After getting tired of hauling av-gas, I dropped the compression in my 427 s/o from 12:1 to 10.5:1. No problem with 93 octane pump gas now. Engine is fitted with ci tunnel port heads, which don't soak up as much heat as uncoated aluminum heads. Other that head material, another huge consideration not mentioned is camshaft events and timing. The more overlap you're running allows more compression, due to pressure loss during the overlap period. You can play with camshaft advance, too. Don't forget spark lead, i.e., advance versus retard. We can get into combustion-chamber efficiency and air/fuel mixture, too, so there is no pat answer on the compression one engine can tolerate versus another with a given octane petrol rating.
