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As they say, it all depends
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 8 Feb 2005 08:41:17 Share Post by Email
In Reply to: too little back pressure is a bad ?,, Tino, Tue, 8 Feb 2005 07:40:43
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If engines were simple beasts that ran at one operating point, life would be much easier. But it isn't.

First, if a car is running with an overly-restrictive exhaust, a bigger one may help. The exhaust may be restrictive because the manufacturer decided to keep the diameter small for cost reasons, or to fit it under the car, or to contain noise. There's a lot going on other than pure performance, or a Saturn would run like a Corvette.

Next, the engine is an air pump. It's the total flow from intake to exhaust tip that counts, and over the range of operation. And it's the transient operation - acceleration - that gets the most attention. The physics of intake flow, valves, combustion, and exhaust are wildly complex. Really. So exhaust are tuned for certain operating conditions. Yes, there is even exhaust tuning to improve extraction from the cylinders, based on standing waves set up in the exhaust.

As an example, one very good way to ruin the performance of a two-stroke is to put a less restrictive exhaust on it. Saab went to very great lengths to tune the exhaust for ideal extraction. Put a less restrictive exhaust on, and the exhaust stays in the cylinder.

With a turbo engine, the turbo acts as a restriction, so the downstream exhaust is less of an issue, but still there. A lot depends on the spool-up time constant of the turbo, exhaust flow, valve timing, and intake restriction. A bunch of pretty bright engineers study this, and have lots of engines to play with.

As a very general rule, unless the exhaust was overly restrictive due to packaging, cost, or noise, adding a free-er flow exhaust isn't always a great idea. It may very well improve performance at one or two operating conditions, or certain RPM. But in general, where it gains at one point it loses at another. That adversely impacts overall driveability. You end up with a 'peaky' engine. Lots of power in a smaller powerband. For racing, that can be good. For street use, not so good.

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