hey guys, i just got done putting my fd back together with a fresh motor swap.
I stumbled over a very intresting theory.

i am running the borg warner s366 with the .91 a/r housing. right after the turbo thers is about 2 inches of 4'' pipe and then cuts down to 3'' pipe all the way out to the muffler!

So How much am i being restriced? i am planning on running 22lbs of boost and i feel like thats going to be a huge holdback.

 

Running that much boost you would probably see a significant power increase by switching to a completely 4" exhaust.

That said, a 3" exhaust is probably not all that bad. A gradual step down might actually help with exhaust velocity compared to a 4" setup.

 

its not really gradual though. there is about 2 inches long right off the v band and then it just bridge welded down to 3 inch. so its like a instant restriction hit.

 

Can you take a picture?

 

yes when i get home from work around six i will take a picture

 

Turbine exducer is 3 inches, right? If you donīt want to redo whole exhaust setup, you would be better with 3 inch system right from the turbine. As it is now, its over expanding exhaust gases which isnīt good for flow and abrupt reduction back to 3 inch pipe only worsens it.

Only way how to determine if your exhaust system is sufficient for given flow ie power level, is to measure it - post turbine pressure. You just simply canīt say that x inch system is good for xxx HP and in no way you can say that upgrading from x size to y will make certain increase.

Mostly it depends on how picky your engine configuration is. Main factors are porting and turbine expansion ratio. And of course flow rate on its own.