Hello everyone, I have been searching for the stock compression ratio for my 70 350. I am bringing the engine to a builder tonight and want to know what the original compression ratio was. It is the numbers matching engine for my cutlass supreme vert. So I am sure it was the 310 hp V8 option engine. The only thing I could find doing research was a thread on this site that said 10:25 to 1. Is that correct? From what I have read you don't want over 10:1 to run todays pump gas. Its not that I don't trust the guy I'm bringing it to but rather this will be the first time I've met him as I got his number through some other guys I know. I took the heads off and from what I can see I am not sure it even needs to be bored out now, I was expecting it to need that but there are no ridges that you can feel in the cyclinders. If that is the case I assume it would be a good Idea to re-use the stock pistons? So if that is what I do, is the compression going to be too high? Any advise would be appreciated Thank you

 

Finn5033 - The 70 Cutlass Supreme 350 (310 HP) has the compression rate of 10.25 to 1 while the 350 (250 HP) has the compression rate of 9.0 to 1

 

Mine was the stock 4-barrell 310 horse. Turns out my cylinders do not need to be bored and my pistons are re-usable as well. Do we need to get the compression down so I can run pump gas? Should I get lower compression pistons, or will thicker head gaskets get me down under 10:1?

 

Yes. The original HGs are the thin shim type. Just about any replacement HG will lower the C/R...

 

So that should get it down enough to be ok running pump gas?

 

Re-jet the carb a little richer, lower thermostat, and timed by ear [-2 degrees when you hear pinging] should do it, and hardly effect performance.
Maybe ask Cutlass EFI for a cam suggestion.

 

Just had my 69 350 rebuilt. Originally a 10:1 motor but reduced it to ~9.5 with thicker head gaskets. No pinging with 94 octane.

 

Thanks guys I appreciate the information