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Do-It-Yourself High Performance Car Mods: Rule the Streets by Matt Cramer

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Exhaust Modification

Pushing the exhaust out of the engine requires power. Removing restriction from the exhaust can reduce the power needed, leaving more power to turn the wheels. Many heavily modified cars have each and every one of the parts of the exhaust system replaced in a quest for more horsepower. Improving the exhaust will decrease back pressure, the pressure of the exhaust where it leaves the engine.

Sometimes a less restrictive exhaust can flow too well, but only for low RPM. This usually occurs on engines tuned to run at very high RPM. In this case, at low RPM, the gas flowing out the exhaust valve can actually suck the incoming air and fuel out the exhaust valve at low speeds. As the engine speed increases, this no longer happens ...

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