These BBK direct replacement 1999-04 Mustang 4.6L X-pipes feature 2.5″ diameter CNC mandrel bent aluminized tubing 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 for weekend track or race use. 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. Please note: BBK or Any of its Distributors – CANNOT SELL THIS PART TO ANY END USER WITHIN CALIFORNIA. THIS PART CANNOT BE SOLD OR INSTALLED ON ANY POLLUTION CONTROLLED VEHICLES IN CALIFORNIA. OFF ROAD pipes do not have any catalytic converters and are manufactured and represented ONLY for the purpose of OFF ROAD-RACE USE ONLY. In some cases will require a tuner to switch off rear O2 sensors.