Akrapovic Slip-On Line (Titanium) for Porsche Carrera /S/4/4S/GTS (991)SO Slip OnIt's a challenge to develop an aftermarket exhaust for the great 911. The Akrapovi engineers have prepared a superb system with a dual nature. By using the optional Akrapovic sound kit, the driver can fully control the exhaust valves, which are controlled by the cars electronics if no optional part is used. When the valves are closed, the 911 sounds deeper than stock, but not louder. When the valves are open, expect a completely opposite,