How is the compression any different than in a car? You've got a 4 valve DOHC engine with a piston, compression rings, etc... You could have 11.1:1 compression and if you're timing is at a base 0 degrees (for example) you could run 87 octane. Really, if you want to know what gas to run, read your motorcycle manual (RTFM). If it says 87 octane, you can run 87 octane just as long as you haven't fucked with the timing or compression (or have a massive carbon buildup, but you guys wreck your bikes WAY before that happens).