Flowmaster vs. MAC vs. Magnaflow
 

Which is the best out of these 3 exhaust for horsepower gain and sound?

Flowmaster= great sound, drone(2 chamber), and head ache
Mac= decent sound, rust, and moderate drone
Magnaflow= low quiet rumble, quality, and no drone or head ache
That is my experience.
As far as HP, I don't think it matters unless your motor is pushing 400+ HP. Anything is better than stock.
Just FYI

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1998 Mustang Gt
C&L 80mm
Steeda Underdrives
Steeda Tri-Ax
Magnaflows