Quote Originally Posted by CatFan View Post
You have it backwards. Octane retards ignition. Higher octane gas is for high compression engines where the fuel tends to ignite from compression alone. When that happens before the plug fires, you get a knock since the piston may still be on its way up.

Running higher octane fuel in a lower compression engine will result in carbon deposits in the engine. Chunks of carbon flying loose inside a 2-stroke is a bad thing.
You are correct, the higher the octane the more compression it takes to ignite. However a common misconception is to associate the size of the engine with the compression ratio. In fact most larger engines only require 87 octane, but smaller engines are tuned to get maximine HP from a smaller size so the jack up the compression to achieve that. For instance I have a Motor Home with a 8.4 Liter V8 GMC engine. It is designed for 87 octane. My Acura MDX has a 3.4 liter V6 and it requires 91 octane to achieve the HP rating. The Honda Pilot with same size engine only requires 87 Octane, but it has about 50 less HP than the Acura. A more extreme example is the Mini Cooper. It takes high octane gas because of the output of such a small engine.
That is also the case with my 25HP Mecury. In order to get the HP up, the compression is higher to compensate for the smaller displacement.
Best way to determine what is required is to consult the owners manual. After all, they designed the thing and they know what it requires.
SeaRay