The closing point of the intake valve has some of the largest influence on octance tolerance vs. compression, as well as combustion chamber shape and quench area. The later the intake valve closes, the less cylinder pressure the engine will develop, and the more tolerant of increased static compression ratio the engine will be. For a general rule of thumb, engines with cast iron heads will usually handle up to 10:1 on 91 octane, and engines with aluminum heads will usually handle up to 11:1. There are more extreme pump gas combinations out there, but this is a good general rule to follow. Cranking compression is a little bit more accurate determining factor for pump gas use. 170-180 psi of cranking compression is about the max for iron heads, and 200 psi for aluminum heads.