What is a compression point worth?

[dd74]

Given all things equal in two 3.0 engines, what do any of you guess would be the horsepower difference if one engine had stock 8.5:1 compression, and the other stock 9.3:1 or maybe something like JE Pistons at 9.5:1?

Thanks.

[kstylianos]

Approximate HP increase per compression point is 10%/point.

[Tyson Schmidt]

I've heard more like 3% per point. But it is a parabolic curve. The higher you go, the lower the gains. Diminishing returns, as they say.

[69911e]

Here is the formula from a past link for 2 points increase.

theoretical thermal efficiency n(rc) = 1-(1/rc)^0.4 (See curve above)

Then:
8.5:1 gives n=57.5%
10.5:1 gives n=61.0%

making for a theoretical jump of ~6% (180HP -> 191HP with 6%)

Here is the link.

The effects of higher CR

[nostatic]

I believe you're thinking of a logarithmic curve, but it depends on what you're plotting. If you're doing hp (y axis) v. cr (x-axis), maybe it would look like this:


But then again, would hp start to drop if you kept increasing cr? So maybe it would start back down...

[Wayne 962]

Just do a comparison of some older 911 engines. Some models had varying compression ratios with all other components being nearly the same. That will give you a good baseline to start with...

-Wayne

[Bill Verburg]

Here are the curves for a 3.2 published by Bruce Anderson.

[Tim Walsh]

very interesting.. I'm thinking of upgrading my ~9.0:1 2.4E to 10.5:1 and twin plugs. So in reality it should only make another 10-12HP out of it?