well, damn, the question is to simple for me to vote definate. i know i would personally switch to strait race fuel around 175-180, but that doesn't account for abmient conditions such as heat,humidity, nitrogen content, or flow iinfluences, such as the exhaust charge or crankcase/intake efficiencies. of caurse, timing, too. the cranking compression really only gives you the condition of the cylinders in regaurds to the compression ratio. it's like asking the same question on a car that can be running n/a, or up to 30# of boost, but not specifying which. you can have 40# difference in cranking compression in 2 different engines that have the same octane requirements. also, are you asking which point you start loosing power from lack of compression for the fuel? :shrug: