short stroke vs long stroke
#1129926
12/08/11 03:37 PM
12/08/11 03:37 PM
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WO23Coronet
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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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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129927
12/08/11 03:50 PM
12/08/11 03:50 PM
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Joined: Aug 2003
Posts: 43,719 Bend,OR USA
Cab_Burge
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Quote:
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.
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
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129928
12/08/11 03:56 PM
12/08/11 03:56 PM
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Joined: Oct 2007
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polyspheric
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It's a good question, but it's very difficult to compare like with like. Leaving out chamber shrouding, the rod ratios will be different. If the head flow is sufficient, the diff may be small but if the head is too small the shorter stroke wins. They're going to need slightly different cams, and which one makes more power may just be "which cam came closest to what the motor wants". The dome shape will be slightly diff for the same static CR, and since the dome is the "floor" of the chamber some minor diffs are expected.
Boffin Emeritus
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129929
12/08/11 03:58 PM
12/08/11 03:58 PM
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sixpackbee
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Too many variables for a direct comparison. Internal weights will differ quite a bit if using the same block. Using a like rod will result in a heavier piston in the short stroke application for example.
1959 Bugeye Sprite 1967 Charger Black L code 1967 Coronet R/T Convert Green 440 auto bought from original owner 1968 Charger R/T Bronze 440 4 spd console AM/FM 1969 Super Bee WM21H B5 A40 D21 N96 1969 Barracuda Formula S 340 Convert pilot car 1969 Hemi Road Runner RM23J D32 Omaha orange 4.10 Dana N96 N85 1970 Super Bee WM23N FE5 V1X 3.91 axle package, N96 1970 Road Runner RM21N B3 V1X D13 1971 MG Midget 1971 Road Runner RM23H GW3, A57 1972 Road Runner RM23P FY1, D21
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129931
12/08/11 04:40 PM
12/08/11 04:40 PM
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sixpackbee
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If there is a CID rule and you start with a clean slate then a large bore/short stroke engine is desirable in most cases.
1959 Bugeye Sprite 1967 Charger Black L code 1967 Coronet R/T Convert Green 440 auto bought from original owner 1968 Charger R/T Bronze 440 4 spd console AM/FM 1969 Super Bee WM21H B5 A40 D21 N96 1969 Barracuda Formula S 340 Convert pilot car 1969 Hemi Road Runner RM23J D32 Omaha orange 4.10 Dana N96 N85 1970 Super Bee WM23N FE5 V1X 3.91 axle package, N96 1970 Road Runner RM21N B3 V1X D13 1971 MG Midget 1971 Road Runner RM23H GW3, A57 1972 Road Runner RM23P FY1, D21
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129933
12/08/11 05:19 PM
12/08/11 05:19 PM
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Joined: Nov 2003
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sixpackbee
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All I can tell you is that a large bore/short stroke engine will go down the race track quicker.
1959 Bugeye Sprite 1967 Charger Black L code 1967 Coronet R/T Convert Green 440 auto bought from original owner 1968 Charger R/T Bronze 440 4 spd console AM/FM 1969 Super Bee WM21H B5 A40 D21 N96 1969 Barracuda Formula S 340 Convert pilot car 1969 Hemi Road Runner RM23J D32 Omaha orange 4.10 Dana N96 N85 1970 Super Bee WM23N FE5 V1X 3.91 axle package, N96 1970 Road Runner RM21N B3 V1X D13 1971 MG Midget 1971 Road Runner RM23H GW3, A57 1972 Road Runner RM23P FY1, D21
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129934
12/08/11 05:23 PM
12/08/11 05:23 PM
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Joined: Jan 2003
Posts: 9,924 Weddington, N.C.
Streetwize
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GENERALLY and for a conventional 2 valve wedge design. All else being equal( displacement, head flow, compression, cam timing, bobweight, etc) : For the same cubes the longer stroke will produce higher average torque but the peak will occur at a lower RPM. Bigger bore will be lower torque below the long strokes peak and nearly equal achieved at a higher RPM and (again GENERALLY) will achieve slightly higher peak (but not necessarily higher average) horsepower. When you take "all else being equal" out of the equation you can get excellent results from either combination by selecting parts to 'overcome' the torque or HP "weakness" and some people are very good at this, so the becomes "muddy" whenever exceptions are taken For Most hemis and canted valve motors and 4 valve motors the bore/stroke difference will generally be more extreme in terms of torque and less extreme in HP. These type cylinder heads are less dependent on adequate valve to bore clearance and, in actuality, even bore diameter for good flow and torque/power production, most production motors are undersquare (smaller bore than stroke) in an attempt to maximize part throttle torque but still achieve excellent top end power. A little discussed constant of torque in a 4 cycle engine, beyond peak torque (where the highest VE% is achieved) the rotating and reciprocating assembly (ring and bearing friction) begins to act as a brake on the torque output, HP goes up with RPM but the rate of rise (in rpm/sec) begins and continues to slow. This is why (particularly) on the street broad flat torque curves tend to "feel" faster than peaky high RPM "horsepower" motors. Race motors are typically designed and tuned to never have to operate below peak torque so that's quite a different animal.
