A 3" downpipe is sufficient. The downpipe doesn't need to be any larger, as it gives little extra effect and will scrape on the road too easily. The downpipe removes BOTH cats and results in huge horsepower increases of 45-55rwhp. You must be careful, if you have an intake and exhaust, as the car may hit 15psi and invoke the fuel cut limiter which is designed to shut down the engine to protect against overboost. We have had problems with this at about 6300rpm. In the event the car does hit fuel cut, the ECU needs to be reset and a GReddy Boost Cut Controller installed. Please note too, that if you have a OBD II vehicle, removing the cats will result in a constant-on check engine light, which has no effect.