I voted 165 cutoff for Prem. gas w/4 degrees timing. I have run 170 w/4 degrees timing BUT you have to have it jetted on the rich side, and you have to have GOOD Prem. fuel (out of a tank that has its own designated fuel hose , not a fuel station that has one hose does all three 87,89,93 etc. as you may get some of the 87 fuel that the last guy pumped out. Some other variables that come into play are squish band width/angle, dome shape, etc... but for the general answer 165 is cutoff.