DYNO Day Figures :D

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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.

Well, you've got a few generalisations there, and you've contradicted your previous post - there aren't too many exhausts with 65mm tubing You are correct (in your latest post), that most systems have 60mm piping, but probably not as prevalent as you'd think. Doing some quick research of 86 exhaust systems for the S2000 - of the exhausts with reported sizes - 37 are 60mm, and 23 are 70mm. There are 3 of 63mm size.

Sure, 70mm exhausts tend to be louder than the smaller ones, but this doesn't mean that they were designed for the track, and vice versa. A lot of the manufacturers make exhausts in both 60mm and 70mm tubing intended for either or both street and track use. Note, that these Japanese manufacturers tend to design these exhausts for their largest markets - Japan and North America - where respective local laws usually differ significantly from those in Australia, and may be less strict. A common theme for N/A cars, however, is that single exhausts tend to dyno/produce more power, making them more attractive for race tracks where noise restrictions aren't in place. Sure, we all know that a lot of mods make no or less power to the F20C, but FWIW, after having the intake and exhaust installed a few years ago, I dyno'ed significant gains in power and torque over stock, across the entire rev range - there was no loss in mid-range and big gains past 6K.

But seriously, if you want to make the most power-per-buck, you need to go F/I.

Some of you might find this thread useful from S2Ki, which as usual is a great source of information for our car.

S2Ki Official Exhaust Thread