The CTS Turbo 02 pipe is designed for use with a T31 style T3 flanged turbine housing (T3 2.5" 4 bolt) and CTS Turbo tubular cast exhaust manifold for the VW/Audi 1.8T transverse 20V engine. Our 02 pipe accepts all T31 flanged turbos and 38mm TiAL or 39mm PTE external wastegate. When used in conjunction with the CTS Turbo manifold and a stock placement downpipe this will effectively act as an intermediary pipe to connect your current stock/aftermarket K03 turbo back exhaust to your new big turbo. The flange that bolts to the turbo is T31 (2.5"), however shortly after the connection the piping flares to 3" as can be seen in the product photos. This product allows for maximum flow, while requiring minimum installation effort.