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Should I de cat the 2.5 tdi or not?

Discussion in 'A4/A4 cabriolet/S4 forum(B6 chassis)' started by boya, Jan 30, 2010.

  1. boya
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    boya Member

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    [Jan 30, 2010]
    am in two minds lol. cant find a replacement pipe anywhere so it will be a powerflow special. i have heard many plus sides to doing it and apparently its legal, so what do you guys think? its a 2003 a4 2.5 tdi fwd (163bhp) cheers
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  3. stoakseya4
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    stoakseya4 Active Member

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    [Jan 31, 2010]
    Why do you want to do it?
    Taking out the pre-cats is fine and makes a performance gain, also doesn't affect emissions re MOT.
    Taking out main cats makes further performance gains, but means it's likely to fail it's MOT.

    I've done both, and will have to get sorted at next MOT...
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  4. BILKO1
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    BILKO1 Member

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    [Feb 1, 2010]
    Don't think you will gain that much being a diesel. It won't effect the MOT emmission test as it stands at the moment for diesels as the diesel cat is not tested like a petrol engined car is.
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  5. BahnStormer77
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    BahnStormer77 BahnStormer

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    [Feb 1, 2010]
    I think the rules still stand that a diesel can only fail an "emissions" test if the emissions create such a cloud of smoke that they create a hazard to vehicles behind you... that said: limited gains in terms of performance as gas-flow to TDI's is much lower and very reasonably well catered for...
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  6. boya
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    boya Member

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    [Feb 1, 2010]
    its just what i have read on google searches etc, apparenly the cat on a diesel acts more as a smoke catcher and it doesnt need one for a mot as you have already stated, correct me if i am rong but from what i have been reading a de-cat pipe on a diesel will help the turbo spool up a little faster? the only reason i am looking into it is because i am looking at getting the car remapped in a month or so, so every little helps if you get my drift. surley the cat will be really clogged up after 85k of black derv poo going through it??? it must give some sort of gain?? plus have you heard the v6 tdi's ith no cat?? check out this link....

    http://www.youtube.com/watch?v=DW0zzTq5gqE

    thats with everything removed tho! cat and mufflers with one stainless muffler added. so what do you's think?
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  7. V6 Quatt
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    V6 Quatt Active Member

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    [Feb 1, 2010]
    Sounds Awesome. Wonder if its too noisey on a cruise.

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  8. clough103
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    clough103 Member

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    [Aug 6, 2010]
    i am interested to ressurect this thread. My prev mondeo tdci i decatted and it definately made a noticable difference, made the engine rev a lot freer, more like a petrol with power in the top of the revs. It passed the MOT fine and cant really say it made the engine louder because the 2 other backboxes were still in there. You could hear the turbo a lot more than before which is pretty cool actually.

    Has anyone actually done it to the 2.5 TDI? If so can you tell if it made any performance differences and volume differences

    cheers
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  9. paul20v
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    paul20v Audi fanatic

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    [Aug 7, 2010]
    Theres a bit more to it than that ,
    Vehicle first used on or after 1 August 1979

    1. The emissions cannot be measured because a tailpipe accessory is fitted or a deliberate modification has been made which prevents insertion of the smoke meter probe

    Note: There is no reason for rejection for vehicles that do not meet the fast pass criteria

    2. After 6 free accelerations, the mean of the last 3 smoke levels is:

    a. for a non-turbocharged engine, more than 2.50m-1

    b. for turbocharged engines more than 3.00m-1


    3. Exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users

    Note: The criterion is density and not volume of smoke. The description 'dense smoke' includes smoke or vapour which largely obscures vision
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  10. clough103
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    clough103 Member

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    [Aug 11, 2010]
    back on topic, who has done it and what difference did it make lol
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  11. paul20v
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    paul20v Audi fanatic

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    [Aug 11, 2010]
    It had not gone off topic as all is cat and emission orientated.
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  12. Phil MC
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    Phil MC SerialAudiOffender

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    [Aug 11, 2010]
    I'll let you know in the next week or two when my 2.5TDi Quattro Sport(remapped to 220) gets de-catted and twin tailpipes fitted....hopefully, all going well :yes:.
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  13. murran
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    murran Active Member

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    [Aug 11, 2010]
    right, for a start, the're not catalytic converters on diesels. they are particulate filters!
    and id be very surprised if it will pass an mot test with one "knocked out" or removed.
    plus the mot regs are changing as per the smoke dencity next year i think. down to 1.50m-1 mean over 6 accelarations. for all diesels 2004 onwards. alot of diesels are guna fail this!

    im a tester and ive read it in a special notice a couple of months ago.
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  14. Phil MC
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    Phil MC SerialAudiOffender

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    [Aug 11, 2010]
    Right, for a start.........(Nice way to start a reply to a friendly thread 'murran'???)

    1) We all know that only petrol engined cars have 'catalytic converters', but we also recognise that this is the diesel equivalent.

    2) I have had my last 3 diesel cars all 'de-catted' and they do indeed pass the mot. My other VAG diesel, which has also been 'de-catted' by the way, passed this year with a test result of 0.29 1/m (Fast Pass).

    3) This years mot had a Fast Pass test limit of 1.50 1/m, so under stricter tolerances you mentioned it'll still be well within the parameters.

