Two of the most important factors for maximizing engine performance are fuel and air. Getting more of both to your 1999 to 2004 Mustang's engine is the best way to get the kinds of extra horsepower and torque it is truly capable of making. Some of our best-selling upgrades for the 1999 to 2004 V6 Mustangs are cold air intakes combined with a larger throttle body. Your next best bet is to pick up an upgraded cat-back exhaust kit.