Not long ago, I went riding with a bunch of cool guys. Most of them rode Harleys, and at the gas pump, I noticed they all used Super Unleaded fuel (91+ octane) in their Harleys. I ride a Valkyrie with a special tuned horizontally opposed six cylinder engine displacing 1520cc and have 6 carburetors, and I use regular unleaded fuel (87 octane) eventhough the Valkyrie Owners Handbook recommends 91 octane or above for high performance. But if you read on, 87 octane may be used. You just don't get the maximum performance, but it's still powerful and no detonation. On the other hand, if your Owner's Manual says, "91 octane or above required," then you have no choice. It would definitely detonate and may cause damage to your engine. I checked Harleys specification for fuel and it is indeed recommended but not required with the exceptions of the modified engines which are required to use 91+ octane fuel. Rest assured, if I have to unleash the beast, I will definitely use super. He he he! 
