You won't see a significant horsepower gain, maybe 1-5 hp at most, from just adding a Flowmaster 44 muffler to a 2006 Dodge Charger. Exhaust modifications like this primarily affect *exhaust flow* which can *indirectly* lead to small power gains at certain RPM ranges, but the effect is usually minimal and easily overshadowed by other factors. The advertised gains from aftermarket exhaust parts are often optimistic.
Any perceived power increase will more likely be due to the change in exhaust note (louder, more aggressive sound) which can *feel* like more power, even if there's only a slight or negligible actual increase on a dynamometer. For substantial horsepower increases, you'd need more extensive modifications like a performance intake, tune, headers, and possibly other components.
