[Kahuna]

The compression ratio of your engine determines the octane rating of the gasoline you must use in the car. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. Think of Octane as an ignition inhibitor -- the higher the Octane rating, the less likelihood that the air/fuel mixture will spontaneously ignite. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can potentially damage an engine. Lower-octane gas (87-octane gasoline) can handle the least amount of compression before igniting. If you have an engine with a compression ratio of 10:1 or greater, 91-octane gasoline or higher is recommended. If you have a low-compression engine (8:1 ratio), high octane fuel would be a waste of money.

I use 89-octane in my '62 Olds but that's because I have the lower-compression (8.75:1) 394-2 bbl. This engine was a no-cost option that allowed the use of regular gas. I also use a lead substitute additive.