LottaBallsCamaro

and is a shorter stroke better for autocrossing because it can rev faster??

Ragin'87

longer stroke = more torque b/c of the definition of work.. which torque is. w=Fd. The greater the distance(lenth of torque arm, or in this case stroke) the force(in a lever: you pushing on it, in an engine: piston go up.. BOOM.. piston go down.) is applied to.. the more work it does.. and the more torque. I keep loosing my connection so I'll have to end here. Someone else can add to or correct anything i've said.

Anthony

AlkyIROC

A good example of a short stroke engine is the 302 that was in the early camaros. Large bore and short stroke produced big HP at high rpms. It could easily buzz to 8000 rpm and was produced to compete in the TransAm series.

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87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
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Best results before the 383 blew up
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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

Vader

LBC,

A longer stroke crankshaft means the crank throws are longer, giving the connecting rod a better mechanical advantage over the crankshaft. An equal combustion pressure will exhert more twisting force on the crank, but at a lower relative speed (more torque, lower RPM).



Conversely, a shorter stroke engine with an equal combustion pressure creates less twisting force (torque) but rotates the crank faster because of the shorter distance travelled.

Looking at the other parts of the engine cycle, the shorter stroke engine can have a higher effective compression pressure and not bleed off as much power from the engine to make the compression because of the greater leverage of the crank over the piston. Conversely, a higher compression pressure would tend to slow the crankshaft more in a longer stroke engine.

The long stroke engine does have a more effective intake stroke at lower RPMs, but loses more power because of the longer period of exhaust gasses resisting piston travel on the exhaust stroke. A shorter stroke engine must get its fuel/air charge with a little less veccum assist but also loses less power on the exhaust stroke.

In essence, a long stroke engine theoretically makes more torque at lower RPMs but has some limits to power at higher RPMs, while a shorter stroke engine may lack torque at lower RPMs but can produce greater and more constant torque at higher RPMs, (therefore, more horsepower).

I hope that explains it more thoroughly. It's a lot like putting a "cheater" bar over the handle of a wrench - you can create more force with the longer stroke, but there's no way you can turn the wrench as fast.

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Vader
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zippy

a shorter explaination is all in the piston speed. longer stroke engines have higher pistion speeds causing much more vacuum at a lower rpm than a short stroke engine. if you punch something close to you, very little effect unless your hitting fast. punch something further away and your hand is moving faster and has more inertia behind it. problem is the long punch just can't be done as fast. such as 302 vs. 383. 383 don't have to rev high, 302 does.

Matt87GTA

Couldn't have said it better, Vader. That cheater bar analogy is right on!!!!

JoelOl75

A short stroke large bore motor also has less frictional losses because the rings don't move as much of a distance at a comparable rpm.

Kyle F

Actually its perpendicular distance that causes the twisting force. That is why 90* engines have more torque that a 60* and yes the translated wok to the crank is also proportional to the stroke. That is why you see diesels with 90* blocks and stoked long strokes

nFORM91

Joel, good point, however wear and tear on rings is an inevitable factor on all engines regardless of bore/stroke.

Vader, ...and I say again, you are the man! Excellent illustration.

zippy, the punch throw theory is about the best way to explain why longer stroke engines have greater torque.

Low end and overall torque ratings on the older big blocks also give an indication as to why longer stroke produces more torque. Alot of other factors play a role as well;

Air/fuel induction (such as TPI vs TBI or for the more seasoned of us - Tunnel Ram vs low rise or stock intake manifold).

Valve train and (intake/exhaust port size),

Cam specifications and timing,

Compression ratio,

exhaust setup, etc...

...for these and other reasons our 5.7 litre with slightly larger BORE has impressively greater torque than the 5.0 litre.

LottaBallsCamaro

Awesome explainations everyone, thanks!!!

