Can anyone explain why back-pressure would be a good thing??? NA or Turbo. I would think that pipe diameters are totally valid in designing the header (my thought being smaller creates higher velocity airflow resulting in momentum so that when the Exhaust valve closes, it builds a vacuum drawing the exhaust out when it opens next.) But restrictive higher up. But I would think that past the header, a 30' exhaust would be way more gooder.

When I went from stock exhaust to 3" pipe on my N/A Ka24e, I definitely noticed a bump in high end HP, I was able to rev was faster when I floored it on the highway. On the other hand, the car was 10X louder which isn't that bad if you don't mind it. There was little loss in low end torque but its not that bad and I also noticed there was a drop in mileage. I definitely recommend it to who ever who's not sure if they should go for 3"!

Id also agree by having your exhaust made for you..

Back pressure is not a good thing on any exhaust despite it being NA or Forced induction. This backpressure theory has been thrown out the window.

You need an exhaust with the right diamter to maintain a high velocity stream which in term has the scavenging effect on the exhaust. It helps "suck" out the left over exhaust gas from the combustion chamber.

An exhaust too large will not maintain a high exhaust velocity and the scavenge effect will be lost, and too small of an exhaust will have too much back pressure which in term kinda has the same results.

Choosing an exhaust isnt exactly easy, dont compromise, get what you want, and do it right once.

See but I think that scavenging is only valid like up to and including the collector, maybe a little beyond that as well. After that, no exhaust is better.

Well I would say the Exhaust manifold/header would have the biggest effect on it, but the rest of the exhaust will play a significant role too.

According to magnaflows site, some OEM mufflers can create up to 18PSI of backpressure. I would say thats enough to rob some significant power! I Do agree with you though

yeah for sure, obviously we wouldnt want to put too small of an exhaust on the car, but yeah I think we are clear here, but just to make sure, what I am saying is that there is no such thing as too big of an exhaust (in terms of perofrmace deterioration) past the collector area. At this point all the individual runners come together (depending on the header) to aid in scavenging and increasing performance. I would think that not far past the collector, any back pressure is un wanted or should I say exhaust velocity is no longer an issue. I dont know for sure, I would love it if someone was really good with exhaust theory could come tell me the exact dealeo on it.

The reason i would not go 3' if i were u is noise. I have a hytech header with straight 3 inch exhaust. It is just brutal loud. If i could make the power i want out of a 2.5 i would have stayed 2.5. but if you have your heart set on a 3 inch then do it up