Last edited by Streetwize; 12/08/11 06:25 PM.
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Re: short stroke vs long stroke
[Re: sixpackbee]
#1129935
12/08/11 06:20 PM
12/08/11 06:20 PM
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dogdays
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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 is a lot of evidence that suggests a shorter R/S ratio is advantageous when using a port with large cross-section. 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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Re: short stroke vs long stroke
[Re: dogdays]
#1129936
12/08/11 08:27 PM
12/08/11 08:27 PM
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polyspheric
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"a big engine under moderate flowing heads is you don't have to spend gigabucks on a high rpm valvetrain or rear gears"
X2
Boffin Emeritus
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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129938
12/08/11 09:30 PM
12/08/11 09:30 PM
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Joined: Apr 2007
Posts: 2,028 Trumbull,CT.
jim sciortino
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Quote:
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.
IMO, there is a bottom line with everything and in this case, that would be ability to cope with the maintenance schedule dictated by rpm.
Pick the engine design you want to run, choose the desired rpm range you want to operate in.....this will help you determine the dimensions.
Personally, in a race application, I favor as big a bore as is practical within the bore space and enough arm to allow the cross section/induction side to reach my rpm goals. Do not overlook the mechanical advantage of launch rpm and gear ratios to accelerate a vehicle. It isn't JUST rpm, but useable rpm, including AFTER horsepower peaks. If the cylinder heads and induction can support BOTH icreased cubic inch and rpm, now we're talkin'.....but, there is the diminishing returns problem as engine size grows. Look at the hp per ci in NHRA prostock vs mountain motors. More overall steam for the huge engines, but look how much bigger they need to be vs their NHRA counterparts.
Advances in valve springs, rockers, lifters, pushrods, etc., have raised rpm levels without the associated carnage of just a few years ago.....although meticulous inspection/cycling is still required for elevated levels.
In a street application, where "low rev" seat of the pants and decreased maintenence are higher up the "importance" ladder, increased stroke will lower the curve allowing for more civilized gear ratios and converter flash, also resulting in fewer issues with valve springs, lifters, questionable pushrod angles, etc......
In your example of 4.25 x 4.25 vs 4.500 x 3.75 , I don't expect anyone would build two engines so different that "all else being equal" would ever apply.
Engines that radically different would need different build approaches to optimize its own internal dimensions.
I'd be racing the big bore combo and street driving the big stroke deal.....but that's just me.
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Re: short stroke vs long stroke
[Re: AndyF]
#1129939
12/08/11 09:33 PM
12/08/11 09:33 PM
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Posts: 4,330 Lynchburg, VA
Leon441
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Back to the question. It all comes down to what you are trying to acheive. If you are looking for the most power. Make the most you can in bore. That is not to say use a 4.500" bore with a 452 head.
I like to build the largest bore and then stuff the most stroke affordable to get the most bang for the buck with the rules at hand. I post quite often on this subject because beginners are always wanting to run R3 blocks that can go 4.200" and run forever but want to choke thier Race W7-9 heads with a 4.100" bore. Then brag on how big their engine is because they stuffed a 4.250 stroke in it. There is a place for a 4.250" stroke crank in a small block. I have been wanting to build a small bore small headed small block with a before mentioned crank. Because I would love a stock stall speed converter with a highway gear ratio and a torque monster engine. Can you say driveability with big power.
I know very little about generation II Hemi's but I would think they would benefit less than a wedge from increasing bore.
Leon
Career best 8.02 @ 169 at 3050# and 10" tires small block power.
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Re: short stroke vs long stroke
[Re: dogdays]
#1129940
12/08/11 10:24 PM
12/08/11 10:24 PM
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Joined: Jan 2007
Posts: 3,695 nc
emarine01
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Quote:
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.
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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Re: short stroke vs long stroke
[Re: WO23Coronet]
#1129941
12/08/11 10:40 PM
12/08/11 10:40 PM
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Joined: Jan 2003
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gregsdart
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Ring friction is high enough to tip the scales to a large bore short stroke, all else being equal. Thats the info from an engine design book used in the past at Cornell University. Narrower rings in use today may have reduced that some, but crank case windage will be higher with a longer stroke also. Modern engines like the Ford Coyote 5.0 have a square bore/stroke arrangement to suit emissions. It gets real hard to keep a motor clean with a larger bore due to too much dead space around the top of the piston of a large bore cylinder.
8.582, 160.18 mph best, 2905 lbs 549, indy 572-13, alky
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Re: short stroke vs long stroke
[Re: Bob_Coomer]
#1129943
12/09/11 12:27 AM
12/09/11 12:27 AM
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Joined: Aug 2003
Posts: 43,719 Bend,OR USA
Cab_Burge
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Quote:
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.
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
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: short stroke vs long stroke
[Re: Cab_Burge]
#1129944
12/09/11 12:46 AM
12/09/11 12:46 AM
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Sport440
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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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