It's different for every car, but typically expected gains are in the average range of 10%.Wide Open Performance
Replacing the muffler/exhaust with a MagnaFlow will help your engine increase power by being able to expel exhaust gases quicker. This creates less workload for the engine, makes it run more efficient, and results in better power.
Dyno's - Trucks & SUV's
Dyno's - American Muscle
Dyno's - Sport Compact