Nobody's pointed out the more obvious reason for higher octane fuel: It has more chemical energy. With the timing set to take advantage of this, you get more power, otherwise you'll be exhausting burning gases. I think there are obvious differences in the way the octane rating is expressed between here and the US Cheers Martin #1458 B Benson wrote: > If an engine is designed to use 86 octane that's what you should use. Higher > octane fuels are blended to combust at a slower pace while lower octane > fuels burn much faster. This is opposite of what many may think. If you use > the higher octane fuel the slower combustion can, over a long period of > time, produce carbon deposits in the combustion chamber which raise the > compression ratio. Higher compression ratios require higher octane, slower > burning fuels. Therefore it's possible that your car can develop a > dependency for high octane at some point. The slow burn of high octanes is > necessary for turbo cars because the slower burn helps retard detonation > which is when combustion takes place too early and catches the piston still > trying to finish the compression cycle while the combustion is trying to > drive it back down. The higher intake temperatures that come with turbo > charging create that situation. The bottom line is you're wasting money > using higher octane fuel if your engine wasn't designed for it. > > Bruce Benson [Non-text portions of this message have been removed]