you need to know the compression ratios of the engine.....UCCR (uncorrected compression ratio) and CCR (corrected compression ratio). UCCR is full stroke compression ratio (like a 4 stroke) and the CCR is the ratio of volume, between the roof of the exh port and above, compared to what's above the piston, when its at top dead center. usuaally 15:1 is when you need 110 octane. guys, also remember that the squish band is still very important. a squish band that yields higher MSV (squish velocity), will require higher octane too. throw some advanced timing at it too, and you need better fuel too. you can have 2 engines with the same bore, stroke, and head cc volume, but 1 will have a higher exh port, and that one will show less psi on a compression gage (lower cranking psi), but running comprssions will be higher than cranking. see, don't get caught up on comp gage for dictating octane. this is what trinity does and they suck.