[Matt Cramer]

It'll improve cold start performance... if you're running methanol.

I think this myth came about from a garbled chain of valid or at least partially valid claims.

1. Methanol vaporizes very poorly.

2. Adding acetone to methanol helps it vaporize significantly.

3. Improved vaporization can help with drivability and mileage with the engine cold. Not so much on a correctly tuned and warmed up engine, though - you shouldn't be having fuel puddling issues when the engine is hot.

Somebody apparently took the above and figured acetone would help with gasoline, too. Slight problem: Acetone vaporizes a lot better than methanol, but a bit worse than gasoline. It may have some value as a fuel system cleaner, but that's about it.

I've tried a couple fuel additives on work related projects. The only ones I'd use on a personal project are (1) Toluene is a bit more cost effective as an octane booster than the usual stuff you buy at a parts store, but you'll want to use a gallon or so to do anything, and (2) Oxygenated fuels like ethanol or methanol can give a lot more horsepower, but at the expense of horrible mileage. I haven't seen any concoctions that do well as mileage boosters.