87RS402

Actually, with the exception of international's powerstroke and the DT466 (and chevy's izuzu made 6.5 POS) 99% of all diesels are straight 6's. The design has more torque with a broader, flatter curve. the torque is is about 3:1 vs. the hp. The 600 cat and cummins put out 610hp/2050lbs and 600/2050 respectivly. Peak numbers are at 1200 and 1800 rpm's redline at 2100

Matt87GTA

Chevy's Isuzu diesel a POS??? You could not be any more wrong. That engine is the best thing to hit light duty work trucks since bed liners. That thing will out-pull a Ford or a Dodge any damn day. And that Allison Transmission is an awesome piece of machinery. And what about all of the Detroit Diesel V6's and V8's??? Crapatillers and Cummaparts blow.

[This message has been edited by Matt87GTA (edited April 19, 2001).]

Mach5

Torque is the instantanious value of work and horsepower is the first derivative or timed rate of change of work. Think o fa pry bar. The longer the bar is than the esier it is to pry up on it. This is called the moment arm and the longer it is then the more work can be produced with the same force on the end of it. This applies directly to the stroke becase the longer a stroke is hten the easier it is to make more power with the same force. Horsepower is just a function of torque and is calculated from it.

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82 Firebird with a 2.8 but a 350 is on the way

87 Chevy K20 4x4 TBI pickup

71 Olds Cutlass S My ongoing project. Olds
350 with a th350

JoelOl75

I'm not talking about ring wear, just that ring 'resistance' isn't linear, but more of a logorithmic increase in drag as rpm increases. It's another drawback to trying to spin a long stroke motor. Since ring friction is one of the major parasitic losses in the motor, it really adds up.

This loss is minor to what vader descibed but it does occur.

Plus piston velocity is increased in a long stroke, therefore terminal velocity is reached much sooner and fatigue/failure set in much quicker. When the piston 'slaps' out on the exhaust stroke and quickly changes direction is when the rod is most likely to let loose. RPM's that kill a long stroke can be tolerated with no problems using a short stroke using similar rods in both motors.

irocbsa

Au contrair grasshopper. I work on the railroad, GMs (EMD - Electro Motive Division) new locomotive diesel is a V-16 displacing 10,800 c.i.


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"If they do not run, Then they will die." - Stonewall Jackson
#3
'87 IROC, 355 TPI, Converted to SD, 700R4, B&M Megashifter, 3.42, Trick Flow Twisted Wedge Heads, Edelbrock Intake, Accel Runners, Comp Cam, Crane Gold rockers, Lucas 23# injectors, Edelbrock headers, Flowmaster 3'' exhaust, Mallory Ignition, Transgo Performance Shift Kit, Corvette Servo, 2,000 Stall Torque Converter

87RS402

Irocbsa, you got me there. I should have said "on road". Show me a semi with more than 6 holes and I show you 1000 with only 6. I have seen alot of V-12's and V-16's. I even saw a V-12 cat in a peterbilt.

Matt87, You liked the 6.2 didn't you? I'm sorry but I drive a 6.5 everyday and I've driven both the ford and the dodge also. When you load any of them down to the 15,000 lb range (GCVW) you quickly relize the 6.5 isn't half the engine the powerstroke is. The powerstroke and the 5.9 cummins are both tuned way down for the application. The cummins will put out over 275 hp and 800lb/ft of torque, but the rest of the truck will break from that power. I'll agree that the 6.5 feels more powerful, it's when you put a big load on them that you find out which one is the winner.

Don't confuse me with liking the fords. The powerstroke isn't a ford, it's an international.

LottaBallsCamaro

Youre not talking about the duramax are you???

89BlwnRs

Hey 87RS, I was wondering why, from a Physics standpoint, a straight 6 would be better for producing torque than a V8? If you have two engines, with the same displacement, the same internal friction, and assuming everything else equal, they should produce the same power right?

Jason

87RS402

89blwn, physics isn't my department. And it's been over 10 years since I was given the answer to that same question. I do remember that the answer was very convincing.

BTW, what about the horizontaly opposed design? Anyone know a good automotive engineering instructor?