UrS4 exhaust
 

Can anyone tell me what functionality/reason Audi split the exhaust into 2 separate streams just after the dp? I would venture this was done for acoustic/flow reasons... but am unsure. If you have any insight into this, please let me know! I haven't been able to find anything else out about this. I guess a 3" system would replace both quite nicely?

Erik

RE: UrS4 exhaust
 

Assuming your talking about the 1.8t....
In short, flow is increased which thus increases hp at higher rpm's. A 3" cat back is the max you can go without compromising back pressure. There are a couple of guys I know that had success with 3" systems.

It's a delicate balancing act between opening up the back end for extra hp versus keeping the torque up with necessary backpressure.

RE: UrS4 exhaust
 

You are bringing a lot of things to my attention that I hadn't thought about before. Can you explain the whole philosophy behind back pressure for me?

RE: UrS4 exhaust
 

a good exhaust has the perfect balance between 2 things.

while im new to the turbo scene, i dont see why it would be any different:

the 2 variables you have to balance are 1)backpressure and 2)exhaust gas velocity.

basically what ive gathered, is you want ZERO backpressure...

i understand you guys think you need some backpressure to keep low end tq, but thats not the case.

what you do when you do a huge exhaust (>3") is you eliminate alot of backpressure (good) but you lose almost all of your exhaust gas velocity, basically your exhaust gasses are spinning around inside the pipe (almost as if there were turbulance) causing more restriction than your dinky stock pipes. this is where you lose your low end torque, not from the "missing" backpressure.

what you look for in an exhaust is 1) the lowest amount of restriction (backpressure) while raising your exhaust gas velocity.

i hate to contradict what Imolas4 said, but this is what ive learned from my reading throughout the years.

RE: UrS4 exhaust
 

Legal reasons :)

RE: UrS4 exhaust
 

you realize that if you cut off the exhaust system after the headers 1.) your engine/ car would be filthy 2.) it would come in through your air condition and ruin the inside of your car and 3.) it would be louder then a dump truck