there is debate on this as far as i can see but for the most part most builders that you talk to will tell you around 160 you wanna run 100 octane (or half 110 half 93) and any compression above that race fuel. I ran 93 on my old 4 mil stock cylinder setup at 160 psi with no detonation issues, but the first time i took it to the dunes I was backing plugs out all day, higher octane would have helped in that case ( or less timing), terrain can play a role.