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What is the real world difference of an engines power band, given that they have equal displacements but different strokes?

For example: If you took a 4.25" bore x 4.25" stroke HEMI it comes in at 483", and if you took a 4.5" bore x 3.75" stroke HEMI it comes in at 477". All else being equal, ie same heads, cam, headers, intake, etc, would there be any difference in the dyno sheets? Theoretical answers are welcome, but do any of the engine builders have any examples of actual builds?

I chose the HEMI because it's not a suseptible to shrouding with the smaller bore.

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I'm not sure if the compression and bore combination or if it is the bore and stroke combination make the dfference in the results I have noticed that wedge motors with less then 10.5 to 1 compression, regardless of stroke, make more torque than HP on pump gas Higher compression motors make more HP than torque at a higher RPM on peak HP and peak torque RPMs I have seen more HP and torque per C.I. with similar heads on the larger C.I. motors during engine dyno testing If it is a street pump gas motor use the longest stroke you can, regardless of bore size

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What is the real world difference of an engines power band, given that they have equal displacements but different strokes?

For example: If you took a 4.25" bore x 4.25" stroke HEMI it comes in at 483", and if you took a 4.5" bore x 3.75" stroke HEMI it comes in at 477". All else being equal, ie same heads, cam, headers, intake, etc, would there be any difference in the dyno sheets? Theoretical answers are welcome, but do any of the engine builders have any examples of actual builds?

I chose the HEMI because it's not a suseptible to shrouding with the smaller bore.

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In a pure race scenario like NASCAR (no comments on NASCAR's purity, please) the engine builders tend to go bigger bore and shorter stroke. So much so that there is a maximum mandated bore spacing on new engines. Bore spacing tends to limit piston size. So a bore spacing limit is a de facto piston size limit.
The closer your engine application is to a pure race engine the more you'll want to find that bigger bore, assuming displacement is limited.

I remember back in the day I spent a semester reading all the old SAE journals in our engineering library. It was awesome! Articles by Fred Duesenburg on the advantages of double overhead cam engines with power curves, articles by many of the big guns in American engine development. At the time I was interested in Ford flatheads. The trick as I saw it was to get that elusive Mercury crank and offset the rod journals to get the biggest flathead one could drive on the street. Then I read an article from the '50s which had dyno curves for the same block and compression ratio but with 3.75, 4.0 and 4.125" cranks. Remember that on a flathead most of the breathing is controlled by the ports in the block. I was flabbergasted to find the three horsepower curves maxed out at the same power, nearly exactly. The lesson for the flathead was that power stays the same, displacement only determines what rpm it occurs at.

Now for a modern street engine that all goes out the window. As ports get bigger and bigger and flow more and more the limitation of an engine's power tends to become the displacement of the engine. This was apparent back in the 426 Hemi days where one had to rev the snot out of the 426 to get big power. It should have been a 488 right from the start, then it could have been really something as a street engine. But now with 350-400 cfm heads all over the place it seems one has to build the shortblock as big as possible just to use the heads available. And the increased power from using the heads' flow potential is greater than any added inefficiencies from R/S ratio. There are also the mountain motors that are making unreal hp/cubic inch with R/S ratios of 1.38 or so. There are also the Enginemasters winning combos which tend to be short rod engines.

The last part of this lecture is about the myth of stroke vs. torque. Like so many things in the mechanical world, It doesn't happen exactly like one would think. Given a piston/cylinder combination with a certain displacement, the longer stroke would "seem" to produce more torque. But for the same displacement, the longer stroke makes necessary a smaller diameter piston. Smaller diameter means smaller area for the pressure to work on. Conversely, a larger diameter with more area has less leverage because of the shorter stroke. I think it evens out. We need to look back at our lives and remember all the times we heard something like "long stroke means high torque" which was actually written by the advertising boys. It wouldn't be the first time that the general population believed a falsehood because of advertising.
In point of fact because smaller bores generally inhibit head flow the longer stroke production engines were usually designed to have maximum cylinder filling (meaning peak torque) at lower rpm. Larger bore engines promote head flow and could be cammed to have maximum cylinder filling at a faster engine speed producing more horsepower to boot. On the street the "high torque long stroke" engine would have most of its maximum torque available from off-idle to the torque peak. It'd set you back in the seat. A short stroke engine would have its torque peak at a much higher rpm even though it may be the SAME number, but because most street driving occurs at a much lower rpm the shorter stroke engine operates most of the time far away from its peak torque. So it seems to prove the short stroke low torque idea even when the two engines produce the same peak torque.

So I don't believe your question has merit on the street. The real answer is build the biggest shortblock you can afford. Then you put heads on it and increase flow when you get more $$$$. One more benefit of having a big engine under moderate flowing heads is you don't have to spend gigabucks on a high rpm valvetrain or rear gears. Thanks for listening.

R.

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There is a lot of evidence that suggests a shorter R/S ratio is advantageous when using a port with large cross-section. Can anyone take this statement a little further

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Every serious race engine that I know of uses the biggest bore allowed by the rules. Most rule limited racing has a bore size limit (or a bore spacing limit) for this very reason. The stroke is just there to get the displacement allowed by the rules.

If the displacement is unlimited then the big buck guys go with the biggest bore and the biggest stroke that they can buy.

