Where does the 300EX require a step up ?

8.9:1 = REGULAR / 87 - 89 Octane (stock recommended)

?:1 = 93 -94 Octane

?:1 = 100 - 112 Octane

Thanks

 

I can tell you what has worked for me, and what I have heard talking to racers, ususally anything over 10:1 will need Premium, which should work up to 11.5 , anything 12:1 or better should probably be running race fuel,or a Prem/Race blend. To be honest, I ran a 10.25:1 .040 overbore in my Wolverine for years on 87 regular, probably would have ran a little better and cooler on premium, but I was a cheap a$$ back then. Now that I have my 446 kit, I am running premium to be safe, as it runs a higher comp piston. The race pistons in the 13 + range will require pure race fuel to prevent it from melting the piston. Some new Sport quads run 12:1 or more from the factory, but the cams are set up with enough overlap to prevent high cylinder pressures from building up and causing detonation.

 

Heat contributes to detonation. Air cooled engines run hotter than water cooled, so they always need a higher octane than just their compression ratio would suggest! This is especially true if you run in hot weather or pull long hills, etc.