# Secondary Cat Delete 2.0 TFSI



## TALBOTL (Aug 30, 2017)

Hi Everybody

Just a quick question, I have seen bits and bobs about what pipes to get mainly for the RS having done a search. 
I was just looking for a straight answer on whether the EML would be triggered if the secondary cat is deleted?

I know that it will for the main cat, but I'm unsure whether the lambda sensor(s) are, I'm looking at exhaust options so if deleting it will cause EML. I will probably just stick to the centre section delete.

Thank you

LT


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## Barr_end (Oct 19, 2016)

"secondary" cat on the TFSI downpipe is the main 800 (or so) cell cat under the car. Deleting this (the correct way with a full new downpipe) will cause EML and without mapping it out will cause poor running.
However if you want to release a few extra HP and flow, removing the precat on the TFSI is a good mod if you're not looking to replace the downpipe with a sports cat. Takes out the smaller cat right at the turbo allowing better flow, some small gains in spool and response and few extra HP. Can be down for 'free' if you're willing to do it yourself if you remove the downpipe and hammer the cat yourself.
Got a spare TFSI downpipe sat in the garden with the precat bashed out :roll:


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## TALBOTL (Aug 30, 2017)

Barr_end said:


> "secondary" cat on the TFSI downpipe is the main 800 (or so) cell cat under the car. Deleting this (the correct way with a full new downpipe) will cause EML and without mapping it out will cause poor running.
> However if you want to release a few extra HP and flow, removing the precat on the TFSI is a good mod if you're not looking to replace the downpipe with a sports cat. Takes out the smaller cat right at the turbo allowing better flow, some small gains in spool and response and few extra HP. Can be down for 'free' if you're willing to do it yourself if you remove the downpipe and hammer the cat yourself.
> Got a spare TFSI downpipe sat in the garden with the precat bashed out :roll:


Thanks for that Barr End, great advice there and the process you mention sounds exactly like what I want in fairness.

I'll have to check pictures to make sure I delete the right one, I'm guessing it's the one visible from the engine bay so to speak as opposed to directly under the car near the centre section? I'm only looking for a slight noise gain and the extra benefit of the pre cat removal might give me a few little burbles I'm guessing. I spent big on a well known exhaust system for my cupra and it was shocking so now I just think you may as well go to a trusted exhaust shop.

Will the precat also throw up the EML? if I do as you suggest and get the downpipe whipped off before hollowing it out?

Cheers

LT


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## repsol (May 1, 2015)

Removing the secondary cat i would think this would trigger a fault code simply because its not getting a reading on idle. If you replace it with a sports cat this too will throw up a fault, however it will not run rough as its only monitoring at idle.
The main lambda sensor that does all the work is the 1st one directly after the turbo.
To over come this you need to fit a lambda spacer ideally with a built in cat. What triggers the fault is the gas flow is speeded up now the exhaust gas is less restricted. This make the ecu thinks theres a fault. If you need a lambda spacer i have a brand new one with built in cat. This fits onto the secondary cat not the 1st.


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## Barr_end (Oct 19, 2016)

repsol said:


> Removing the secondary cat i would think this would trigger a fault code simply because its not getting a reading on idle. If you replace it with a sports cat this too will throw up a fault, however it will not run rough as its only monitoring at idle.
> The main lambda sensor that does all the work is the 1st one directly after the turbo.
> To over come this you need to fit a lambda spacer ideally with a built in cat. What triggers the fault is the gas flow is speeded up now the exhaust gas is less restricted. This make the ecu thinks theres a fault. If you need a lambda spacer i have a brand new one with built in cat. This fits onto the secondary cat not the 1st.


Wrong unfortunately on the sport cat, it will not bring up an EML light.
Lambda spacers are for the secondary lambda sensor. The first sensor measure everything you need to know, I.e Air/Fuel mixture is correct, but the second is the bastard that causes lights on the dash if you go full decat.
However I could be interested in just that lambda spacer, depending on brand and cost...