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I disagree..
99% of the people out there the block is a limiting factor on bore size. They run the largest bores they can without going to thin.. very, very small % or racers are running class rules. 85-90% are your average bracket racer in the real world.... And they are plenty serious too..
As for stroke, I will say head volume plays a much greater role in peak RPM.
Build a decent stock 440 with stock stroke and some compression with stock heads see what it will RPM.....
I know about 6K with a decent solid flat tappet cam say a MP 590...
Take the same engine and throw a set of well prep'd 440-1's and see what it will want to turn?
More, a bunch more...
Now build the same engine and test with a 4.15 stroke crank, or even a 4.25 crank...
RPM wont be much diff than the same 440 with the same heads etc.
Your heads and cam/valve train dictates peak RPM.
I turn a 4.5 stroke 8K with a set of 440-1's. I know a 4.15 stroke will go that too. The 4.5 stoke has way way more cycles per minute, which wears out parts though.
Im am not so sure a 4.5 stroke is faster or makes more power than a similar engine with a 4.25 stroke...
I truly believe after 4.25 + in stroke it just doesnt impact a engine performance wise as much as overall parts selection.... I know this to be true, despite what the text book racers may think.

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I turn a 4.5 stroke 8K with a set of 440-1's. I know a 4.15 stroke will go that too. The 4.5 stoke has way way more cycles per minute, which wears out parts though.
I know this to be true, despite what the text book racers may think.

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Bob, are you sure that what you wrote is what you meant? RPM are RPM regardless of stroke, 5000 RPM is 5000 RPM with two inch stroke or a four inch stroke The ring and piston travel, distance traveled, is more on a long stroke motor than a short stroke motor 5000 RPM on a two inch stroke motor equals 10,000 inches per minute, the four inch stroke motor rings will travel 20,000 inches in the same minute Not trying to start or make a war, just trying to make sure what we are discussing is accurate and understood the same by all readers

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Your right Cab, Coomer mispoke, that will happen.

To the original question, in most cases, with the same CI, The bigger bore is the way to go.

Technically, with the same PSI combustion pressure, both equal CI engines would produce the same output if the A/F intake charge would remain the same.

But typically the heads are always shrouded by the bores with the production motors that we use for racing. Therefore, the bigger bore will always rule VS longer stroke Same CI.

It will have a lessor effect on a smaller HP motor say 500 HP and a much larger effect on say a 900 HP motor.

Dogday, and others good posts.

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i have to agree.
aside from that, in the real world nobody would build one of each with identical parts anyway, would they?

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For racing I would go with bore... but for the street
I would go stroke due to the rpm factor and the leverage

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... in the real world nobody would build one of each with identical parts anyway, would they?

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Does a Chevy-oriented magazine count? IIRC, I may still have copy of one where they actually built two versions of the same SBC engine, one as a 383 (3.75" stroke + stock bore) and the other as a 377 (stock stroke + big bore) to see the outcome. (Even the same cam was used, which IMO means if it was "right" for one of them, then it was also "wrong" for the other. )

The two engines performed pretty much as people would expect w/ the 383 making more torque and the 377 pulling a little more peak HP. From what I recall, the differences weren't major, yet fairly predictable.

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I turn a 4.5 stroke 8K with a set of 440-1's. I know a 4.15 stroke will go that too. The 4.5 stoke has way way more cycles per minute, which wears out parts though.
I know this to be true, despite what the text book racers may think.

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Bob, are you sure that what you wrote is what you meant? RPM are RPM regardless of stroke, 5000 RPM is 5000 RPM with two inch stroke or a four inch stroke The ring and piston travel, distance traveled, is more on a long stroke motor than a short stroke motor 5000 RPM on a two inch stroke motor equals 10,000 inches per minute, the four inch stroke motor rings will travel 20,000 inches in the same minute Not trying to start or make a war, just trying to make sure what we are discussing is accurate and understood the same by all readers

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Yea thanks for clarifying, I got it backwards guys... I get into a hurry and dont proof read none..
Just to clarify my point.
Stroke or Bore size has little effect on peak rpm, compared to the impact of head volume. People do NOT select bore size in reguards either...
Most go with the largest bore size possible cause we know it helps. I think Bore Vs stroke, bore is the clear winner. We usually start with 3.75" of stroke as a baseline. Stroker cranks are had now it just doesnt cost hardly any more to build a bigger engine, so...
I think the engine I outlined before is relevant, there is lots of guys with flat tappet cams and Indy heads, lots and lots more than larger- big bore, long stroke engines.
It falls back on the availability and cost of a aftermarket block. 90% of Mopar bracket racers are still using stock blocks. So figure bore size range from 4.32-4.375.. Which leaves stroke to increase cubic inches. I think the 4.25 stroke is "Best" along with the largest bore size possible.
This coming from a guy who's last two engines where 4.5 stroke, large bore aftermarket block engines. So Im not exactly speaking out of my butt here. These engines arent dyno queens, or test beds for bolt on parts....
matter of fact they were NOT dyno tested at all. Only numbers that matter to me is the ones they generate shortly after they are first fired at the local track.
The engine Im slowly slapping together will be the largest bore possible with a aftermarket block, and a 4.25 bottom end..
I believe it to be more effective than my previous setups, when you look at HP per cubes.

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... in the real world nobody would build one of each with identical parts anyway, would they?

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Of coarse not, thats the point... This is the type of engine that 90% of the racers are building though, so it matters plenty...
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Does a Chevy-oriented magazine count? IIRC, I may still have copy of one where they actually built two versions of the same SBC engine, one as a 383 (3.75" stroke + stock bore) and the other as a 377 (stock stroke + big bore) to see the outcome. (Even the same cam was used, which IMO means if it was "right" for one of them, then it was also "wrong" for the other. )

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The two engines performed pretty much as people would expect w/ the 383 making more torque and the 377 pulling a little more peak HP. From what I recall, the differences weren't major, yet fairly predictable.

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