Octane rating is simply a way to rate a gasoline fuel/air mixture's resistance to spontaneously igniting in the cylinder. Since detonation can be disasterous - especially in aircraft engines which run a lot hotter then automobile engines. Air-cooled aircraft engines can run at temperatures that would cause a liquid-cooled auto engine to seize solid. At the peak of piston aircraft engine power, there was actually a 145 Octane avgas - The famous purple gas. It was used in engines like the Pratt & Whitney R-4360 28 cylinder monster.
