The part is used to bypass the catalyst from the exhaust to create a decat system. Eliminating the catalyst enhances gas flow and performance by removing the restriction caused by the catalyst, resulting in a significant increase in horsepower. This product is intended for motorsport use only and requires a Stage 2 ECU remap. - Bypasses the catalyst for improved exhaust flow - Designed for motorsport applications - Requires Stage 2 ECU remap Specifications: - Pipe Diameter: 63.50 - Race Product: Yes Kit comes with: - MSAU233REP: LH Downpipe and Catalyst Replacement Pipe - MSAU233REP - MSAU234REP: RH Downpipe and Catalyst Replacement Pipe - MSAU234REP Vehicle Compatibility: - 2003-2005 Audi S4 4.2 V8 quattro B6 Saloon/Sedan, Avant, Cabriolet - 2004-2009 Audi S4 4.2 V8 quattro B7 Saloon/Sedan, Avant, Cabriolet