3" or 2.5" downpipe?

I'm not an expert on turbo exhausts, more knowledgeable on NA, I've the 1.3. Yeah 3 would be too big for the NA, but I think it might be for a little engine like the Copen turbo. Although they say big bore exhaust is good for turbos. I think because its post turbo you just want to gases out of there. You are not tuning it for laminar flow and scavenging quite like NA can as the turbo will block that effect back to the cylinder. Pre-turbo I think is a different matter as you want gas flow at some optimum speed to spool the snail. To fast and probably give you too much back pressure, too slow flow/low pressure and the turbo might not move much. The 0.7 only emits so much gas volume, putting a drainpipe on it and the gases might wander around like in a big mansion.

her is what I have grabbed...

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

So 2.5 inch DP is not needed. go for 2 inch.

 

How can the manufacturer do the bend then and he can't? does he just use his hands? Can he not do a long piece and cut it shorter? Get some of that aluminium ducting!
I think its not very low because its an ickle 0.7! Now if you had the big boy 1.3

 

thats probably the area it makes the difference! to get that high pressure/velocity gas out the turbo. Back it up means its static so the turbo doesn't spin. Or it goes through the wastegate and that means no boost. Allow the post turbo pipe to accept all the gas the turbo is throwing at it.
Big 9 inch tail pipes do sweet fanny adams. Once its passed the silencer there is little to gain or lose in comparison.

 

although twin tail pipes looses hp

 

I had mine in at an exhaust place the other day and they said they wouldn't even bother with making a downpipe as the price would be comical due to the flange on it having to be custom made so they're just gonna smash all the cat out instead then do me a 2" system with no silencers. Already has no silencers in but the guy who made it was possibly the worst welder in the world and used crush bends D:

 

I have made a 3" system on my VX 220 turbo using a Garrett 28/67 that is pushing out in excess of 400 BHP and 375 lbs of torque with no problems. 2.5" systems on the same cars are only a restriction approaching 300 BHP. I would think a 2" system would be plenty large enough.

Dave

 

http://www.davehardwick.co.uk/photo-albums/vx220-albums/black-vx-220-turbo/

Hopefully the link above may work.
Gives you some idea of the exhaust I made.

Dave.

 

For those wanting to modify the exhaust just stick to a 2" down pipe, cat back.
It is easily fitted and re welded to the original flange by a capable exhaust shop. Remove the middle muffler in the process and run the pipe all the way to the back muffler and weld it there.
Leave all the rest as is. Makes no difference with the slightly smaller diameter pipes and distance to the rear.
Super cheap mod to gain more power.
You will find the exhaust note will change after its run a couple of times and I felt more power after 100kms or so.
If its a bit loud for you, just add a small silencer.
Seems the ECU needs time to adjust to the better exhaust air flow.
Make sure they put the O2 sensor in the exact same place/distance it was before.