Fits S4 with or WITHOUT Sport Differential. System contains a merge X-Pipe for increased performance and improved sound. Features polished T-304 stainless steel tips. Borla(r) Cat-Back(tm) & Axle-Back exhaust systems feature patented straight-through and multi-core technology designed to unleash hidden horsepower. Increased exhaust velocity adds power driving excitement fuel economy and the distinctive Borla(r) Sound of Power respected by motoring enthusiasts everywhere. Each system is built from premium T-304 stainless steel superior to T-400 series knockoffs (such as 409) to give you the absolute best in performance and durability. Ultra-smooth mandrel bends ensure maximum flow & power and precision computer numerical control (CNC) manufacturing ensures an accurate fit. Best of all Borla(r) stands behind every system with a Million-Mile Warranty. Just what you'd expect from the world's most experienced (and most winning) exhaust brand.