Air cooling is not as efficient as liquid cooling, so air cooled engines run higher combustion chamber boundary surface temperatures than liquid cooled engines. The higher the combustion chamber boundary temps, the more tendency to detonate.

Also, larger cylinder bores increase the tendency to detonate. Combine these two trends and a Harley engine cannot tolerate as much compression as a four-cylinder sport bike on the same octane fuel.

Combustion chamber design is another factor. The compact pentroof chamber of a four-valve head with a centrally located plug is inherently more detonation resistant than a semi-hemispherical chamber on a M103 or Harley engine.

There are many issues that influence detonation. Unfortunatley, it often gets "dumbed down" to erroneous statements like "premium gas burns slower".

Apart from engineers who have specialized training or working experience in the field of IC engines, very few understand what detonation is, and the various design and operating parameters that influence it one way or another.

Duke