Paul, you’re misunderstanding what I wrote. No doubt that’s why you misquoted me. I wrote “low low” (twice, not a typo) as in torque available at the lowest rpm, just off idle. I’m not making WAGs and Googled data of maximum torque rated at higher rpm won’t address this.
In “clutched” industrial applications available power just off idle is extremely important.
With heavy loads the optimal way to start things into motion is to put the load on at a dead idle, and then, when the clutch is fully engaged, put the power on.
Anything else is a recipe for clutch burning and powertrain “chatter,” which of course are destructive. Remember, many industrial applications don’t have the luxury of things like springs and soft mounts or light weight, that allow for “lurch” starts.
Again, I am speaking of available torque just off of idle speed. In the class of engines that also lived in light highway vehicles none will match the lowly Continental for low end grunt. And yes, I would bet.
I’m confident that others with experience are going to chime in and second what I wrote, meanwhile, please humor me.
There are industrial trucks that are identical in every way, gearing, weight, etc. EXCEPT, for their engines. The trucks with Continental engines will take on beyond maximum loads with greater ease and finesse than the same trucks with other engines, it’s common knowledge.
Many engines were a compromise, basically “drafted” into industrial service because they were available, they are far from ideal. Put a heavy load on at idle and “car” engine says “conk.” The old flathead says “aaargh, but I can do this.”
Look at the Tornado chart. At 800 rpm torque is already above 200 ft lb. At dead idle, 600 rpm, torque is at 200 ft lb.
I’m attaching GM’s certified graph for their 230 cid “High Torque” 6. High Torque has a few more cubes, but close enough.
Notice that at 800 rpm the High Torque 6 shows a paltry 100 ft lbs?
Fnally at 1600 RPM the High Torque 6 climbs to almost match the tornado, however, High Torque immediately begins to drop off. Meanwhile Tornado torque continues to climb from 1600 all the way to governed max rpm.
With this CID at low rpm the air requiment is going to be under approximately 75 CFM. Hardly a challenge for atmosphere to jam through even the most paltry ports and valves.
Hope this clarifies my point.
I misunderstood your original comment, because you didn’t word it very clearly. “Low torque” sounds like what it says: a lack of torque.
You’re obviously referring to the torque peak concurring at low rpm, and the torque band being very strong t very low rpm. That’s a bit different.
I’m not sure what you were getting at with the various cylinder heads, but as you can see, improving breathing on the Tornado improved its torque output, although as predicted, it occurs at a higher rpm.
I understand your point about “lugging”. Yes indeed, long stroke engine, because they invariably have a fat torque band at low rpm, are well suited to lugging. No surprise there.
So what was your original point. if it’s to confirm that long stroke engine have their torque peak at lower rpm than short stroke engines, I covered that and confirmed that in my post. It’s inevitable and predictable.
My original point was that it’s difficult to draw conclusions when comparing apples to oranges. IE completely different engine designs and their performance characteristics.
Whereas the Continental was a design that had worn many different hats under the same basic architecture. It had been spec’d from a load lugging industrial engine to a supercharged high speed passenger car engine, and about everything in between.
It was spec’d for various fuels and with various induction systems.
It also carried several valve arrangements. The original L-head. An OHC version. And, an OHV pushrod version.
( The latter which I mistakenly called out as an F-head, despite having owned one.)
That said, my original point was that the engine would be an ideal candidate for comparing what different iterations of the same engine did to performance characteristics.
BTW, while hunting for a horsepower / torque chart I ran into something that seems to confirm that fudging of published specs went on.
I could be wrong when I wrote that at 200 ft lb the Tornado had double the certified 100 ft lb torque output of the High Torque 6
“Jim Allen wrote of the Tornado:
In fact, the engine’s 140 hp at 4,000 rpm and 210 pounds-feet of torque at 1,750 rpm was underrated.
When Sampietro wrote about the Tornado in November 1962, dyno testing of the two-barrel engine was yielding 155 hp and 230 pounds-feet of torque (gross) as well as BSFC (brake specific fuel consumption) below 0.45 pounds/hp/hour from 1,200 rpm all the way to 4,000 rpm.”
That seems to underscore my perception of their tremendous low rpm grunt. They’ll practically twist off an input shaft while simultaneously turning a slanted engine backwards. LoL
It was never a “supercharged high speed motor”. The supercharged 226 made 140 hp @3900 rpm, and its torque peaked at 210 lb.ft @ 1800 rpm. It revved no faster than the regular version.
