I run 12.5 pistons in a 427 BBC all roller & Lunatti (healthy cam). I am certainly no expert, but here is what I DO know... With extensive port & polish, my comp is about 10.2 now. I run 50/50 93 & 110 leaded... occasionally straght up 93 & ZERO problems at low & high RPM's. It has 488's & 3500 stall. No pings, hestitaions, etc... again, I am not an expert. I will say I have over 100 hours on this motor w/ fuel above. It's a hell of a topic that everyone has thier opinions on though!! Good luck.