Compression ratio for 91 octane

[bobcowan]

What you're really interested in is cylinder pressures. You want to calculater Cold Cranking Pressure (AKA- Dynamic Compression). For pump gas, you want to keep the pressure below about 190psi or so.

Static compression ratio is part of the equation - but only part of it. You're also interested in intake valve closing point, altitude, and a couple of other things.

RB Racing has a nifty calculater for this. It's a harley site, but the calculater works for nearly any engine. http://www.rbracing-rsr.com/comprAdvHD.htm

A good racing cam will also increase cylinder pressures as the rpm's rise. Even though the engine will run pretty good on pump gas, you'll need true racing gas for the track.

Aluminum heads can withstand higher pressures because they're better at transferring heat through to the coolant. Excess heat will increase detonation.

[Tommy]

I learned a little about detonation after ruining the pistons in my engine. As has already been said, static compression ratio and octane are only parts of the equation. You also have to look at timing advance, air and engine temperature, and how hard you're running the engine (e.g., will you be taking it on the track?). After my rebuild, I went one step below the automated knock detection system described above. Because sidepipes made it impossible for me to hear detonation, I installed a MSD Knock Alert system that gives a visual and audible warning in case of detonation. I then adjusted distributor timing to where I thought it should be and went for a ride. It didn't allow me to optimize the timing constantly as would the automated system, but it did give me confidence that I wasn't experiencing detonation under ususal conditions.