It’s a very common myth that super/turbocharging makes an engine rev higher. In fact, it’s generally the opposite; most turbo engines have a lower rpm hp peak. Supercharging increases torque, and thus there’s no need for high revs.
What OHV version of the 226 are you referring to? Never heard of such a thing except for the Tornado, which of course had ohv along with an ohc.
“High speed” being relative.
3900 is screaming for that engine, more than double it’s typical governed maximum speed when “at work.” For everything that shines about the engine at low speed, there are two blemishes that show up when revved. At 2500 they seem stressed.
There was a pushrod OHV version of the engine.
Tin cover, awkward massive cylinder head, horizontal spark plugs. Once you see one you’ll understand why all these years later I mistakenly recalled it as being an F-head. I couldn’t find a picture. Go ahead, you can call me full of it until one shows up. LoL
“High speed” being relative. 3900 is screaming for that engine,
No it’s not. The flathead 226 had its hp peak at either 3600 or 3800 rpm, depending on the verison. 3900 is exactly 100 rpm more. And 100 rpm less than the 4000 rpm hp peak of the Tornado.
I’m not talking about the speed they were governed to run in certain industrial applications. I’m talking about automotive applications, and 3600-4000 rpm was its hp peak. Sorry.
And I’m going to need to end this thread. Unless you can find that ohv 226. 🙂
How long do you want it to last?
There’s a chart for that too believe it or not. lol
How far do you want to go? All the way to Memphis, or do you want to drive home too? LoL
Hey, that new short stroke ’65 Ford 240 really starts to come on at 3800. What’s the holdup, how come you’re not enjoying your truck to its potential?
Get it?
I probably have paper for the pushrod Continental. One day I’ll try to dig that up and we can have a gander. How much is that wager, by the way?
So long thread, for now.
Looks like my replies are posting out of order, or not posting?
Anyway, my original point was simply that the Continental engine family was unique in having had such varied applications and configurations, yet with some constants, that it it would be a good study.
The 226 pushrod OHV images are in. The engines are obscure, not surprised if their existence was questioned. However, there is some credible online reference available.
One image and reference was found at a site called
CurbsideclassicDotCom, not sure if that would be considered credible? 😊
I had hoped for an actual capture in the wild. I took a field trip to track one but winter has it locked in, if, the engine is still there. Maybe in the spring? Meanwhile other peoples’ images.
Anyway, here it is. An engine cobbled together in the depths of the depression using some existing carryover of even earlier components. In various iterations it lived on for 50 years. Including being installed (and rated) in pre-war tractors as well as exotic supercharged ’30s cars. The later pushrod version was advertised as 145 horsepower.
A perfect platform to compare the output potential of various valve arrangements and “breathing” systems
…
Another image, per the one-per-reply limit.
As good an explanation of the subject as I have read. You yield nothing to Jason Fenske’s “Bore vs. Stroke” Youtube video. Well done!
Just watched it. It’s pretty good, but I’m not so sure about the section on efficiency and heat rejection. Heat isn’t only an issue at the moment of combustion; it’s a factor throughout the whole combustion cycle, and heat is very much transferred from inside the cylinder (and piston) to the cylinder walls and then the water passages. Which tends to negate his argument that a very long stroke engine is intrinsically more efficient. In reality, a square engine would tend to be more efficient in terms of heat rejection, as the internal volume is maximized in relation to surface area.
So I’m a bit dubious about some of his claims, especially about the very long stroke engine being most efficient. He doesn’t even bring in the factor of significantly higher friction, among other issues.
It gets pretty complicated. Many interacting aspects. Which is why I stuck to just the one or two most basic principles here.
The theory is that the smaller the surface area when combustion occurs the higher the peak pressure in the cylinder. The higher the cylinder pressure you can get with a given amount of fuel the more energy you have extracted from the fuel.
As you point out there is more happening than just the combustion event and as more of the cylinder is exposed you reach a point where the surface area between the large bore vs long stroke cross paths.
The other point he just briefly touches on is that you want pressure to be as high as possible during combustion and as low as possible at the end of the stroke.
That was Atkinson’s theory way back when and of course he created an engine that had an expansion stroke that was longer than the intake stroke. That was to minimize pressure at the end of the expansion stroke. Lower pressure at the end of the stroke means less energy is lost out the tail pipe.
So the long stroke engine extracts a higher peak pressure improving efficiency in the first part of the cycle and doesn’t let as much out of the tail pipe, increasing efficiency at the last part of the stroke. Yes more is also going into the engine as heat too but the net is still an improvement.
