Just wanted to clear this up. Higher octane fuels (generally) burn slower, or rather at a more controlled rate, than lower octane fuels. They are used in higher compression (and hotter) engines to help reduce 'knock' - that being detonation. Ignition advancers can help increase HP by reducing the additional time the higher octane fuel requires to complete the burn by starting the burn process a few degrees sooner than normal. In a car engine, the ignition advance is usually vairable, controlled by spring tension against RPMs (as in older cars with weights and springs under the distributor cap), but on motorcycles, the ignition rate is fixed, whether it's stock, +2, +4 (or whatever) throughout the entire RPM range. Basically, in a tuned, higher compression, hotter running motor that needs a higher octane fuel to eliminate detonation, an ignition advancer can help get back horsepower lost to the slower burning fuel.jim schmidt said:If you have modified your head by porting or shaving, you may need to advance timing to meet the ideal burn completion point since a b***** or more compressed charge burns slower. This is why higher compression engines require higher octane (faster burning) fuel.