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anybody here gutted their cat (1.9 diesel)

Discussion in 'A4/A4 cabriolet/S4 forum(B6 chassis)' started by deano1978, Dec 3, 2013.

  1. deano1978
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    deano1978 different gear

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    As above. Not sure weather to cut flap, empty,and weld back up?
    Poke,hammer,chissel and batter till empty?
    Any other ideas.

    Im not going down the custom pipe route,at the min!
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  2. DieselJake
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    DieselJake God Mode

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    Allot of people will disagree and there will be an input on the turbo not having enough back pressure to function correctly without one but personally think it should be fine and will work out cheaper than buying the whole new system you were thinking of getting :happy:
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  3. deano1978
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    deano1978 different gear

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    I agree with you.
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  4. Mike.M
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    Mike.M Well-Known Member

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    If it works fine at the moment I would buy a bolt on straight through pipe which are about £60 and keep the cat for any future issues.
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  5. Mike.M
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    Mike.M Well-Known Member

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    #5
  6. deano1978
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    deano1978 different gear

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    Thats the one i bought (have) and is not right part?
    Mine goes from turbo to under my seat,much much longer than THAT part. Cannot find any info on this.

    So knock outs my best option.
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  7. pauldazzle
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    pauldazzle Active Member

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    Had the guts taken out of mine a few months ago by Charlie at Prospeed in Cardiff. He said he's done it on dozens of VAG TDI's & never known there to be an issue with back pressure killing turbos. Made a big difference to my car as the CAT was really blocked up with cr*p.
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  8. desertstorm
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    desertstorm Well-Known Member

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  9. JimmyH
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    JimmyH Member

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    Turbos do not require back pressure to work, the less back pressure the quicker the turbo will spool.
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  10. DieselJake
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    DieselJake God Mode

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    Well duh, fifty seconds of google explains turbo's spool better with less restrictions. Argument is that the cat helps restrict the 1.9tdi's tiny turbo, without it the ECU has trouble controlling overspool and will not be able to control surges. Surges can bend rods and snap the shaft when you step on it at low revs due to the boost spikes. This information has been regurgitated countless times across these boards and others with it's fair share of disaster stories.
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  11. Ian2012
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    Ian2012 Member

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    What about emissions when it's mot time? don't they test the cat function?
    #11
  12. leegsi
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    leegsi Member

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    Not on a diesel
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  13. bradh
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    bradh Active Member

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    Iv got a 2nd hand car that's had the innards removed iv just not had the energy to fit it plan to over crimbo though :)
    #13
  14. Gaz!
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    Gaz! Member

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    Of course removing the restrictive cat (particulate filter on a Diesel) will give you better acceleration. The turbo will spool faster, the problem with the Pd engines is that the turbo is a variable vain turbo, that is regulated by vacuum pressure rather than a conventional turbo that would have its boost pressure regulated by the waist gate controlled by manifold pressure. The ECU adjusts our turbo's by vacuum, so allowing it to spool up very quickly by removing the cat, the ECU reacts slowly so surges occur. The shaft is weak and can break.

    I asked a local tuner to remap my car and mentioned that I was thinking of fitting a decat, they said they would refuse to map it with it fitted as they would not be held responsible for any damage.
    #14
  15. desertstorm
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    desertstorm Well-Known Member

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    I removed the cat on my Passat, I know a lot of other people who have done it as well and they have not had any issues. A tuner who knows what he is doing could alter the N75 map and fuelling / boost to cope with a situation such as a missing cat.
    The ECU will alter some of the PID parameters so that it is better able to control the boost.
    On a standard car I really dont think this would be an issue, A car with an aggressive map may be more of a problem.
    I have seen several cars with faulty N75 valves or sticking VNT's and when logged they have been making 3 bar of boost before they go into limp mode. These kind of issues are more likely to cause damage to the turbo than a decat and a decent map.
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  16. AlexGSi2000
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    AlexGSi2000 Well-Known Member

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    Just to add my 2cents worth;

    Back pressure shouldnt be an issue with a turbo car - the free'er flowing the better.
    I have a 2.5TDi with the precat and both normal cats removed when I had a custom exhaust fabricated.
    If I remember correctly there were three silencers on the exhaust too.
    The difference was notable straight away - the car came "on boost" much quicker.

    You wont boost any more as the map is still specified to give a certain amount of boost unless re-mapped.
    As for limp mode, bending rods.etc - I wouldnt worry, the ECU has very tight tolerances (unless re-mapped) if anything is thrown out of line anywhere it will cut back.
    I wouldnt worry about breaking turbos, as for bending rods - I very much doubt the turbo would produce enough boost to even bend them, especially on a diesel - not sure how much boost the turbo produces on a 1.9 but im guessing around 17-19psi?
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  17. adamss24
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    adamss24 Well-Known Member

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    Jake is on the money there Alex, however the phenomenon is a little more complex than that ! When a turbo shaft breaks- due to over speeding, the seals get damaged and allow oil to pass on the exhaust- very smoky, or in the intake- which will lead to engine runaway and bent rods. The 1.9Tdi has a very quick spooling turbo which, by removing restriction and poor control of the N75 will allow the shaft to break at it's weakest point. Just remember it's simple physics- cold side is compressing air and it's slowing down and the hot side is still driven by the exhaust flow. The harmonic differences (aka surge) is what places huge strain on the compressor shaft and sometimes the compressor blades do deform as well. Spool down surge is not as bad but the best option when tuning is to fit a larger turbo. The extra mass on the turbine will be better for building boost gradually but won't feel as snappy as the gt17 at producing boost, unless you go for a later gen. GTB1756vk which has a better hot side and bigger compressor so you can have best of both worlds.
    In pd130 case with a remap is better to leave the cat in place to smooth surges out IF you keep the gt1749Va turbo, if you go larger then by all means, get rid of it !
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  18. pauldazzle
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    pauldazzle Active Member

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    My PD130 is mapped & decatted, & I've had no issues or surging at all. 193k miles on the clock, & running like a dream.:thumbsup:
    #18
  19. MooLard
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    MooLard Member

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    I bought in to the idea of having a straight through pipe fitted of a tiny bit larger diameter and then back to the standard diameter under the car with removing the centre box of the exhaust and I'm really happy with the results. The reason I prefer the straight pipe over decat because air flow should potentially be smoother by not having higher volume space for a few inches before being squeezed back in to a narrower pipe.
    My car doesn't suffer any surging either, to my relief.
    #19

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