Today: What's the Advantage of Having Two Tailpipes?

Dear Car Talk

Dear Car Talk | Aug 27, 2015

Dear Car Talk:

When I was a kid back in the late 1940s/early '50s, you had to have a dual exhaust system. I think most of the new cars that have two tailpipes use the "cat back" system, running the dual exhaust only from the catalytic converter back. My question is: Does the "cat back" system give any improvement in performance, or is it just to look cool? Also, do any production cars have a true dual exhaust system? I expect that a true dual exhaust would require two catalytic converters and would not be cost-effective.

-- John

Gee, I'm not aware of any true dual exhaust systems on production cars these days, John. At least, I haven't seen any in the shop since the Nixon administration.

The vast majority of cars you see with two tailpipes out back have what I'd call "faux dual exhaust." That does nothing to improve the car's performance. It improves the car's appearance, but doesn't make the car go any faster.

The theory behind real dual exhaust systems is sound: You send gasoline and air into a cylinder, it detonates, then you have to clear the exhaust gasses out of the cylinder. The faster you can get the exhaust out, the faster you can get a fresh charge of fuel and air in.

That's why engines with four, and even five, valves per cylinder are popular. With more valves, you can get more stuff in and out of a cylinder quickly.

Similarly, if you have two real exhaust pipes running all the way from the engine to the tailpipe (one each dedicated to one-bank cylinders), you can clear your exhaust more quickly, get your fresh charge in more quickly and get more power out of the engine.

But it is expensive. You need a complete second exhaust system, with its own a muffler, catalytic converter (or converters, in many cases) and every thing else. And it adds weight, which cuts into fuel economy.

That's why most manufacturers just go the cheaper and lighter way, splitting the tailpipe after the catalytic converter and muffler, and sending two tailpipes out the back of the car.

That does give the impression of performance. You see a car with two tailpipes, and subconsciously, you say to yourself, "Wow, that car must have a lot of power." But all it really has is five extra feet of tailpipe.

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