[Mike Croley]

Racing gasoline allows you to increase compression, timing, and RPM without detonation or pre ignition. That's accomplished with Motor Octane. So to keep your engine alive, you have to first meet it's motor octane requirements. Too much octane isn't really a problem, but too little octane will be expensive.
Burn characteristics would be the next important determining factor, and then down the list a little way would be specific gravity.