FREE Shipping! The Flowmaster American Thunder Cat-back for the 1999-04 Mustang GT and Mach1 will make give your Mustang the classic muscle car sound that everyone wants! It produces an aggressive, deep rumble that will take you to 60's muscle car heaven. This cat-back is louder than the factory exhaust system but still adequate for everyday driving. The Flowmaster Cat-back has the sound you want at a great price!