I can give you some
help here.
I have a 3" decat downpipe custom made. I butchered my old downpipe to use the flange because they are like gold dust. I wish i hadnt now because i had to make my own out of 10mm steel, to fit my old downpipe back for warrenty purposes and mot.
Moral of the story, keep original parts!!!!
Good points:
1. Engine temps have improved massivley, engine seems smoother and mpg are
increased by 2mpg
2. Peak power feels very strong
Bad points:
1. Due to the lack of backpreassure, although not confirmed it is suspected that overspool happens at full throttle when using kickdown or during a 1/4mile drag race in 3rd gear. This causes a spike in torque and dsg to change gear on its own. In the case of the kickdown you can feel the cars power and im assuming throttle being feathered for a second before the acceleration resumes at the expected rate.
This could be solved with a map designed for a decat. revo stage 2 although advertised as suitable for decat, is not. It was only tested on a milltek system which has the 200cel cat.
2. I have been getting the odd puff of white smoke after coasting to a junction and re-applying the gas. I believe this is again due to the way the turbo bearing seals have been set up. Because of the lack of back preassure, some oil is pushed passed the seals on occasion. Only noticed on short journeys of 3miles or less. probably because the seals havnt expanded due to operating temp not been reached yet.
My recommendations:
1. Keep your existing downpipe. Get the custom exhaust place make a bolt on bolt off flange under the car where your milltek cat back starts. You can then jack the car up come warrenty time and quickly unbolt your custom unit and replace with the original.
2. The best exhaust for a turbo car is no exhaust, the standard downpipe is something like 2.3". I would go for 3" or your gain in performance will be minimal and its the size of the downpipe that gives the gain in performance.
3. To overcome my problems I would get the 200cell cat with the 3". That would give the back preassure that im lacking and would solve the problems I have had.
4. The cat needs to be as far away from the turbo as possible, ie just before the start of the cat back milltek section. That will give better performance. Supersprint have their cat under the car. If you look at the boost pipes, they almost touch the current cat on the car so you are getting heat transfer from the cat to the boost pipes. You want max temp in the exhaust for faster gas flow and minimum boost temp because the colder the air, the more dense it is which equates more power.
I paid 270 for my decat downpipe. Before it is fitted, give them some high temp paint and exhaust wrap made of titanium and it will be better insulated. It will speed up the flow and decrease engine bay temps and therefore decrease intake temps.