TALBOTL said:


> Thanks for that Barr End, great advice there and the process you mention sounds exactly like what I want in fairness.
> 
> I'll have to check pictures to make sure I delete the right one, I'm guessing it's the one visible from the engine bay so to speak as opposed to directly under the car near the centre section? I'm only looking for a slight noise gain and the extra benefit of the pre cat removal might give me a few little burbles I'm guessing. I spent big on a well known exhaust system for my cupra and it was shocking so now I just think you may as well go to a trusted exhaust shop.
> Will the precat also throw up the EML? if I do as you suggest and get the downpipe whipped off before hollowing it out?
> ...


Removal of the precat at the top of the downpipe won't cause EML either as the main cat under the car is still present and before the secondary lambda sensor.
Downpipe has to come off to remove it and make sure that you do so with the turbo opening on the downpipe as the lowest point facing downwards and point the entire downpipe downward whilst doing so. You don't want any dust/chunks failing into the main cat and melting and blocking it when you reinstall and it gets warm.
Depending on where you're located could always purchase the one in the garden I have spare and save yourself the work/hassle haha. However I can get pictures of this if it would help you understand?


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## TALBOTL (Aug 30, 2017)

That makes perfect sense to me, in that case it is the one visible from the engine bay. I'll have to check what the access is like it looks quite easy to get to but in practice it probably won't be.

What's the noise difference like with it removed?

I am based in Nottingham, what about you? As it might be an option to grab yours!

Cheers

LT


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## Barr_end (Oct 19, 2016)

TALBOTL said:


> That makes perfect sense to me, in that case it is the one visible from the engine bay. I'll have to check what the access is like it looks quite easy to get to but in practice it probably won't be.
> What's the noise difference like with it removed?
> I am based in Nottingham, what about you? As it might be an option to grab yours!
> Cheers
> LT


That's the one man, if you're fwd not Quattro, its piss easy to do, had the downpipe off, cat smashed out and back on the misses in around an hour (although I am qualified, not that it means a lot :lol: )
Only a slight increase in noise, but its a deeper note that more noticeable, well on a milltek catback which was already deeper.
Ahh, just a little journey, down in Deal, Kent! So might be best if I grab you some pictures instead?


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## Vanu (Oct 2, 2016)

so, in short - gutting the first cat is without any negative consiquences? No smell of petrol on start? No bad noise or vibrations? No lambda errors or anything?


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## Gaz78 (Mar 15, 2017)

Vanu said:


> so, in short - gutting the first cat is without any negative consiquences? No smell of petrol on start? No bad noise or vibrations? No lambda errors or anything?


I'm Interested in this too for my TTS. Reason I havnt gone for de-cat/sport cat was cost and potential mot issues, but hadn't realised there was a "pre-cat" next to turbo!

So as Vanu asks......no issues??? Easy enough to do on a TTS??

Gaz


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## Barr_end (Oct 19, 2016)

Vanu said:


> so, in short - gutting the first cat is without any negative consiquences? No smell of petrol on start? No bad noise or vibrations? No lambda errors or anything?


Exactly that, no issues or differences to with the pre cat in terms of negative.


Gaz78 said:


> I'm Interested in this too for my TTS. Reason I havnt gone for de-cat/sport cat was cost and potential mot issues, but hadn't realised there was a "pre-cat" next to turbo!
> So as Vanu asks......no issues??? Easy enough to do on a TTS??
> Gaz


As above nothing
But on a TTS, no, precat removal on a TTS is a ballache. Purely down to the haldex/prop. Far less room and a lot more effort involved. moving the prop across/or taking it off completely.
I'd save the money and go sportcat downpipe to make it worth pulling the old downpipe off. Even with the precat gutted you still have the biggest/worst restriction in the exhaust, which is the bend directly after the precat, inner diameter tapers down to something shockingly small (read 1.5-1.75 inches). Aftermarket downpipe is 3" all way through if you buy a decent brand, if not can even get 3.5 decat ones for TFSI.
Will never have an mot issue with a sportscat though.


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## arron99 (Mar 2, 2019)

I know its bumping an old post but it has relevant information.

How much of the exhaust do I need to remove to get the pre cat off and hammered?


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## Barr_end (Oct 19, 2016)

arron99 said:


> I know its bumping an old post but it has relevant information.
> 
> How much of the exhaust do I need to remove to get the pre cat off and hammered?


Full exhaust..

You can just disconnect the midpipe/backbox from the downpipe and then unbolt this from the turbo and remove.
BUT if you remove the full exhaust or the mid pipe, you allow yourself alot more room to pull the downpipe out, especially if doing this DIY on a set of stands rather than a ramp.


