I see a lot of references on post on here about using the dynamic compression ratio to determine the quality of fuel used, I have never done that. So my question is where is the cut off point at 2500 ft above sea level on using pump gas with 10 % ethanol, or do you use cranking compression off of a good gauge to determine which fuel you use or do you use the the old school way of using different fuels and wait for the pinging to start before upgrading to a better fuel The reason I'm asking is I'm considering switching heads on my 518C.I. street pump gas motor to a bigger set that has smaller combustion chambers that will change the mechanical(static) compression ratio from 10:29 to 1 to 11.07 to 1, I used KB and Wallaces calculators sites and they both come up with less than 9.0 to 1 dynamic compression ratios all commenst wanted