I get that, and had already. There’s a good reason why all modern engines are longer stroke than was once the case.
I was referring to his extremely undersquare “model”; I have doubts whether that would actually be as efficient as he claimed.
I didn’t hear any claim about his models which were exaggerated to make the point using the now common 500cc cylinder size.
The claims of 3-5% improvement was reporting the measured results of the University’s experiments, which weren’t as crazy as his examples.
Thanks for the great explaination Paul. There’s one additional consideration I find very useful to add to the discussion: mean piston speed (MPS)
MPS = 2 * stroke (mm) / 1000 * RPM / 60
Mean piston speed ranges between 8 m/s for large marine engines and 30 m/s for racing engines. A standard car engine has piston speeds between 15 and 20 m/s. According to Pounder’s Marine Diesel Engines Book, “the piston speed is limited by the acceleration stresses in the materials, the speed of combustion and the scavenging efficiency: that is, the ability of the cylinder to become completely free of its exhaust gases in the short time of one cycle”. I compared the three engines and got following numbers:
Honda RC116
RPM 21000
Stroke (mm) 25,14
MPS (m/s) 17,60
M-D Offy
RPM 6500
Stroke (mm) 114,3
MPS (m/s) 24,77
Hall-Scott 400
RPM 2000
Stroke (mm) 177,8
MPS (m/s) 11,85
I didn’t expect to see that the M-D Offy would have a higher engine speed than the small Honda engine, but in any case, we can see that all of them are in the typical range. Each design is limited by engine durability, materials, scavenging efficiency and speed of combustion (which also sets limits to piston speed, but in this case, as all examples are gasoline engines, should be the same).
Thanks for the addition on piston speed, an important one.
Lubrication is also a big factor in acceptable piston speed and how that effects durability. Too fast and the oil essentially can’t keep up.
Another interesting aspect of bore/stroke has more to to with packaging than any other requirement. Horizontally opposed engines, especially aircraft, but also VW air cooled, Corvair, etc. have extremely short stroke to keep the engine narrow. Paul’s favorite, the 1300cc VW, is the closest to square. At the extreme the most prolific piston aircraft engine of all time, the Lycoming O-360,
150-160hp@2700rpm depending on compression ratio.
Type: Four-cylinder air-cooled horizontally opposed engine
Bore: 5.125 in (130.18 mm)
Stroke: 3.875 in (98.43 mm)
Displacement: 319.8 cu in (5.24 l)
I don’t think width had anything to do with that. An extra half inch or inch on either side would have been immaterial. The original VW engine was advanced at the time, being slightly oversquare. That was the result of their experience as well as seeing the benefits. It was simply a modern engine at the time, and its bore and stroke ration confirm that.
In small aircraft, width is a big deal. Yes, it is quite likely that the VW designers were aiming at an “aircraft style” horizontally opposed engine, which was new technology when the VW was first designed. So I do agree with you on that.
Nikita, width is a big deal, but only for single seater aircraft which are a tiny proportion of the market, both then and now. Almost all two or more seat light aircraft have cabins of at least 38 inches in width (C-150). Almost all the opposed aircraft engines from LyCo are families with increasing cylinder bores and strokes as the years went by.
The Honda Fit engine had the exact opposite packaging goal. I read a paper written by a Honda engineer, but cant find the link. To “fit” the engine in the space they wanted, bore needed to be small, so we have a relatively high revving long stroke engine. A pent roof combustion chamber and four valves makes this possible.
SOHC 16 valve i-VTEC (This particular version has variable intake cam lift.)
Displacement: 1.5 L; 91.4 cu in (1,497 cc)
Bore x Stroke: 73 mm × 89.4 mm (2.87 in × 3.52 in)
Compression Ratio: 10.4:1
Power: 120 PS (88 kW; 118 hp) / 6,600 rpm
Torque: 145 N⋅m (107 lb⋅ft) / 4,300 rpm (GE8 Fit)
All modern engines, especially the smaller fours, tend to be undersquare. Yes, it makes them more compact. But it also improves low end torque, and breathing is not so compromised due to the four valve heads. And undersquare engines are intrinsically cleaner.
The Honda engine with 16.5 HP @21,500 RPMs requires the torque to be 4 lb-ft @21,500 RPMs.
Excellent article! Thanks so much, Paul, and all of the thoughtful commenters. Fascinating information.