BBK Performance mid X pipes are specifically designed to be used with BBK full length headers to complete the installation to the cat back exhaust system. All BBK mid X-pipes feature BBK's exclusive die stamped x center section and 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. They are available with catalytic converters or as an off road version. Please Note Off road versions are for weekend race use and not EPA street legal! Each BBK mid X pipe offers performance, quality and value that guarantees years of top performance for your Ford Mustang. Please note: *ALL - OFF ROAD pipes do not have any catalytic converters and are manufactured and represented ONLY for the purpose of OFF ROAD-RACE USE ONLY. *These High Performance pipes are NOT EPA Street Legal in any state. *Part Number 1636 is made from 2-3/4" Tubing for built engines and high performance applications