Highest compression ratio

[coyote42]

Looking for advice on what the highest compression ratio I can run on 92octane pump gas. I dont want to deal with eng ping and dont want to go too high. Im building a 68 340ci with cast iron X heads, Around a 268 cam. I want to true deck the block and heads and need to pick the right piston height. Whats the average CC of an X head chamber?:banghead:

 

[marksmopars]

I have 9.25:1 on my 340, bored 40 over, but running solid cam, 550 lift at the valve, with ported T/A heads. Provides 400 HP on pump premium. But am running T/A head which I don't think will be any different than the X head for this consideration.

 

[1970Duster]

Usually it's ~10.1 using cast iron heads with a good quench.
You can get 11.1 using Aluminum heads with a good quench (I'm running this on my 340 at .030 over and a full roller setup using Eddy closed chambered 63 cc head, 60779).

 

[Coyote Jack]

There are to many variables at play to say one particular compression. I run 10.58 to 1 compression with iron magnum heads with 91 octane and can even run 89 if I want. I did run 87 once in a pinch but I babied it. You have to look at the fact you have open chamber heads and your cam timing to come up with what kind of compression you can get away with.

Jack

 

[blue missile]

Without getting to crazy here, your static compression ratio, is one variable in the Dynamic compression ratio equation. Cam timing and overlap is the other major one. Find Pat Kellys DCR calculator, and enter in the info for a much better picture of how your combo will react to the 92 oct gas. Try to keep it under 8.0 for that octane level. Aluminum heads will also help, as well as chamber shape.
Andrew

 

[RustyRatRod]

What's your budget? If you have deep pockets, you can build a 10:1 or so motor for pump gas, but it takes MONEY. Most people just leave that part out. On a budget, 9.2:1 is about it.