iv been considering a 3" dp/header combo to a 2.5 " exhaust..now my friend with a tsi sais go 2.5" dp/header combo with 3" exhaust becasue you want the back pressure by the head not far away..does this sound right or wouldnt you want the less pressure right off the head then in the catback some back pressure......(maybe there the same i dont know)...this is not a 2.5" vs 3" thread either so no flamming for not searching...

 

I agree with you.

IMO the 3 inch at the header/dp will allow the exhaust to exit away from the head quickly.... and then the reduction to 2.5 will increase exhaust velocity helping to extract the gasses quickly.

Think of it like how a wind tunnel works.


(this is my opinion)

 

seriously, i would NEVER take modding advice from whoeve rtold you that. you dno't want backpressure, backpressure = bad. if anything, start off bigger then go to smaller. not the other way around

 

yah thats what i thought..why was i listening to a tsi guy..lol no that thing is quick though...

 

There are two sides to the exhaust equation. Backpressure is BAD, but is a byproduct of exhaust design.

Large diameter reduces backpressure (adds power) and reduces exhaust velocity (reduces power)

Smaller diameter increases backpressure (reduces power) and increases exhaust velocity (adds power)

There is a good median here and it seems to be 2.5". Going from a 3" downpipe to a 2.5" catback if simply bolted together will reduce performance as the transition will add turbulance and unsettle flow.

Also the farther away you get from the head the less backpressure has an effect. Consider that the GMPP Exhaust starts at 2.5" at the downpipe but narrows to 2.25" right at the muffler. Yet the GMPP exhaust gains a dyno proven 12whp/10wtq and no other cat back has yet to prove a greater gain.

 

all good points. I agree.

 

I have GMP Manifold, goes into my ZZP 3" downpipe with highflow cat, reduces to 2.5" then into the GMP exhaust. I like myset up and I see nothing wrong with it. After 300whp+ I would change my GMP exhaust to a custom 2.75" or so. Good luck.

 

Why is it that everytime I read a header/exhaust quesion i get even more confused??

 

it depends on your mods and personal goals. it always does. There is no absolute solution to any modding path in the car world unless you know what is wanted as a result, and what there is to begin with.

 

Actually, your friend is correct to a point.

Ever heard of step-tube race headers? Some concept.

As exhaust moves away from the engine it begins to cool, slow down, and require a small tube to increase velocity. But that's only one example.

Another is to end the restriction at a specific point, such as jumping from a 2.5" pipe to a 3" pipe. Yes, the exhaust will lose velocity, but there comes a point where the reduction in restriction outweighs the need for velocity.