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TDI exhaust options, de-cat or not?


jon_273
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I just want to weigh up all the options as my full exhaust system is ready for replacement. As it stands as the moment the flexi looks like its starting to split, the exit pipe from the cat is severely corroded (already had to get it welded for MOT), the exit pipe from the centre box has holes in it, and there is no backbox as it had split.

Since removing the backbox I've noticed a bit of an annoying resonant sound when cruising at about 60mph, but I think this is from the heatshield, since the exhaust now finishes just under the back seats rather that at the back of the car. And also my rear wheel keeps getting covered in soot!

I've been quoted £160 for a new flexi and decat section in stainless, to which I could add a new OEM centre and rear section. Alternatively I could get the flexi replaced and cat rewelded (again), and get a new centre box and straight rear pipe.

Now I know Al used to run his with no backbox, I'm just wondering if anyone has any experience of a decat on a 1.4 TDI and is it worth it?

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Decat, then rest custom, straight through rear section?

Custom centre section would cost same as OEM? £160 for flexi and decat sounds alot?

Al's must of had a rear section of some type?

Just idea's as I know nothing about TDI's :lol:

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Since removing the backbox I've noticed a bit of an annoying resonant sound when cruising at about 60mph, but I think this is from the heatshield, since the exhaust now finishes just under the back seats rather that at the back of the car. And also my rear wheel keeps getting covered in soot!

I've been quoted £160 for a new flexi and decat section in stainless, to which I could add a new OEM centre and rear section. Alternatively I could get the flexi replaced and cat rewelded (again), and get a new centre box and straight rear pipe.

Now I know Al used to run his with no backbox, I'm just wondering if anyone has any experience of a decat on a 1.4 TDI and is it worth it?

I didn't as such run it that way and wouldn't ever recommend just driving around with the backbox removed. I'll get some pics up a little later on but my system consisted of a custom made straight through rear section which got rid of the rear silencer and replaced it with a straight pipe from the centre silencer to the rear bumper.

De-cat's on TDI's are completely fine. Although the turbo on the AMF engine is only tiny and spools good anyway, de-catting will help to reduce back pressure on the compressor wheel and will help to make the spool up quicker.

In terms of MOT, a TDI will almost never fail an MOT because of a De-cat. I know recently some decisions have been made regarding the removal of the DPF on the very late cars which were DPF equipped as standard. But for anything pre 2009, there is no legal obligation to have a cat fitted to a diesel at all. Nor will removing the cat increase the emissions by any amount.

Custom centre section would cost same as OEM? £160 for flexi and decat sounds alot?

A downpipe with a flexi section custom made for £160 is good for the money! :)

It's around £165 normally for an off the shelf milltek downpipe for a Golf 1.9TDI!

Yeah, pretty sure Al had a full length system, but with no backbox, if you know what I mean. A straight through.

It was quite loud though, from what I remember.

:P Wasn't that loud Phil!

Mine had the standard Cat (wish I'd de-catted it), a centre silencer (5" round about 12" long) and a straight through rear section.

Video clip here:

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Seem to remember it being loud coming over Keilder Resevoir! In a good way though. :)

LOL I have a picture from that day too:

DSC00016.jpg

Here's some pics of the exhaust to give a clearer impression of what it was.

PICT0177.jpg

PICT0175.jpg

Edited by nfbr
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This is quiet a good thread as I have been wondering what to do exhaust wise on my Tdi Lupo when it comes to that part of the build.... now I know :)

now I just need to get cracking with the rebuild!

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Thankyou for all the replies, I have decided now I'm going to go for the custom flexi and decat. Then while I'm there I'll ask them to make me up a straight pipe in place of the rear silencer. And if I do find it to be too noisy then I can always get a rear silencer fitted.

Phil, a straight through would probably sound quite something on a 3 pot but this is my economical motorway workhorse too and it's got to have a bit of refinement :lol:

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Boring! :lol:

Where are you going to get the pipe made up? I'm tempted to get my centre box removed and replaced with a straight pipe.

I'm gonna make a start removing my exhaust now, you might have to come and rescue me from the mess I've made soon. :lol:

Edited by Philplop
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Exhaust-A-Fix in Darlington is where I got the quite from. Just not sure I trust them with my car. They very nearly bent the sills that I have just had repaired by jacking in the wrong place. We had a little argument but after some swearing on the guy's behalf he did as I asked :coffee:

Funny you should mention a straight through pipe though, because I have a stainless one sat in my garage.. And I'll try not to bend your sills either...

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Thankyou for all the replies, I have decided now I'm going to go for the custom flexi and decat. Then while I'm there I'll ask them to make me up a straight pipe in place of the rear silencer. And if I do find it to be too noisy then I can always get a rear silencer fitted.

Should be good Jon! :)

Edited by nfbr
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  • 3 weeks later...

Well I've got it all done now. The decat has made it miles louder compared to just having no backbox, especially when you start it up. I would much prefer it if it was quieter. But it definitely feels quicker!

I've already been through the Tyne tunnel with it and it sounds like nothing I've heard before.

It pulls like a train, even more so than it did before. The only problem now is because the engine moves so much it's making the exhaust catch on full throttle at 2200rpm, and then again at 3200rpm when the second wave of torque kicks in. Think I'll go back and see if they can adjust it upwards. The real solution would be a stiffer rear engine mount, but nobody makes one.

Is this for the Exhaust its self? or the fitment?

If you dont mind me asking how much is this costing you jon?

They do everything. It was £160 just for fabricating and fitting just the front downpipe/decat section. They quote for the exact work depending on the car so I had to take it down for a quote.

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Richard did in not make a difference to the noise at all? But you noticed a little improvement in performance? I might put a standard backbox on if that's the case, we shall see.

Is your decat mild steel?

In the end I got a new stainless flexi and decat section welded to the standard turbo flange. Then a new OEM VW centre silencer, and a custom mild steel rear pipe without silencer.

I hope it's not the stainless that is causing it. The Raceland manifold (stainless) I used to have on my GTi made a lot of noise :coffee:

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