I was just wondering why most japanese downpipes are only 2.75in(70mm) in diameter compared to most U.S. downpipes that are 3in? Some people say that it may not produce a lot of hp but it help increase torque.

Does anyone really have a real answer for this?

 

The Japanese are not stupid , if they were then the 4G63Ts and Evo Lancers would not be as good as they are would they ?
If I sad to you that 2.75 is 11/12s of 3.00 would you feel so short changed ?
Mitsy got 276 quiet emissions legal horsepower out of a two litre (approx 121 Cu In) four banger which I think is pretty good for a registerable production car . They did it with a metrified 2 1/4" pipe from dump to rear bumper .
Maybe you should do some cross sectional area calcs fror 2 1/4 , 2 1/2 , 2 3/4 and 3" tube .
Many Japanese aftermarket exhausts are not 3" all the way and as far as I'm concerned whats the point having them if they don't work as a 3" exhaust .
Corky amongst others said that exhaust gasses don't like to be slowed down and sped up along the system .
I think a lot of people would be happy if they had and exhaust that worked well and didn't destroy their hearing . The deal breaker would be realising that it might not be 3" all the way - because everyone knows even a turbocharged lawn mower HAS to have a 3" exhaust .
If anything the worst place to neck down an exhaust is at the front where the gases temperature and velocity is , or should be , highest . Yet some name brand systems are 2 1/2 past the cat and grow to 2 3/4 or 3" from the middle to the back . The bit out the back may look big but if its fed off a garden hose whats the big bit achieved - SFA ?
The advantage of 2 3/4" systems would be slightly higher gas speeds which probably helps the mufflers work better and the size makes it easier to fit around things .

 

japanese care about all around power a 2.75 has a little more restriction resulting in better boost response and less lag, the obv trade-off is less max hp.

 

excellent response--^^^ yep that 1/4 of an inch split 2 ways, so roughly an 1/8 inch on each side or 3mm dia difference side to side.. nough said here.

 

More restriction is NOT going to improve boost response. Got that a little backwards.

 

2 3/4" or approx 70mm is hard to find here in Oz and I think the reason is that people like to have a BIG looking exhaust and 2.75 as a number isn't as big as 3 . Actually from aftermarket stuff I've seen here on JDM imported cars the closest tube they make to 3" is 80mm . 3" tube fits neatly inside it .

A .

 

The reason why I was wondering about this is because I've seen most aftermarket downpipes from Japan is about 70mm(2.75in). For example: Tomei, Fujitsubo, Monster-sport, Tanabe, Mine's, etc. have 70mm downpipes then opens up to 76mm(3 in) after the cat.