If its a dual exhaust "tip", that just bolts onto the end of the tailpipe, nothing will happen. If you put a real dual exhaust in the car with 2 pipes, yes that will be a bad thing. You will lose backpressure and torque. Basically, a mod where you would think it would give you more horsepower, it actually gives you less. Go get a high performance muffler or catback exhaust system.
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