Im looking at the HKS High Power exhaust which is 3.5" on a V6 STi, but i see alot of you guys have 3". Why 3" and why not go for the 3.5"?

__________________
STi Version VI

Too much back pressure, and hot gas from your turbo/manifold don't flow too well

this is the piping, not the tip i'm refering to

are you talking the tip or the piping turbo back?

I don't know of anybody with bigger than 3" turbo back here, there was some big discussion about this on nasioc a while back with some people trying 4" etc. It seems there is a point where it can get too big...

__________________
Mazda CX-7
Mitsubishi Evo X MR

3.5" piping is an unnecessary expense as opposed to the 3", and all it does is make the car louder, with no real benefit to performance.

If you're talking about the exhaust tip, then plenty of people on here have bigger than that!

unless your making more than 400hp then 3" turbo back is just fine. Anything more will just add noise and drone and it may even cause a loss in low end torque.

__________________
RS Liberty - Because WRX's are only good for parts...

People are making 600HP on 3" exhausts, so I dont think anyone will ever require anything bigger.

As for low end torque, its not an issue on a turbo car. There are plenty of people make good power on open exhausts and side pipes on their turbo cars.

Yeah, i was refering to the piping, and this is from catback all the way to just before the tip, not turboback. Also having a 3" (mayb xforce) down pipe (i think thats the one from turbo to cat ) n 3.5" catback...will that effect the flow?

__________________
STi Version VI

Going the wrong way really..... the aim is to keep the exhaust gas flowing as fast as possible. As you increase diameter from 3" to 3.5" the exhaust gas will expand and cool, most probably slowing the overall gas velocity. Thats why you often see systems with a 3" dump mated to a 2.5" cat back system. Apparently gives good low end torque and 2.5" is still enough to flow at higher rpm. Bigger pipes make noise but hey, no resonator and a cannon and its all good

No the goal is to minimise back pressure at the turbine to allow the turbine to spin faster and earlier. Yes you increase velocity by decreasing diameter but it is at the cost of pressure loss, which equals higher back pressure at the turbine. You don't get something for nothing in the laws of energy

Also low end torque is at the cost of high end horsepower, which in reality makes the majority difference in the acceleration of your car. Low end torque is just for driveability. (Don't go confusing the two things though, hp is a result of torque force in a time frame)

As others have said it actually comes down to economic reasoning, which is even true in a large chemical plant (which I work on). The larger the diameter the better but cost skyrockets. So look at what hp application you need it for and go from there in choosing a happy medium.

If the exhaust you are looking at is a bargain 2nd hand purchase then go for it.

I thought it was diff back 2.5" gives you best results

normally dump and mid sec is 3"

__________________
M POWER