The Invidia 03-06 Evolution 8 & 9 cat deletes are 3" tubular 304 stainless steel with thick flanges to prevent warping. The Invidia test pipes can be used with the stock downpipe and cat-back exhaust or with most aftermarket components. The 03-05 Evolution 8 test pipe has no bungs in it since the O2 sensor is located in the downpipe. The 06 Evolution 9 test pipe has an O2 bung to mimic the stock cat's location. Gaskets are included.