# De-Cat for TTS



## Jasonw10 (Jan 29, 2017)

I've had the resonator delete for couple of weeks now with the standard exhaust for the TTS (2016) but I''m just looking for a more aggressive tone and "fart" with the S-Tronic.

I've been looking at installing a 3" milltek downpipe and decat. The only issue I've got with is - with most cars the engine management light will come on due to being de-catted (so I'm guessing it will be the same for the TTS) and I don't fancy seeing it on all the time.

So I'm wondering if anyone on here have installed and by-passed it with the likes of a MIL eliminator? If so, which one has worked?

The only one I've managed to find online is this: http://www.magnumtuning.com/en/detail/e ... or/audi/tt but it doesn't state if it is for the 8S TTS.

Any advice will be appreciated


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## noname (Aug 20, 2015)

With a decat you'll get the light but if you already have deleted the resonator, a 200 cell would be perfect for the sound and to avoid the light.
Consider that I have milltek 200, resonator and TT exhaust and and sound it quite more than the original and farts very loud.

Secondly,but I think most important, with a decat and without resonator, I think you have to push the car to start!!
You'll loose a lot of reactivity at low revs and turbo lag will increase
Anyway, if you want to go for a decat, an emulator or a stage 2 is required


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## Jasonw10 (Jan 29, 2017)

@ManuTT where did you get one from?  have you installed a downpipe with it or kept the standard one? any sound clips?


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## noname (Aug 20, 2015)

I have the milltek 200 cell and posted a thread..
the only sound comparison was the cold start but trust me don't buy a decay for reasons above!

viewtopic.php?f=98&t=1446154&hilit=milltek


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## Jasonw10 (Jan 29, 2017)

ManuTT said:


> I have the milltek 200 cell and posted a thread..
> the only sound comparison was the cold start but trust me don't buy a decay for reasons above!
> 
> viewtopic.php?f=98&t=1446154&hilit=milltek


Hey I've just looked at your thread! Great info thanks very much. However I'm wondering if the part number for milltek SSXAU605 would work also?


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## noname (Aug 20, 2015)

That is the race cat, not my sport car. you'll probably get the light and don't pass the exhausts check with it


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## Jasonw10 (Jan 29, 2017)

@ManuTT yeah that's why I asked for your opinion because that race cat was 200 cell (same as your sports cat) so was wondering if it would worked as it's £200 cheaper!


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## noname (Aug 20, 2015)

I'd suggest to spend a bit more for the sport cat, it's an hjs, very good (and will last more than the race cat) and you won't have problem with the light and gas exhaust check
The sound will be 95% the same so why risk?!


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## Jasonw10 (Jan 29, 2017)

ManuTT said:


> I'd suggest to spend a bit more for the sport cat, it's an hjs, very good (and will last more than the race cat) and you won't have problem with the light and gas exhaust check
> The sound will be 95% the same so why risk?!


Ok perfect! Have you mapped your TTS after getting it?


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## noname (Aug 20, 2015)

I've installed before the stage 2 so for few months I had only stage 1 and downpipe


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## Jasonw10 (Jan 29, 2017)

Thanks for the help


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## fahdriyami (Sep 14, 2016)

And here I was thinking you probably wanted to know how to get cat fur out of the seats.


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## BauhauTTS (Jan 8, 2017)

fahdriyami said:


> And here I was thinking you probably wanted to know how to get cat fur out of the seats.


May want to check the Jaguar forum for that.


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## James cole (Sep 3, 2007)

ManuTT said:


> With a decat you'll get the light but if you already have deleted the resonator, a 200 cell would be perfect for the sound and to avoid the light.
> Consider that I have milltek 200, resonator and TT exhaust and and sound it quite more than the original and farts very loud.
> 
> Secondly,but I think most important, with a decat and without resonator, I think you have to push the car to start!!
> ...


Why wiill it have more turbo lag? arent decats suppose to help the turbo spool quicker?


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## noname (Aug 20, 2015)

Turbo line, from air intake to exhaust works in pressure, so the cat, but also central silencer and the exhaust, are projected to keep a pressure inside the exhaust line otherwise when you press the pedal, the turbo will overspin.
Try to blow in a tube, then try to blow in the same tube with a web or something to block a bit the flow.
The air that goes out is the same but you make more effort to do the same operation, you're the turbo!
Problem is an empty tube will have no pressure or at least very close to nothing


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## brittan (May 18, 2007)

Hmmmm, that's not my understanding of basic exhaust system requirements.

On a naturally aspirated engine the exhaust system can be designed to create a negative pressure at the exhaust valve to aid scavenging during the exhaust stroke. That means getting the exhaust gas velocity correct which means getting the exhaust manifold and system sizes correct. Raising the gas velocity means making the exhaust tubes smaller which also raises back pressure. Hence there is a balance to be struck, a happy medium.

On a turbo engine, and ignoring anti-lag systems, the exhaust manifold needs to deliver high speed exhaust gas to the turbo to get it spinning fast, quickly. When you put your foot down you want boost, you want the turbo to spin up, and fast.

That means the the manifold needs to be designed such that the tube sizes are small enough to give high exhaust gas velocity, but large enough not to strangle the turbo at higher power states: again there's that compromise thing. There's no scavenging effect from the exhaust system on a turbo engine since the fuel/air mix is delivered under pressure.

