How can you inexpensively make a truck sound like it has more power than it does

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You want to start with at least a 2" system. At the motor, you want a set of headers, followed by high-flow Cats. If you car is pre-emission, then just go with dual exhaust and glasspacks; they get louder as they get older. if you have anything stock, forget exhaust sound. At the end, put a tip that is slightly larger than your exhaust tubing, such as 2" tubing to a 3" to 3 1/2" tip, it will give the exhaust sort of an echo that makes it a lot deeper.

Or you can just buy a tip to go on the tail pipe. It's cheap and will not mess up your exhaust system.

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