This is obviously a hot topic and I thought I would throw in my $0.02. Since I'm Canadian I'll throw in $0.04 for the dollar conversion.


Gas is made up of a mixture of hydrocarbons that have a variety of Auto-Ignition temperatures. The octane level of gasoline is a posting of that particular mixtures ability to resist knock (pre-ignition) in comparison to 2,2,4-trimthylpentane (isooctane) which is the standard for knock resistance (it's octane value is 100). Isooctane has an auto-ignition temperature of 396'C at atmospheric pressure. As gas is compresed in a closed cylinder it's temperature rises (the temperature of the gas,not it's AIT), approaching it's auto-ignition point. This is the same for the poor knock resistant hydrocarbons in the gas mixture like Heptane which has an auto-ignition temp of 204'C. So in theory the octane level that you should run your carat is dependant upon the temperatures that the engine is running at and the compression ratio. Since the temperaturs of the daily commuters vehicle can be considered a constant value in this calcualtion (assume that everybody runs their engine at about the same overall temp) then the factor in the equation that determines the octane level that the vehicle should be running with is dependant on the compression ratio of the combustion chamber. At high comression ratios 9.3:1 the poor knock resistant hydrocarbons will ignite simply due to the pressure in the chamber raising the temperature of the mixture above the autoignition temperature of that hydrocarbon. So the hydrocarbon for that condition should have a high auto-ignition point so that it resists pre-ignition. The timing of ignition is also important, you want to ignite the fuel mixture at the proper compression value so that all the hydrocarbons ignite. If you allow the timing in the chamber to appoach TDC then the gases will be even more compressed and the fuel should burn cleaner, although a lower torque value will be obtained.

So...if you agree with this, your timing should be according to spec and the octane level you run your vehicle with should be relative to the compression ratio of your engine. Then again this is all a mute point if you want to run the highest octane in your vehicle to avoid knock at all costs.

My recommended octane levels per compression ratio are;

7.5:1 = 87 OCTANE
8.5:1 = 89 OCTANE
9.0:1 = 91 OCTANE
9.5:1 = 93 OCTANE

If you're interested in racing your brick, check out this link for suggested Octane levels in relation to compression values.

http://www.klotzlube.com/support/tech_information/ratio.html