Hi-Power Series 304 SS Cat-Back Exhaust System with Single Rear Exit (31006-BM002) by HKS. Exhaust Type: Cat-Back. Exhaust Tubing Material: 304 SS. Main Tubing Diameter: 2.95″. Exhaust System Sound: Aggressive / Loud. Professionally developed with a racer in mind, this HKS exhaust system provides an easy opportunity to upgrade both the horsepower and sound of your precious vehicle. Made from first class materials, it will serve you for years to come.