They may be advertized as 12.5 : 1 pistons, but they aren't.
I cc'd everything when I put my engine together. I have 4V iron closed chamber heads. It came out to 11.3 : 1. I run 93 octane pump gas with no problem. I have about 36-38 degrees total timing. When at the strip I advance it 2 or 3 more degrees and run some race gas. It runs 11.70's.
If you leave your slugs down in the hole and run the CC heads you will be OK (my opinion based on my actual engine). The cam you run could have some bearing on this also. I have a solid flat tappet 242/248, .579/.609, 108 LSA. Runs great !
I ran the same cam and pistons with 4V open chamber heads too. No problem there either. It ran low 12's if my memory is correct (gettin old).
I like the closed chamber heads better
Danford1
