[joedls]

Originally Posted by Vettezuki

You'll probably get a more detailed answer from BRUTAL64 later, but here's my 2 cents.

Your idle quality has nothing to do with the octane rating of the gas you're running. (With the possible exception of very low octane on a very high compression engine, but that's a different story with much bigger potential problems.) Is that gas that's in there "fresh" or has it been sitting in the tank for a year or more? If it has, it may have begun to separate, or there might be a lot of water in it, etc., which would certainly could effect how smoothly the engine is running. Assuming the gas is fresh, the idle quality has to do with the engine and most likely the carb itself.

The only reason to run higher octane fuel is because of higher compression, more boost, and/or more timing. The octane is simply a resistance of the fuel igniting. For example, low octane fuel is dangerous (won't really work) in a high compression engine because it will knock, or possibly pre-detonate, potentially damaging or even destroying the engine. A higher octane will allow the air/fuel mixture to be compressed without spontaneously igniting.

What he said. And to add a little, your car will not run any better without some tuning for high octane fuel. Simply adding the fuel without tuning for it will net you nothing.