The lower grade fuel usually has a lower ignition temperature and is more combustable. If you are running forced induction of any type it can actually ignite the fuel just by pressure and heat before the engine wants it to. If thats the case, a higher grade fuel is needed. which in turn will be harder to ignite making things better.

The weird thing is, with a high performance engine, sometimes, not ALL the time. Performance can actually be greater with a lower grade fuel seeing as how it is more combustable.

If you run high grade gas in a cheap car made for the lowest grade gas, it can have very bad effects. Killing your catalytic converter because of the increase in un burned fuel making it's way in there. Among other things.

Best to just stick to what your manufacturer says. Preignition will cause more damage to your engine than you could potentially save going with cheaper fuel.

Don't know why I just spewed all of that info. Guess im bored.

BTW. Anyone ever read "MAXIMUM BOOST" by Corky Bell?