Manifold Vacuum Advance what am I not getting?
I bet lower, this is one of those so I was told engines.
Could very well be less. In fact I had forgotten that the TRW/Sealed Power catalog said the piston should be .017-.018 positive.
Even at that, with a 68 cc chamber you won't get 12:1.
And that's why every build should be poured and calculated. Way too many books and catalogs are wrong.
The end result is this. Or worse yet when you in a compression limited class and you trust what the piston guy says. And they are off and you have more compression than you thought.
Cost a guy a track championship because he didn't measure. And he was about .5 over the limit.
Obviously he was pissed but the sanctioning body did not care. I think (this was 1989 I think) he measured it and the dish was slightly smaller than what the book said.
You really need to know your compression ratio to get even close on cam timing. Otherwise you are just guessing.
It's no different than not degreeing a cam.