The APR Race Midpipes with Catalysts are both 2.5" and are designed as a simple plug and play upgrade. We use T304 stainless steel piping that's brushed, mandrel bent, and TIG welded to the included catalyst and CNC machined ultra-flat OEM-style 3-bolt-flanges. The system includes new gaskets, bolts and copper plated nuts. The system includes two proprietary GESI high-flow catalyst, which are ideal for forced induction applications. They feature a fully captured substrate brick, which is furnace brazed to the outer mantle and physically captured by the diffuser cones. This helps to prevent rattling, rotation and damage to the substrate, especially on higher horsepower applications. The APR Race Midpipes with Catalysts are compatible with the factory DP and our Race DP. Race Midpipes (catted or non-catted) are required for Stage 2 along with our Race DP.