Fact or fiction: Adding mothballs to your gas tank increases the octane.

Dear Car Talk

Dear Car Talk | Feb 01, 1997

Dear Tom and Ray:

I have been told that adding seven or eight naphthalene mothballs to a tank of gasoline increases the octane. Is this true? If it is true, what will it do to the engine of the vehicle? -- Donald

RAY: That's an excellent question, Donald. This "mothball" story has been around as long as we have. So immediately upon receiving your letter, we called in the illustrious Dr. Jim Davis, Ph.D., director of the chemistry labs here at Car Talk Plaza, to try to get a definitive answer. And Jim said he'd get right on it.

TOM: Two months later, he called us and apologized for the delay, which he said was unavoidable due to a two-month-long faculty meeting that had just ended at Harvard, where he moonlights.

RAY: Anyway, after several months of study, and the complete depletion of an otherwise useful NIH grant, Jim has concluded that this mothball story is basically a bunch of horse pie.

TOM: There are several different types of mothballs on the market, none of which, to his knowledge, do anything to improve the performance of gasoline. They WILL burn, so you will get some power out of them. But since mothballs are more expensive than gasoline, this is not a very economical way to get to work, Donald.

RAY: If there were some magical performance-enhancing mothball, Jim says, don't you think Exxon and Mobil would be selling it to us as an expensive gasoline additive i.e. "Mobil Super ... Now with Mothballs!"

TOM: The kind of mothball you mention, Donald, is made of "naphthalene" which is a hydrocarbon, like gasoline. For those chemical engineers reading today, it's C10H8, and it looks like two benzene rings fused together. Jim says that benzene makes a very smoky fire when burned, so his guess is that naphthalene would make a lousy gasoline. On the other hand, he says, since it's just carbon and hydrogen (like gasoline) naphthalene probably wouldn't do any harm to the engine either.

RAY: Another type of mothball which COULD potentially hurt things is made of dichlorobenzene. That won't improve your car's performance either, but since it throws chlorine into the mix, it can produce HCl as a byproduct when burned.

TOM: For those of you who don't remember your high school chemistry, HCl is hydrochloric acid, the stuff that burns through almost anything it touches. And pumping HCl through your engine and exhaust system is probably not very good for its longevity, Donald.

RAY: Not to mention what it does to a) the people who happen to be breathing anywhere near the end of that exhaust system, b) your catalytic converter, and c) your manufacturer's warranty.

TOM: So, based upon Jim's research, we feel confident in summarily dismissing the notion that adding mothballs to your gas tank does anything to improve performance.

RAY: The only thing Jim will guarantee is that, if you put mothballs in your gas tank, any sweaters you store in there will come out without moth holes in them.

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