Originally posted by Port:
Is there a reason nobody has mentioned compression ratio in this discussion? You run a certain octane based on your compression ratio, not based on timing. Ocatane is the ability of the fuel to resist combustion under compression. If you run cheap fuel in a high compression engine the fuel will ignite before the ignition lights it strictly from being compressed. That is detonation. You can cause detonation by advancing your timing because it ignites the fuel before the optimal time in the engine's stroke.

9.8:1 is the cuttoff for running 91 octane pump gas at sea level. Higher compression than that you need to start thinkin about race fuel. I am not sure what these contours have, I don't have one, YET, but I doubt they require 91 octane unless you are right at sea level. The higher in elevation you are the less air going into your engine wich lowers compression in the cylinder, so you can use cheaper fuel.

If you run high octane where you don't have the compression to get it to ignite, you are wasting fuel and potential energy in the wasted fuel every time the cylinder fires. Resulting in lost power and fuel mileage.

If your car gets better mileage on the cheap stuff, and the "seat-of-your-pants" feel is better, run it, that means it works better. Higher elevation and lower humidity will allow you to run cheaper fuel.



Compression ratio is too simplistic of a concept to make a full determination about octane.

The most critical element is cylinder PRESSURE. Cylinder pressure is determined by much more than just compression ratio. Cylinder presseure is effected by compression ratio, valve timing, ignition timing, combustion chamber shape, port configuration, number of valves, any restrictions in both the intake and exhaust systems, fuel quality, fuel mixture, flame propagation, flame temperature, engine load, and so on.

Jim Johnson 98 SVT 03 Escape Limited