Akrapovic Slip-On Line (Titanium) for Porsche 911 Carrera /S/4/4S/GTS (991) 2015Akrapovic Slip On Line (Titanium) for Porsche 911 Carrera S 4 4S GTS (991) 2015 It'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