John is the YS engine a 69? Looked at your profile, guessing that it 350 hp with 62 heads? (Nice car collection btw). I have calculated around 9.6 when trying to use ours 62s. The 62 heads we have measure 75 cc untouched. Seen some folks say they run 77-78 sometimes. I had a set of 13s years ago off a 70 gto 350 hp, seemed like they ran closer to 77cc. Thought they were all 72cc for years, until some others said some castings ran more cc’s and I checked what I had. Two sets of 16s have run 72. Have single nice 48 laying around I will never use to if some needs one. I think it was 72cc.
I think I posted a few weeks ago my 70 455 Olds has 10:1 with factory pistons with a 214/224 cam. Got to looking the other day at some engine specs I had down, it is 9.6. Pontiac was not the only one over rating stuff.
FWIW, My 452 with the roller cam is a pump gas F.A.S.T. type build. So I have different reason for going on the high side of compression. If I was running a big intake, carb and headers it would been fine some place in the 9s. Factory appearing is a little different deal for big hp.
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