Q Dear Tom and Ray:

I went to get my 1987 Toyota Tercel inspected, and the inspector told me that my exhaust system was bad and had to be replaced -- the whole thing, from the exhaust manifold to the muffler. I took it to a discount muffler shop, where a guy who said he was the assistant owner told me that that he didn't have the right-size piping to fix it and that he'd put a larger-diameter pipe on it. I asked him whether the larger piping would affect my gas mileage, and he said the only thing it would do is give me more horsepower. Well, after being charged $600, I left. Since all this, my mileage has fallen from 35 miles per gallon to about 20. Is there any correlation between gas mileage and exhaust piping? If so, do you think the shop is obligated to fix my exhaust system and make it work the way it used to?

-- Scott

A RAY: First of all, I'd like to know what an "assistant owner" is. In fact, I think I might like to be one. But let's put that aside for the moment.

TOM: The answer to your question is yes. A larger exhaust pipe could possibly lower your gas mileage.

RAY: Engines "breathe." Cylinders breathe in fresh charge (gasoline and air) and breathe out exhaust. And the better an engine breathes, the more power you can get from it.

TOM: In fact, that's why multivalve engines have gotten so popular. They allow more stuff to go into and out of each cylinder quickly, improving performance.

RAY: But it works only up to a point. And that's the key, in your case. If the engine breathes too well, some of that fresh charge will get pulled out through the exhaust system before it even has a chance to be used. And that's wasteful.

TOM: So if your muffler guy didn't have the right-size pipe for your car and put on one with a bigger diameter, he might be responsible for a drop in mileage.

RAY: Even so, you said your mileage has fallen 40 percent, which is a huge drop. That might be possible if the "assistant owner" went to a pipe significantly larger than the original, such as 11/2 inches instead of an inch. It's not easy to fit a pipe that much bigger in the front of the Tercel, but I suppose it's possible.

TOM: A pipe big enough to cause a 40 percent drop in fuel economy would also have produced a significant increase in power. So if you've been leaving Corvettes and 350ZXs in your wake, that might be a hint.

RAY: On the other hand, the drop in your mileage might have nothing to do with your exhaust system. It could be something like a thermostat that's stuck open or a bad vacuum leak. So I'd take it to a dealer or a Toyota specialist, and ask him to have a look.

TOM: If he says your exhaust pipes are 50 percent bigger than they're supposed to be, then you have a good reason to go back to the muffler shop and ask the guy to take out what he installed and put in the correct-size system.

RAY: And in the future, consider having your regular mechanic get you a factory exhaust system. While there's nothing wrong with the muffler shops, the factory mufflers (the ones your mechanic would buy from the dealer) are exactly the same as the ones they replace. They have the same bends, flanges and pipe diameters, and they never have to be knocked with a hammer or resized with orange-juice cans. So you're less likely to get rattles and noises later on, and more likely to be able to buy individual replacement parts instead of replacing the whole system again. And you might be surprised to find that the costs are not that different, so it's worth checking out.

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(c)2002 by Tom and Ray Magliozzi

and Doug Berman