American Thunder™ 409 SS Cat-Back Exhaust System with Single Rear/Side Exit by Flowmaster®. Exhaust System Type: Cat-Back. Exhaust Tubing Material: 409 SS. Main Tubing Diameter: 3". Exhaust System Sound: Moderate. American Thunder gives you the classic AGGRESSIVE or MODERATE sound (depends on application and exhaust type) of a Flowmaster chambered exhaust, which is the classic sound of classic American big performance engines. This is a system that gives the most sedate vehicle a performance rumble. American Thunder is perfect for a wide range of cars, from ponies to sleepers.