On the downstream side of the turbo the ideal is just a straight pipe of increasing diameter. Often the first part of that increasing diameter is the "bellmouth" on the exhaust outlet of the turbo. Cats, resonators, silencers and even the pipe bends all add unwanted restriction. 
Of course that is also compromised by the requirements for road legal cars to be fitted with exactly those devices that restrict exhaust flow!


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## noname (Aug 20, 2015)

If you have the turbo back decat, gas will goes out from the turbo too much fast and it will increase the turbo lag since the turbo is empty at the moment you press the pedal again..it's not because you need that gas to have power, but because the engine need all the line, air intake to exhaust, in pressure.
Consider that just switch from a 70mm to a 76mm makes a loss of pressure, imagine a straight tube!
Try it and let me know if it's odd for you..


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## brittan (May 18, 2007)

ManuTT said:


> If you have the turbo back decat, gas will goes out from the turbo too much fast and it will increase the turbo lag since the turbo is empty at the moment you press the pedal again..it's not because you need that gas to have power, but because the engine need all the line, air intake to exhaust, in pressure.
> Consider that just switch from a 70mm to a 76mm makes a loss of pressure, imagine a straight tube!
> Try it and let me know if it's odd for you..


I don't need to try it but what you've said is certainly odd.

Maybe it's because you are typing in a non-native language and I fail to understand your points properly?

Anyway, a decat turbo back exhaust will tend to decrease turbo lag. The exhaust turbine within the turbo creates a pressure drop across it. The lower the back pressure on the downstream side of the turbo, the higher that pressure drop will be and that will make the turbo spin up faster so less lag.

A turbo engine will work perfectly well with no exhaust system at all.


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## James cole (Sep 3, 2007)

ManuTT said:


> If you have the turbo back decat, *gas will goes out from the turbo too much fast* and it will increase the turbo lag since the turbo is empty at the moment you press the pedal again..it's not because you need that gas to have power, but because the engine need all the line, air intake to exhaust, in pressure.
> Consider that just switch from a 70mm to a 76mm makes a loss of pressure, imagine a straight tube!
> Try it and let me know if it's odd for you..


IMO... if the gas goes out of the turbo faster = quicker spool, but please explain further.

There were some Subaru STI that had overboost problems when adding a downpipe but I dont believe this is the case with us.


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## Jonny_C (Jul 24, 2013)

Pressure and velocity are two different things.

The turbine element of the turbo will spin up faster, with higher velocity exhaust gases, but higher velocity means smaller area (smaller pipe effectively), so as you increase the size of your exhaust, the velocity of the exhaust gases decrease, leading to greater lag as spool up time increases.

You trade this for being able to shift more exhaust in total, so for ultimate performance you might want a larger exhaust, but you will lose drivability with increased lag.

If you go to far the other way (but who would?) and decrease exhaust size to far (in chasing zero lag), then the turbine loses efficiency & can stop working properly.

Like all things in life; the Lord giveth and the Lord taketh away, or you can't have your cake and eat it.

edit: Suspect within the normal operating ranges of everything but highest states of tune; makes f**k all difference for stuff you can buy from tuners.

In other words; go for it as you're likely to get a small performance increase, with little noticeable difference in drivability.


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## noname (Aug 20, 2015)

Thanks for the help


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## brittan (May 18, 2007)

Jonny_C said:


> Pressure and velocity are two different things.


Which, for a fluid in a pipe, are connected by Bernoulli's Equation.


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## Jonny_C (Jul 24, 2013)

brittan said:


> Jonny_C said:
> 
> 
> > Pressure and velocity are two different things.
> ...


True, but with everything else being equal and for the purposes of the discussion here, and to try and help others understand why sticking a 5" downpipe on their car may not be the best thing you could do (even though it might sound good); thought it might pay to oversimplify. [smiley=cheers.gif]


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## gavinwilson26 (Sep 16, 2016)

I have the milltek de-cat on my TTS and have to say it has made a massive difference to the sound and performance (running stage 2). I would highly recommend, although you would need to remove for a MOT or find a friendly MOT inspector.


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## James cole (Sep 3, 2007)

gavinwilson26 said:


> I have the milltek de-cat on my TTS and have to say it has made a massive difference to the sound and performance (running stage 2). I would highly recommend, although you would need to remove for a MOT or find a friendly MOT inspector.


Looks good... do you have more pictures?

How is the turbo lag?

JC


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## gavinwilson26 (Sep 16, 2016)

James cole said:


> gavinwilson26 said:
> 
> 
> > I have the milltek de-cat on my TTS and have to say it has made a massive difference to the sound and performance (running stage 2). I would highly recommend, although you would need to remove for a MOT or find a friendly MOT inspector.
> ...


Turbo lag is not noticeable at all for me, but I am running Stage 2 @ 410 bhp. 
The only other photo I have is looking down the other end of the pipe...


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## noname (Aug 20, 2015)

gavinwilson26 said:


> James cole said:
> 
> 
> > gavinwilson26 said:
> ...


sound improbable both things..no turbo lag and 410bhp with the original turbo but better than stock is sure. declared hp are less that the real ones


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## gavinwilson26 (Sep 16, 2016)

ManuTT said:


> sound improbable both things..no turbo lag and 410bhp with the original turbo but better than stock is sure. declared hp are less that the real ones


On the turbo lag front it is fairly unnoticeable to me. Won't say I've ever noticed it if it is there. On the power/tourque front my car ran 410.8hp + 438.7ft.lb on the dyno.


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