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Supersprint claims a 7 HP gain with their 'Street' muffler for the 3.5. If you can feel a 7 HP gain good for you. AR Design in Denver wants to develop a 3 inch down pipe, I assume similar to the 335i's. He claims 20 HP gain with catalytic converters and 30 HP gain with them removed. I pulled the catalytic converters off a 328i and got a 14-18 HP gain. Car was dynoed before and after.
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