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November 2001
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BuildYourOwnExhaust
Text by:
Mike Kojima
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Let's say you don't own a Civic, Integra, Eclipse or some sort of popular sport compact car. Maybe you have an old-school Celica or a Cavalier, Neon, Hyundai, Geo, Saturn or similar ride that doesn't have tons of aftermarket support. Maybe you want a spiffy, polished stainless system but can't get a pre-made offering you like. Maybe you have a popular car, but it has a ripping turbo system and no one makes a good pre-made, 3-inch system for your car. What to do? Well, you could always roll your own system. And if you do it right, it will be as good as, maybe even better than a commercially produced system. It will probably be cheaper, too.
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First, we'll explain how the different parts of an exhaust work so you can choose the best pieces, then show you how you can build a high-performance system with perhaps some help from your local muffler shop or a friendly welder. No matter how small a town you live in, you should still be able to get this stuff done.
THE MUFFLER
The key part of your exhaust system is the muffler. The muffler is the can at the end of your exhaust whose main purpose in life is to make the exhaust noise quiet. To be the whisper-quiet device most car owners demand, a typical stock muffler must have an intricate, labyrinthine flow path to help slow and cool the hot, vibrating exhaust gas. It contains baffles that cause the exhaust flow to reverse direction and intermix. These are great for reducing noise but are not so great for flow. The twists and turns the exhaust must endure in a stock muffler are restrictions that cause excess backpressure. You can run in a straight line faster than you can run in a tight, fun-house maze, right? The same goes for your exhaust gas.
To produce the most power, an exhaust should have minimal restriction on the exhaust flow. Restriction hampers the burned exhaust gases from exiting your engine, causing some charge dilution with the incoming fresh fuel air mixture. If all the exhaust gas cannot escape from your cylinders, it dilutes the flammable power-producing intake mixture that is trying to come in. The diluted mixture does not burn as well as a pure mixture.
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This causes a loss of power. You don't feel so energetic at a packed club with lots of cigarette smoke, sweaty bodies and hot stuffy air right? Neither does your engine.
With greater restriction, backpressure is generated, making the engine work harder to pump the exhaust out of the cylinders. That work could be used to turn the wheels instead.
BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
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