To further clarify, the average street car is designed to run on the ethanol which is sold everywhere, but some engine configurations need a higher octane fuel in order to combust the fuel completely. I had a buddy who rebuilt an engine with a compression ratio that required 91 octane, or he'd get knocking. This was during the late 80's, and more than one station around these parts had pumps selling 95 octane (it might have even been 100) for the real hot-rods.