Compression Ratio too much?

You'll be fine, especially with aluminum heads.

RIXX93GT said:

Okay I just picked up a forged 347 stroker kit for my spare block in the garage. I have been researching for a good set of heads to compliment this package. I will be staying NA so a little higher compression than stock is fine but the flat top probe pistons are rated for like 10.2 to 1 compression with a 61cchead. How much compression can I run on high octane pump gas without detonation problems? Most of the heads I have been looking at are 58-61cc. Then if I have the block decked it will raise it even more. I might as well ask for head recommendations while Im at it also. Thanks alot guys.

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I run about a 10.2 or 10.3 C/R on shaved stock heads. 93 Octane does the trick every time. When I ran 104 Octane in it, it ran like a raped ape. Runs a little warmer than it would stock, but I'd say it would be fair to expect that in any high compression application.

I would stay away from 58cc heads if you are going to be 10.2:1 w/ a 61cc chamber. Many times though, the comp. ratios listed are higher than actual - probably so the manufacturer stays on the safe side of things. The only way to know for sure is to run a quick calculation. Determine the cc's of one cylinder, and divide it by the sum of the cc's of the head, valve relief, piston depth "in the hole" and gasket. In other words:

((bore/2)^2*pi*stroke)*cc conversion) / (valve relief cc + quench cc + chamber cc)

Assume a .030 motor w 3.400 stroke (i think that is the combo for a 347 - someone correct me if i am wrong)

4.030/2^2*pi*3.4/.061 / 4cc + 6cc + 61cc = 10.01:1

realize the above is a rough estimator - i've assumed a flat top piston w/ 4cc valve reliefs, the piston essentially at zero deck height, a .030" gasket, and .061 cubic inches/cc. Change the variables to match your situation and you should get a close judge of what your comp. ratio would be.

If you are fuel injected, a true 10:1 or a little more should not be a problem, assuming that you have it properly tuned. It is more difficult to avoid detonation on a hot day w/ a carbed set up. Definately check what the valve reliefs may be though - b/c i have no way of knowing what cc relief those pistons would have.

With aluminum heads you can get away with 11 to 1 just like the LT1 's do....I'm always a fan of hi compression SBF's

We run 11.5.1 in our car now with AFR 165 HEADS all on spec fuel torco(mach 100) race fuel

My motor should be close to 10.5 w/ a 60cc chamber. I have AFR 185s for my 347

yeh like they all said. generally 10:1 on iron heads is the limit and a switch to aluminum heads can do 11:1 on pump gas.

get some trick flows (or afr's)and you'll be golden!!

runningmole said:

My motor should be close to 10.5 w/ a 60cc chamber. I have AFR 185s for my 347

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I have a CHP 347 streetfighter with AFR 185's and it comes out to be 10.5 to 1. I drive it every day (its been over a year now) running 91 pump gas, and have no problems with detonation