De-cat on tdi's

sambarnes

Guest
Ive heard alot of things, both good and bad about de-cat on a tdi, but not sure what the facts are.

What would i gain if i went de-cat on standard exhaust?

Ive also heard that you wont fail the mot on emisions with it de-cat as they test something else. IS this right?

Sam
 

4937Liam

Active Member
Jan 12, 2010
374
0
Lakenheath, Suffolk.
Diesels have to pass a smoke test not an emmisions test.
Decating will create more black smoke (dont know about power gains) so im still sitting on the fence regarding this mod.

Liam.
 
Aug 1, 2005
2,695
0
Cullompton . Devon
Diesels have to pass a smoke test not an emmisions test.
Decating will create more black smoke (dont know about power gains) so im still sitting on the fence regarding this mod.

Liam.

If you de-cat a standard exhaust you will gain nothing to notice as the restriction is in the back box. You can de-cat a diesel car exhaust and it makes no difference at MOT time. Taking the cat off should make no difference to the smoke out put as a CAT filters the exhaust gasses nothing to do with removing carbon particles which is why it makes no difference to a UK MOT.
 

sambarnes

Guest
If you de-cat a standard exhaust you will gain nothing to notice as the restriction is in the back box. You can de-cat a diesel car exhaust and it makes no difference at MOT time. Taking the cat off should make no difference to the smoke out put as a CAT filters the exhaust gasses nothing to do with removing carbon particles which is why it makes no difference to a UK MOT.

Ahh right i see, so you would have to get a different back box to notice anything.

Ive heard that a de-cat on standard will make the turbo spool earlier, meaning more boost. So im guessing this is wrong too?

Cheers
 
Aug 1, 2005
2,695
0
Cullompton . Devon
The rear box is a lot more restrictive then the CAT. If you remove the CAT the exhaust gasses are not going to move any faster as the most restrictive part of the exhaust is fitted right at the end of the exhaust. Even with a decat and a free flow exhaust if your on a standard turbo you will not notice much difference due to the fact the tiny standard turbo doesn`t flow a large amount of air to cause a bir back pressure in the exhaust.
 

Husbandofstinky

Out from the Wilderness
Nov 8, 2007
1,516
12
Temperate Regions
Ive heard alot of things, both good and bad about de-cat on a tdi, but not sure what the facts are.

Tell me what the bad ones are please? (apart from cost).

And no you will never make any noticeable gains, but every little extra helps as they say.

I would only go there if you are having the car mapped with a tailored map (Stage 2 >)
 

adam cupra 20vt

Built Not Bought.
Mar 31, 2005
6,162
2
Mud Hut
I was thinking about getting a decat downpipe for my PD150......were saying its not worth it as the rear box is the restriction?

Seems a bit strange doesn't it? on a petrol the cat back makes no power gains but a decat makes a massive difference, how come its reversed on a diesel?
 

jonjay

50 Years of 911
Jun 27, 2005
5,843
1
Essex
I was thinking about getting a decat downpipe for my PD150......were saying its not worth it as the rear box is the restriction?

Seems a bit strange doesn't it? on a petrol the cat back makes no power gains but a decat makes a massive difference, how come its reversed on a diesel?
Diesels are much louder than petrols so I guess they baffle the rear silencer a lot more which restricts it. You should definitely go for a downpipe which decats the car...this does make a difference.
 

adam cupra 20vt

Built Not Bought.
Mar 31, 2005
6,162
2
Mud Hut
That makes a lot of sense about the noise.....I'm still learning about diesels :cartman:

Does the decat make the turbo spool up faster like on a petrol?
 

jonjay

50 Years of 911
Jun 27, 2005
5,843
1
Essex
That makes a lot of sense about the noise.....I'm still learning about diesels :cartman:

Does the decat make the turbo spool up faster like on a petrol?
I cant remember now but I believe it does slightly earlier. You will get a lot more torque over the range with a downpipe and hold the power better.
 

cupra-rich

Active Member
Dec 28, 2008
423
0
coventry
I cant remember now but I believe it does slightly earlier. You will get a lot more torque over the range with a downpipe and hold the power better.

Agree with jonjay.

Got THS one on mine, turbo spool up is earlier, well worth doing for£130ish:D
 

PAULTDI67

Active Member
Oct 21, 2009
307
0
sheffield s.yorkshire
Ive heard alot of things, both good and bad about de-cat on a tdi, but not sure what the facts are.

What would i gain if i went de-cat on standard exhaust?

Ive also heard that you wont fail the mot on emisions with it de-cat as they test something else. IS this right?

Sam
HI M8:D just had mine mot'd few days ago guy testin asked y i've not de-cat'd said he'd done it 2 wif's audi&it makes NO diffrence to mot aparently makes it more fuel eficient?didn't say power wise.so i'm still sat on fence 'bout this:confused:may look into when exhaust needs replacin as cant b bother'd doin things when dont HAVE 2
 
Aug 18, 2007
350
0
I'm gonna do this next week I think, If it doesn't seem any different then its no massive loss!
 

S3 AKR

livin' the dream!!!
Jun 30, 2004
1,453
1
Colchester, Essex
I had the cat back Milltek for about a year before doing the downpipe too. Once downpipe was done, the turbo spins up sooner, feels more responsive as a result, didn't notice more smoke, mine passed MOT with no issues at all and no major difference in noise inside the car. Do it.
 

seat sean

Active Member
Oct 20, 2008
77
0
Gt. Yarmouth Norfolk
Just had THS decat pipe, but also with a full non res Milltek system. It defo spools up quicker and it just seems like the car just breaths a lot better if that makes sense! It makes the car more free revving and better to drive in my opinion, but it does leave more soot on the tailpipes and rear of the car though!
 
Feb 22, 2009
3,618
0
South Wales
get yourself one of these

Image1164.jpg
 

sambarnes

Guest
get yourself one of these

Image1164.jpg

Haha i have one of them on the side of my tractor!

Just got mine de-cat and have a cherry bomb silencer to replace the back box, sounds LUSH! Nice deep throaty noise, and you can hear the turbo winding down when you let of the throttle.

And i have also noticed, especially after de-cat, that it does rev alot more freely!
 
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