Quote Originally Posted by 9large View Post
Not sure what you meant by this - what did you mean? So are you saying exhausts need to be 65 or 70mm in order to work well, with or without engine mods? What are the sizes of non-Japanese exhausts? The sizes of good exhausts that generate power are related to their respective abilities to optimise exhaust scavenging, based on desired engine characteristics. Some brands just copy. N/A and F/I usually have different requirements.
What i meant was that most Japanese tuners offer an exhaust which will improve engine response, sound, and still remain legal(not always) and are a bolt on fit and are designed for a standard F20C. These are generally 60 to 65mm. 70mm systems are generally designed for track only use as nearly none are quiet enough to comply with any epa laws, and most do not have a cat option.
Obviously a 70mm system without a cat will flow better than a 60-65mm system with a cat. And it will produce more top end power, but you lose some throughout midrange power. IMO 70mm is useless unless your constantly up it. Sure, exhaust flow generally equals power, but back pressure also plays a part in N/A tuning. I'm not saying the 70mm are shit. I think there not intended for street use.