Reply to "Octane vs Compression Ratio..."

I dont believe the question is "what's the optimal CR for 89 octane". It's more like how high of a CR can you run if you want to burn 89 octane. The performance potential of your engine will be somewhat limited by your compressio ratio. Higher CRs generally mean higher volumetric efficiency, which in turn offers the potential for a little more fuel/air per intake stroke, thus more power.

Higher CR will allow you to run a more aggressive cam. The detonation issue is one to pay attention to. Detonation is when your fuel/air mixture auto-ignites before your ignition fires-Like on a diesel engine. It's not just annoying pinging, but the potential to do severe damage to your engine. Many things affect detonation, piston/head materials, CR, Octane, combustion chamber shape, temperature, etc. With 89 octane and iron heads the concensus seems to be a max CR of 9:1. This is about what you get with closed chamber 4V Cleveland heads and the dished pistons from a smog-Cleveland. With aluminum heads, you may consider up to 10:1 CR. With exotic ceramic coatings, perhaps more. I've had people tell me they run 10:1 with iron heads with no detonation problems on 93 octane. Higher octane fuels burn at different rates. If you're above the detonation limit, running higher octane shouldn't hurt, but probably won't gain you a whole lot without some timing adjustments to compensate for the burn rate.

-My 2 cents