These BBK direct replacement 1999-04 Mustang 4.6L GT or Cobra X-pipes feature high-flow catalytic converters and BBK's unique die stamped performance Xtracter center X section to ensure maximum flow and horsepower. OEM original O2 sensor locations and factory style die-stamped flanges ensure a perfect fit and BBK In-house dyno-testing has shown solid gains of 15-20 horsepower depending on application. BBK full length X pipes are designed as direct bolt on to factory headers or any aftermarket shorty headers as well as, of course, BBK shorty headers. BBK Performance X pipes have been the number one performance brand Ford Mustang enthusiasts have enjoyed for years. Each BBK X pipe offers performance, quality and value that guarantees years of top performance for your Ford Mustang.