Originally Posted by jeffs66
Hiya, I have a 66 Catalina.It has the 389 2v engine.well recently I put on #62 heads w/ I beleive the 72 cc combustion chamber.Heres my ?.I know Pontiac used a basic flat top piston and would change compression with head swaps.Well does any body have a guess on what compression I have now,or how I may find out without tearing the motor back down?I know its up there cuz its got a bad case of the rattles if I dont burn premium,even when I back the timing down.plus the little extra power.
The difference in head chamber volume between the heads you removed and the replacements is not that great either way, unless they've been milled excessively. So it should exhibit the same tendancies it did before the head swap.
The exact CR is hard to say as has been said already. Using a quench distance of 0.055" (0.040" head gasket and the piston about 0.015" in the hole at TDC), a piston valve relief volume of 6cc and stock 389 bore and stoke, you're near 9.7:1- which is stretching the limits of pump gas detonation resistance w/an iron head Pontiac if you're using best power total timing- as you've found out.