octane number is the amount of the hydrocarbon Octane the fuel contains. The higher the octane number, the more you can compress it before it spontaneously ignites. When you run a low octane fuel on a high-compression engine, the compression can ignite the fuel/air mixture before the spark gets a chance to. This is what is commonly called "knocking" (= bad!), which leads to piston and valve damage. So when you have a high-compression engine, you need to seek out these rare fuels or mix your own in order to prevent damage to your engine. Plus, they smell real good when they burn (I think so at least). It is just another facet of "if you wanna play, you gotta pay, speed is expensive, how fast do you want to go??"