The Alchemist

iTrader: (19)

Horsepower is a calculation derived from torque, which is in it's basic definiton is a measurement of cylinder pressure. Increase cylinder pressure over time and you increase horsepower.

disc0monkey

iTrader: (10)

the longer stroke gives more momentum to the rotating assembly.
how does this extra momentum effect acceleration of the piston what does that do to HP?

a longer stroke also gives more leverage to turn the crankshaft. how do you think this will effect the engines output characteristics.

ZMX

More stroke = more displacement.

More displacement = more fuel and air.

More fuel and air = more power.

If you only change an engine to increase its stroke, it will make MORE power at ALL RPM. Period.

joecar

It is believed that the leverage effect is offset by the higher friction caused by increased piston speed (they cancel each other out).

slowride

iTrader: (3)

So let me get this straight would you guys thinking stroke won't increase power on it's own agree that a 4.8l ls will make as much power as a 5.3l with the same top end parts and cam? Only difference between them will be stroke right

KCS

iTrader: (20)

If you increase stroke (and therefore displacement), you should make more power everywhere except peak power. The old cam and induction system will limit the peak power, but the displacement increase will make more torque (and therefore horsepower) everywhere else and cause peak HP to happen at a lower RPM.

There's a older thread where this was already discussed and elaborated more clearly, with some dyno results as an example.

slowride

iTrader: (3)

Well I agree with you that power will increase. I really wasn't asking the question rather pointing out another example for others to look at. There was a post from a guy in the dyno section that just switched to a 383 from his stock ls6 and it did make a huge improvement in torque and still squeeked out a little more peak power at a lower rpm. This is with all the same stock ls6 induction parts. Granted it probably picked up a little SCR from the stroke.

transsam

iTrader: (5)

My experience, same cam, similar compression, 346 to 383 was about a 35-40 horse jump in torque and horsepower across the entire powerband to about 5200 RPM. From 5200 RPM to 6200 RPM, the 346's horsepower began catch the 383's and they met at my rev limit of 6200.

That experience has been repeated over and over again.

Here is a very good example from an extremely old thread that will be hard to find from AFR. They posted it 4 or 5 years ago. I am glad I downloaded the pictures.

Same cam, same heads, 346 vs. 383 -??cc pistons, but similar compression ratio-


Edit:
Notice that the statement made earlier in the thread that the powerband is effectively moved down is indeed precisely accurate. At any given time, "draw" a horizontal line and notice that the power made is between 300 and 400 RPM lower with the 4" stroke. It makes it a helluva lot of fun to drive, that is for sure!

KCS

iTrader: (20)

I referred to that thread the last time this discussion came up.

slowride

iTrader: (3)

I do think hidden somewhere in one of the post about the build Tony said the 383 was about 0.4 less SCR also though. Not that it would make a world of difference, but using the same cam the DCR will drop also.

transsam

iTrader: (5)

I hope I wasn't stepping on any toes. I didn't see the last thread, just this one. I was actually trying to validate exactly what you were saying in your last post.

A.R. Shale Targa

iTrader: (2)

Post 29 sums it up perfectly....Just wanted to throw one more variable in.
When stroke is increased and rod length stays the same, the rod to stroke
ratio worsens. Rod length affects piston dwell time (milliseconds) @ or near
TDC and BDC. Compression ratio limited stock car engines (usually 355 sbc)
make use of the longest rods possible not only to lighten the piston to as few
as 300 grams but to dwell (or stall) the piston @ TDC longer to build more
heat during maximum compression and ignition. Heat (literally in British Thermal
Units) is the transformation of potential energy (fuel source) to kinetic motion
So by increasing stroke, decreasing rod/stroke ratio, and accelerating piston speed @ TDC....the piston's increased change in direction while harder on the
wristpin actually pulls on the combustion chamber/intake port harder. Intake
manifold vacuum and port speed are dramatically increased.....both helping
throttle response and low rpm torque.......just say'n

Msilver

iTrader: (1)

This test seems irrelevant since they ran the same compression ratio on both motors. you should see a higher compression on a stroker motor, because of increased swept volume. This test would be much more interesting if they had just changed the stroke and let the compression ratio be higher for the stroker motor. All they really did was compare how the heads and cam responded to more displacement.

KCS

iTrader: (20)

I'm pretty sure that was the point of the test; to see how an engine reacts to more displacement. Adding a second variable (like CR) to a test onlyskew the results and makes the test invalid.