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An engine is really a big air pump with mechanical bits that turn that air flow into mechanical energy. Because of noise and emmission requirements, the exhuast system is usually less than ideal for optimal air flow and usually one of the biggest obsticles in flowing more air, which in turn provides more power. For example, most aftermarket systems do not have the second set of catalytic converters, a requirement for Porsche. Eliminating them makes the gasses flow more easily through the system which lets the engine pump more air, make more power.
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