Nothing screams old school muscle like a side exhaust. The ultra-cool look also comes with considerable horsepower gains. The design eliminates bends in the exhaust system piping, thus improving horsepower numbers. On average, the side exhausts are expected to add 8 to 10 horsepower to your Ford Mustang. The exhaust note will also be changed; it will sound throatier than ever. The growl will scare your neighbors and onlookers.