My answer would have been a lot different if I knew of the other variables. But the original question was general. Can Compression hurt power. That question by itself I stick with my original response.

But, if you are talking hemi which I know very little about other than you run a hollow dome to keep the piston light due to the huge dome to make compression. I would try and run someththing with the least dome I could and cram all the stroke I could in it making my compression that a way. This theory could be way off due to the hemi-culties but in theory this is the way I would lean.

Jeff, for sure I`m just letting you do your thing on this engine. I`m just trying to learn/understand all this. As most people know I`m the 1st guy to ask questions.

Of course, the irony of a 4 stroke multi cylinder motor is that the compression cycle of the adjacent cylinder takes power generated from it's neighbor on it's power stroke. so increasing compression proportionally increases the internal load on the crank for it to overcome. Normally aspirated, as compression increases, power increases, likely to the motors very ability to maintain seal of the compressed mixture and the heads ability to flow gases in and out. but apples to apples compression alone doesn't tell the whole story, in simplest terms there is likely an optimum combustion chamber and cam timing for each ratio and for any given crank speed. And for example going from 10:1 to 13:1 in a hemi doesn't react like 10:1-13:1 in a wedge, too many variables in static vs dynamic especially with regard to timing events.

I can't see compression ever hurting power as long as these two variables are maximized. #1 No detonation. #2 Dome/dish/combustion chamber are optimised. A 426 style Hemi is a perfect example where using a large dome to increase compression may be worth less than the power created by an optimized chamber/piston.
Doug

I am convinced there is a point where piston dome shape ends up completely negating a given increase in compression but I think you will find that in this case (changing compression from 12.8:1 to 14:1 while at the same time increasing stroke to 4.25) the piston dome may be smaller. I would consider leaving your piston crown unchanged and changing just the compression height to match the crank and see what you come up with. If it is higher than 14:1 I would consider going with it.

Quote:

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i was going to fit the same piston i used in a few s/s engines in daves engine which is 104ish dome,,, would yeild 15.5/1 because the added stroke.

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I should have carefully read all the replies, I see Jeff at modern already said same thing and you end up with 15.5:1.

ANYONE remember the 366 short-stroke Hemi ?

With the ability of a HEMI to overscavenge, I'd pay close attention to the camshaft. Especially in a class that doesn't allow crutching the effect with a properly designed header......or shrinking the ridiculously large exhaust valve diameter.

Has anyone seen a HEMI engine where the entire combination was "worked', actually pick up with cams in the 106/108/110 range vs 114/116/118???

"Dynamic compression" is this year's cliche, and greatly over-valued as an effect.

RIGHT ON MIKE!!!!!!

More compression helps but narrows the tuning window, meaning the slightest weather change is going to affect everything - most importantly damage happens real quick if you miss!

Listen to Jeff

Great post guys...

Any comments on the effects of the exhaust restriction from factory manifolds and 2.5" pipe/mufflers on this combo?

My understanding has been that this exhaust restriction is more of a problem at higher RPM's, so perhaps the added stroke will lower peak power range and have less exhaust pumping losses?

More compression should not hurt this.

At almost 500" would this motor not benefit from a wider LSA ??

Good luck with it Dave! IMO your in good hands with Jeff...


Cheers,

Arnie

It is - until engine speed goes above 1,000 RPM. After this point, and depending on manifold and port volume, inertia, gas speed etc. it approaches rhwn reaches full stroke - at some point before peak torque.
If you still have capture based on IVC anywhere near your power peak your cam is wayyy, wayyy too big.

When the effort to compress takes more power away than any gain by more compression it is a negative gain.Current NHRA ProStocks are around 15.5 CR,Comp Elim Single four engines around 16.2.Bill C.

It has already been mentioned by guys smarter than me-but I would say that compression will help until the dome gets in the way of flame travel. Also you will need a different camshaft--again pay close attention to this with a big change in compression. J.Rob

True - the negative effect of dome intrusion on VE can be somewhat reduced by running a narrower LSA.

I don't think the more successful work done with complex chamber shapes has been made public. Hint: there's squish happening during overlap too...