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: : So hopefully this post will save me some time :)
My opinion is 3" downpipes are too big.
Take a good look at greddy TDO5 downpipes ....They start out less then 2" and slowly get larger as they come down.
THE HOTTER THE EXAUST THE FASTER IT EXITS!
The exaust coming out of the turbo wheel is very turbulant,
: : If you through it into a large 3" pipe it stays turbulant and stagnates slowing the exaust down. If you keep the pipe smaller the exaust stays hotter and is forced out. (Better boost response)
Most of the turbo wheels used in Z32s are smaller then 1 3/4" and the internal wastegate holes 3/4" or less. The casting opening is an odd shape and looks big because of alot of unused area.
A 3" pipe is alot bigger then the whole opening of the exaust housing on Z32 turbo....It is so big that the people that make the 3" downpipes cover the whole bottom of the flange with pipe and they put a dowel going through the center of the pipe (on the turbo wheel side) for the bolt to go through causing even more turbulance.
A 2 1/2 inch pipe makes the most since , It keeps the exaust hotter, It is more in sync with the size of the turbo wheel and wastegate, you can interchange stock cats for smogtests and 2 1/2" testpipes for power, They are the same size as most of the cat-back systems on the market and they fit great.
Any Downpipe will way outperform the stock precats (even gutted ones) They are HORRIBLE! The bends in them are only part of the problem. The casting of the precat where the gasket sealing area is actualy blocks part of the exaust flow right at the turbo wheels! : : Hope that helps! Greg
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