Compression question

[BEAVER.989]

Why is it that with the 400EX once you get over 11:1 you need to run high octane fuel when some of the other quads have 12:1 or 12.5:1 stock and run on pump gas?

I'm sure there's a simple answer to this, but I don't know it, lol.

[GPracer2500]

The biggest single reason is air cooled vs liquid cooled.

Engine knock (aka: detonation, pinging, knock, spark knock) closely corresponds to combustion chamber temperatures. Air cooling just can't shed enough heat to prevent certain areas of the combustion chamber's inner surface from heating up to a point where pump gas won't accommodate. There are other reasons as well, but the engine's ability to get rid of heat is major.

[Snipe]

in a nut shell it burns hotter, and if you bore the head is thinner. less metal and more heat creates hotter temps and dont dissapate as quickly so ya gotta do something to keep from motor failure.

[BEAVER.989]

Heat. Makes sense. Thanks guys.