Billy Boat Performance Exhaust's B6 S4 cat back exhaust is precision crafted using 100% T304 stainless steel. CNC mandrel bent 2.25" throughout the entire system along with B&B's proprietary Purge Resonance Technology (PRT) design mufflers this system offers a smooth uninterrupted exhaust flow for increases in both horsepower and torque along with a deep, rich exhaust note. The PRT design is simple in design yet unique in theory so you can have a deep sporty note on hard acceleration but still a mild tone while cruising without the unwanted resonance. With their Twin 3.5" round double wall tips the B&B system will give the S4 a bold yet distinctive look. The S4 exhaust is back with B&B's limited LIFETIME warranty for worry free enjoyment.