Kooks Custom Headers - 2011 Mustang GT - 5.0L 4V Catted X-Pipe 2011 Ford Mustang 5.0L Catted X-Pipe. Unit is 3" x 2 3/4" with an OEM Style Outlet has Off-Road Front Pipes. System is made out of 304 Aircraft Quality Stainless Steel. This X-Pipe is designed to be used with either 1 3/4" x 3" or 1 7/8" x 3" Kooks Longutbe Headers.