Valved Stainless Steel/Titanium Exhaust Cat-Back For BMW F80 M3 F82 M4 S55. This exhaust system is constructed from high-quality stainless steel or titanium and features a valve that can be opened or closed at the push of a button. The valve allows you to open up the exhaust for maximum sound output when desired while keeping it quiet when not needed. This cat-back system provides an aggressive sound with improved performance over stock systems, making it perfect for those looking to get more out of their BMW F80 M3/F82 M4 S55 engine. Sound clips on youtube https://www.youtube.com/watch?v=S7jmKwMK7xs