exhaust talk. i hear repeatedly that the exhaust will only flow as good as it does at the weakest point. so its pointless to have a 3 inch downpipe to a 2.5 inch cat back or visa-versa. well the smaller 48 AR turbo housing outlets are no greater than 2 1/4 im guessing from remembering looking at the size of mine. so wouldnt it be pointless to run 3 inch turbo back exhaust if the outlet is only 2 1/4 or is there another factor? im just curious
running rich? turn the boost up
The bottleneck of a 3 inch to a 2.5 inch is pointless because at that point you might as well just have 2.5 inch exhaust...The other way around is beneficial although not as good a 3 all the way. Think of a stepped header. As for the 2 1/4 outlet on the turbo I still think a larger exhaust is beneficial. Look at it this way....more exhaust is being forced out of the turbo because more air is forced into the engine. once it gets into the pipes you need room for the compressed gases to expand. Also to prove my point think about how much faster a turbo spools up with no exhaust or just a short dump tube compared to full exhaust.
Big to small = Small all the way is < small to big < big all the way.
Imagine cars are the exhaust and road as pipes. If a highway funels into a 1 lane street, traffic has to slow down to merge into the one lane, eventually backing up and only traveling as fast as the cars on the one lane road. One lane all the way and traffic will maintain speed, but because of the dense traffic the cars aren't driving very fast. If a one lane road opens up to a 3 lane road, traffic can spread out and speed up, but it takes them a while to get there. I think the last onee is self explanatory.
D.Simoes wrote:Big to small = Small all the way is < small to big < big all the way.
Imagine cars are the exhaust and road as pipes. If a highway funels into a 1 lane street, traffic has to slow down to merge into the one lane, eventually backing up and only traveling as fast as the cars on the one lane road. One lane all the way and traffic will maintain speed, but because of the dense traffic the cars aren't driving very fast. If a one lane road opens up to a 3 lane road, traffic can spread out and speed up, but it takes them a while to get there. I think the last onee is self explanatory.
^^^ this.
"Vroom."
"Yo I wouldn't race him, there's not even an interior in his car man."
"Vrooooooom PSSSSHHH!"
"Oh sh*t. F*ck that, dude..."
thanks i get it now. but how much would it really help if your turbo outlet is 2.25 and you have a whole 3 inch exhaust versus a 2.5. is there much of a difference?
running rich? turn the boost up
depending on power level no. I made 277 on 2.5 inch, and I will be keeping it for a while on my new LE61T motor which I have goals for 400, but it is going to be a step by step process. IMO bigger is always better BUT 2.5 inch will suit you fine for everyday driving and a great overall powerband
LE61T PTE6262 Powered 
Just go 3inch if you haven't bought an exhaust yet, it will always make more power, and unless you are worried about losing like 1hp under 2000rpm you won't notice any negative effects vs. a 2.5inch.
With turbo cars you want the free-est flowing exhaust you can get, I vote run a cut-out right off your downpipe. Buy an electric cut-out plate (so you can open it with the flick of a switch) and it will be cheaper than probably buying an exhaust.
It will also make more power than even a 3inch exhaust, plus its louder too!
