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Old 07-15-2009, 08:16 PM   #6
sd_boxster
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Join Date: Oct 2008
Location: San Diego
Posts: 434
Every exhaust has a resonant frequency. When the engine RPM matches the resonant frequency of the exhaust system, you get a huge increase in amplitude at that RPM range, which is what we hear as drone.

One of the reasons the piping diameter in factory exhausts is so small (in addition to all of the conspiracy theories about Porsche not wanting their low-end cars to compete with the 911) is to ensure the resonant frequency of the exhaust system falls outside the normal RPM range of the motor.

As you increase the piping diameter, remove baffles, and otherwise do things to improve airflow, you risk changing the resonant frequency of the exhaust to one that you will hit frequently while driving.
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