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## arron99 (Mar 2, 2019)

Nice one. Makes sense.

Such a big restriction to have so close to the turbo.


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## CurryMan (Nov 20, 2016)

This looks like a very interesting mod. Would it have an affect come MOT with emissions etc doing this? I have the FWD 2.0 BWA setup.


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## Romani44 (Jun 14, 2018)

Bit of a revive here...

Does anyone know how much a pre-cat removal would cost?

I don't think I can manage this myself.


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## Romani44 (Jun 14, 2018)

£120 all in from R-Tech if anyone is interested.


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## ab54666 (Nov 18, 2019)

The Scorpion downpipe has two slots for the sensors. One up by the Turbo and one after the cat.

Getting mine fitted in the week, wonder if I should get a new gasket for mating to the Turbo?


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## Flashy (Nov 8, 2019)

So you can buy a 3 inch down pipe to replace the section containing the pre cat and narrow piping? Who sells this?


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## ashn33 (Oct 21, 2021)

really good quality ebay straight pipe sounds good itam152817531844
STAINLESS EXHAUST 3" DE CAT DECAT DOWNPIPE FOR AUDI A3 8P TT 2.0 TFSI ROADSTER | eBay


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## Rufflesj (Jun 22, 2020)

1st post and, like the pipe, he's straight in there BOOM


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## Pittsy (Aug 19, 2021)

ashn33 said:


> really good quality ebay straight pipe sounds good itam152817531844
> STAINLESS EXHAUST 3" DE CAT DECAT DOWNPIPE FOR AUDI A3 8P TT 2.0 TFSI ROADSTER | eBay


I guess fitting this might give you issues come MOT time?

I am a bit confused when it comes to the Cats and MOT's.


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## FNChaos (Nov 30, 2016)

Pittsy said:


> I guess fitting this might give you issues come MOT time?
> I am a bit confused when it comes to the Cats and MOT's.


A catalytic converter uses a mix of precious metals (palladium, rhodium and platinum) as a catalyst (a substance that causes a chemical reaction to happen more quickly) to convert noxious exhaust byproduct like nitrogen oxide into safer byproducts like nitrogen and oxygen or carbon monoxide into carbon dioxide. The metals are typically embedded into a ceramic 'honeycomb' matrix that your exhaust gases flow through.

Since vehicle emissions are tightly regulated, messing with component that increase pollution will get you in trouble (illegal here in the US or as a minimum, MOT failure in the UK)

On a highly tuned turbo motor, removing the pre-cat or primary cat and replacing it with a downpipe will help the turbo spool up faster. Adding a free-flowing 'sports cat' downstream from the downpipe is a compromise that will allow you to (maybe?) pass MOT testing while still enjoying the performance benefits gained from a downpipe.

Removing a secondary cat is pointless from a performance point of view. Some do it to increase the noise your exhaust makes, but the increase in noise also means more pollution (and possible trouble).

If you do choose to remove your cat(s), don't throw them away or get rid of them. If a MOT failure requires re-installation and you no longer have them, you will find out why precious metals are called '_Precious_”...


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## Pittsy (Aug 19, 2021)

FNChaos said:


> A catalytic converter uses a mix of precious metals (palladium, rhodium and platinum) as a catalyst (a substance that causes a chemical reaction to happen more quickly) to convert noxious exhaust byproduct like nitrogen oxide into safer byproducts like nitrogen and oxygen or carbon monoxide into carbon dioxide. The metals are typically embedded into a ceramic 'honeycomb' matrix that your exhaust gases flow through.
> 
> Since vehicle emissions are tightly regulated, messing with component that increase pollution will get you in trouble (illegal here in the US or as a minimum, MOT failure in the UK)
> 
> ...


Thank you for the detailed reply, good information.

So looks like my best bet for a hassle free (come mot time) solution is to get a down pipe with a sport cat?


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## FNChaos (Nov 30, 2016)

Pittsy said:


> So looks like my best bet for a hassle free (come mot time) solution is to get a down pipe with a sport cat?


Assuming your motor is turbocharged, yes.
Aside from running good tires, a downpipe / sport cat / Stage2 tune is probably your next best performance upgrade.










Daylight showing through a sport cat


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