[McTaff]

Ok, where do I start? Oh yeah... If you are a chemical engineer, by all means go ahead and try this in a lab. Anyone else DO NOT ATTEMPT THIS!

Do NOT, repeat DO NOT believe everything you read on the internet. I have seen countless 'infospots' saying how you should add this or that to your engine, etc etc.

Answer yourself the two questions that the people that put up these sites forget to ask themselves, or delude themselves with:
1) Why don't fuel companies do it?
There would be something in it for them, certainly. They can charge you for AceGas, at a premium, and make a killing because they aren't using their precious oil reserves while working at the market price plus extra for the additive. I call 'bull$#!^' on the fighting tooth and nail against it... if they saw an opportunity like this they'd be buying up every source of acetone and adding it to the fuels. So that raises a red flag right there.
2) Then why is it on the internet? Surely because of the oil companies supressing the bad media, right?
Well, if it's such a brilliant stroke of genius, why aren't chemical engineers doing it all the time? Oh, wait - and has anyone got their check from Bill Gates/AOL, and seen that Taco Bell dog run across their screen from forwarding all those emails??
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- What does Acetone do?

Stick it on stuff yourself. Experiment... but don't leave it on your skin. It's pretty drastic stuff. I've seen a high school science teacher strip paint off a desk with this stuff, and it does no good to seals/rubber hoses and the like. It is used to dissolve fake fingernails. Don't get it anywhere near paintwork. You have been warned.

- What happens when I put it in my fuel tank?

a) It contains water (about 5%). Water = rust. It would also condense in the combustion chamber... ever thrown a couple of teaspoons of water into an engine intake? Ok, it's a small amount, people will say not enough to cause any dramas... but are you going to risk it?
b) It decreases the ignition point (or 'flashpoint') of the fuel. Note the high compression ratio of the SVX make this a HUGE no-no.
c) The various nasties in Acetone will burn to create some pretty big nasties in the atmosphere - carconegens and some half-formed stuff that likes to steal molecules from other compunds. Acetone also contains Ketones which are pretty rude all by themselves. All up, you can get nerve damage, kidney/liver damage... I need go no further.
d) Any plastics/rubbers/polymer substances (teflon rings? not sure) with begin to degrade faster. (It can begin to pit and degrade some metals, apparently??). The the weak link here would be IMHO cat-converters and oxygen sensors. I wouldn't expect them to put up with it.
e) Storing reasonable quantities of acetone around the house is not exactly awesome safety practice, and for the love of (insert deific being here) don't carry it around in the car!

- So shut up McTaff, I just want to know does it work?

Short answer, yes, increased milage.
Long answer, yes, but you could end up costing yourself more than the few bucks at the pump... your car, your money or your health. And don't come flaming at me with the "McTaff is in bed with the oil company" - I have nothing to do with them. If you read very carefully, that article is really an advertisement to sell their products (oil filters and the like).

- But I saw it on some performance thing on TV!

Fuel additives have been used, with varying success, to vehicles modified to take them. Acetone is one of them. This also includes Methanol, Nitrobenzene (yummy, almonds), Nitromethane, Benzole, Ether, Propylene Oxide.... these are tried, tested and work with the correct equipment.

- Man, did someone barf in your cereal this morning McTaff? You're such a cynic! They tell you you don't need heaps! Just a little. That'll be ok, man.

They tell you you only need tiny amounts, and the tiny amounts won't have the detrimental effect in the way I've put it here.... But did the cigarette manufacturers tell you about cancer, emphysema, heart disease? "But it's in oh-so-small amounts! Not enough to hurt you!"

It's a scam. Marginal facts just prettied up to sell stuff, ignoring/brushing off the dangers - and it's been around for years! Don't believe everything you read.

(edit - fixed my totally screwy spelling)