DIY DeCat

**jimnielsen** · 14-01-08, 03:01

I did a quick "decat" job on the Q4 today. I just used a 90 degree mandrel bend in place of the "precat" and welded a couple of flanges onto 2" exhaust tube to take the place of the cats. Take a look inside the precat in the pic.... its a wonder anything gets through!

I also saved another 5kg with this exhaust mod.. That makes 154Kg (thats 339 lbs) in total that I have removed so far (yes I'm weighing everything I remove...)

JimN~

**wrinx** · 14-01-08, 07:00

Originally Posted by jimnielsen

I also saved another 5kg with this exhaust mod..

I'd be more interested in the power gain...

wrinx

**AlfaJack** · 14-01-08, 08:35

wow that is restrictive!

**pavs** · 14-01-08, 11:24

Do you have the measurements of the pipe and the flange/mating pieces?

**Steve Webb** · 14-01-08, 18:14

Originally Posted by pavs

Do you have the measurements of the pipe and the flange/mating pieces?

Flange ,

one of my favorite words.

On a serious note Jim, I'd ditch the middle 2 silencers as well to save some more kg's . There won't be much increase in noise. What are the dB limits on the tracks over there?

**AlfaJack** · 14-01-08, 19:31

Originally Posted by Steve Webb

Flange ,

one of my favorite words.

On a serious note Jim, I'd ditch the middle 2 silencers as well to save some more kg's . There won't be much increase in noise. What are the dB limits on the tracks over there?

I would definately do this. The standard setup is wayy too complex!

**jimnielsen** · 14-01-08, 19:43

..Pavs, the pipe inside diameters that I used are 60mm for the bend near the turbo and 50mm for the cat replacement. The flanges are just weld on bits they are 52mm dia. so that the 50 tube can go inside it.

Steve, the other two 'silencers' were already gone when I got the car - see pics - it just has a single quite large silencer at the very rear. The noise limits here are 75db for "sprints" and 95db for races

**Evodelta** · 15-01-08, 12:20

There are some good power gains to be had from de-catting, the downside is that there is no back pressure for the turbo anymore, it has relied on this for years to hold it together so it can and quite often does 'spit the dummy out'.
Mine expired in quite spectacular style when its inards collapsed and I left a plume of white smoke covering both lanes of the road for 500 metres.

Still, it was a good excuse to bolt on something a little bigger in its place......

**Steve Webb** · 15-01-08, 12:49

I'll second that, after mine was originally de-catted, whenever you lifted off the throttle it spewed a huge smoke screen. Not good when you would stop at a junction and be engulfed by the smoke screen a couple of seconds later.

**jimnielsen** · 15-01-08, 19:45

um, so how did you fix that problem Steve??

JimN~

**Steve Webb** · 15-01-08, 20:56

Originally Posted by jimnielsen

um, so how did you fix that problem Steve??

JimN~

New Turbo, as in the punchline to the famous joke, it had blown as seal. (due to the reduced back pressure of the exhaust)

**pavs** · 17-01-08, 12:44

Thanks Jim

**pavs** · 13-02-08, 20:46

Jim

how long were the decat pipes I was going to order up some pipe and make some myself

**jimnielsen** · 14-02-08, 00:19

I used 400mm long tubes as that was the length of the original cat from flange to tip. You can just cut off the flanges on the cats and weld them onto the tube if you want so save a few dollars!

jimn~

**pavs** · 14-02-08, 06:51

Thanks again Jim

**pitucas** · 17-02-08, 22:48

If i'll remove the cats then due to the reduced back pressure of the exhaust what can happen?

Turbo blows?

Did'nt understand (sorry my poor english).

**jimnielsen** · 18-02-08, 06:08

I removed mine as above - then took it to the track and 50km flat out with out any dramas!

JimN~

**Steve Webb** · 18-02-08, 06:49

If your turbo is already in pretty poor shape, the lack of back pressure can allow oil to pass by the bearing seals and enter the exhaust.

Oil from turbo + very hot exhaust = lots and lots of smoke!

Thats speaking from experience !!

**pavs** · 18-02-08, 07:08

Made the pipes up this weekend but the cars 450 miles away so won't get them fitted for acouple of weeks yet.

But the kit I bought to make them up had 4 flanges and I only needed 2 so if anybody else wants to make up some decat pipes