You won't lose hp but you won't gain much either. The side exit exhaust is pretty restrictive in that it has some major bends to get the pipping back from the rear axle area to the side of the car. It's just it's design. Ask Stefan about his. He has side exit exhaust on his. Also you have to be careful about speed bumps and other clearance issues because that is a big problem with the side exhaust. If you don't mind that I say go for it. You won't lose hp. But you won't gain much, if any, either.
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1996 Mustang GTS <---Must sell. $10,900 obo 56,000 miles