You ask to see their dyno charts and compare them
and ask if you get your money back if you don't achieve their claimed gains...because you won't. I know a fellow who tried over a dozen mufflers on his car looking for one his shop could recommend. He has his own dyno so it was a very controlled experiment. He was really surprised at how many of the mufflers actually lost HP, torque and mileage. He also took the car with each muffler out on the road to see if there were real world problems or improvements. Most of the mufflers had an unbearable drone.
Be very careful of the claims made.
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