The part is used to bypass the catalyst from the exhaust to create a 'Decat' system. Eliminating the catalyst enhances gas flow and performance, adding significant horsepower by removing the restriction caused by the catalyst. This product is intended for motorsport use only and requires a Stage 2 ECU remap. - Bypasses the catalyst for a 'Decat' exhaust system - Enhances gas flow and increases performance - Motorsport use only; requires Stage 2 ECU remap Specifications: - Pipe Diameter: 63.50 - Race Product: Yes Kit comes with: - MSAU321REP: LH Downpipe and Catalyst Replacement Pipe - MSAU321REP - MSAU322REP: RH Downpipe and Catalyst Replacement Pipe - MSAU322REP Vehicle Compatibility: - 2006-2008 Audi RS4 B7 4.2 V8 Saloon, Avant and Cabriolet - V1 & ECE Systems