Can you elaborate on this? The Garret guy has very specific reasonings and explanations behind his statements, can you offer any explanation behind how backpressure helps spool the turbo, or improves torque, or improves engine breathing?

 

So, if I'm under 400hp, and already have 2.5" exhaust....what kind of downpipe should I get?

 

As an engineer, I have to agree with the Garret guy. Everything he said is legit. Turbulent "fuild" flow, gases are considered fluids, is a completely experimental field. There are no laws, or rules or formulas for turbulent flow. Everything we know is from experimental studies. Garret knows their business.

 

When he says "avoid step changes in diameter" referring to other things you can do to minimize exhaust backpressure, does he mean going from large to small? Or does he mean small to large stepping also. I thought that stepping from small to large ,in effect, creates anti reversion chambers . I ask, because I am buying a hks dp (2.76") and want the exhaust to "step up" to a 3" test pipe and cat-back. I was hoping to get the best low end torque, (lord knows our cars have none) and still keep the top end flow going by using this set-up.

Any comments? I like this thread

 

...to quote myself on this "back pressure bull****"... any way you slice it, back pressure is a bad thing in this application. A 2.5" downpipe doesn't perform "better" (if at all) than a 3" on a lightly modded Evo due to backpressure... it actually produces less back pressure due to the increased velocity of the exhaust pulse.