    4) If you say these stricter pass limits are to be applied to 2004 onwards vehicles, then that's fine with mine and the OP's being 2003 models.

    5) Density is correctly spelled with an 'S' and not a 'C' :p.

    To be totally level with you murran, if by de-catting it meant that I'd need to refit a cat every year for an mot I wouldn't have it removed in the first instance. But to come on here and basically preach that we're all sinners for doing such a horrid thing and that we're all gonna come unstuck is just a bit rich for my liking.

    We don't all remove the cat just to try and make the turbo spool a bit quicker and squeeze another ounce of performance from the engine. I know someone who was faced with a rather large bill for a replacement cat because the flange at his downpipe was split, so he decided to delete the cat to save on the unexpected expense, and there's nothing the law or mot testers can do or say about that is there?

    We're all on this forum because we are car enthusiasts, whether it be for performance modding, styling show-n-shine or just to be part of the wider Audi owner community. Please don't shoot down fellow enthusiasts just because you don't share the same interest or opinion.

    There, rant over, but the manner in which you responded warranted my long-winded reply.
    Last edited: Aug 12, 2010
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  15. paul20v
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    paul20v Audi fanatic

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    [Aug 11, 2010]
    Also a tester and they make very little difference when knocked out and removed
    my car always goes through on a fast pass as do alot of other audi/vw i work on and modify
    yes they are particulate filters
    but mot regs change for up and coming cars but they very rarely start knocking limits down on current cars as this would be unfair as your car may pass one year with a specific reading then fail the next year at the same reading if the limits were changed
    what special notice number was this as i dont remember seeing it if you can find it i will have a read .
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  16. murran
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    murran Active Member

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    [Aug 11, 2010]
    didnt mean to sound harsh lol, if it passed, it passed, just suprises me thats all.

    thought id follow up your long windedness with just one sentence. :)
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  17. murran
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    murran Active Member

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    [Aug 11, 2010]
    i'll have a look tomorrow at which sn it was. as soon as ive read them they get forgotten about!
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  18. Phil MC
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    Phil MC SerialAudiOffender

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    [Aug 11, 2010]
    LOL, apology accepted :thumbsup:.
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  19. paul20v
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    paul20v Audi fanatic

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    [Aug 12, 2010]
    Hi mate the limit is still 3.00 for turbo and 2.50 for non turbo
    the fast pass is 1.50 but has to be this or below in 1 flatout acceleration otherwise its upto 6 flat out accelerations till an average reading below threshold is excepted by the smoke machine
    whoever told you the limit has dropped to 1.50 is lying to you mate
    sorry to go on just dont like people taking the piss out of others , just in case they were after work if you went over 1.50
    cheers
    Paul
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  20. paul20v
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    paul20v Audi fanatic

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    [Aug 12, 2010]
    I know what you mean , i dont remember it thats why i asked just incase you had a rough idea of when .

    Paul
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  21. Phil MC
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    Phil MC SerialAudiOffender

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    [Aug 12, 2010]
    Cheers for explaining the reg's Paul. I was just trying to decypher the emissions report with an untrained eye :scared2:.
    It all makes perfect sense that it was a Fast Pass with a limit of 1.50 1/m, and an actual corrected reading of a lowly 0.29 1/m :thumbsup:. Not bad for a remapped/de-catted Ibiza FR TDi :racer:?
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  22. crazy88
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    crazy88 Loving the anonymity

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    [Aug 12, 2010]
    So how would one go about doing this then? I'm intrigued by it, as the sound of that 2.5 tdi was awesome. Only problem is that mine is a 1.9tdi, so only the 4 cylinder. I just want to hear my turbo more, haha!

    Excellent knowledge on here though, it's ace! Cheers dudes.
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  23. murran
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    murran Active Member

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    [Aug 12, 2010]
    had a look today, it was sn 3-2010. 1.50 limit is coming in in july 2011, but its for cars post 2008. not 2004!

    told you my memory isnt good when scanning thru sn's while rushing to log on a test!
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  24. paul20v
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    paul20v Audi fanatic

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    [Aug 13, 2010]
    Cool i will have a read of that one tomorrow
    cheers
    Paul
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  25. paul20v
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    paul20v Audi fanatic

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    [Aug 13, 2010]
    No probs its nice to know what stuff means:icon_thumright:
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  26. Mackers
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    Mackers Member

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    [Oct 7, 2011]
    i;m thinking of de-catting my 2.5Tdi and was wondering where can you buy the de-cat pipes from, also are there only the 2 cats under the car and if so, how do you fit the pass pipe as mine is a single pipe coming from the engine (or so it looks, with splash guards in place... not taken them off yet) this then splits into 2, into 2 cats... then back into 1pipe and back to the backbox.
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  27. adamss24
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    adamss24 Well-Known Member

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    [Oct 7, 2011]
    Just remove the cats and use a long drift to knock them out, making sure you dont leave any lumps in as they will do your head in when rattling. Also if you have a pre-cat after the turbo, then remove that as well whilst at it, that is the biggest restriction in the system as all the flow passes trough that...If you can get someone to fabricate a decat pipe then even better, fjtwelve has a 3" system on his a6, it's tight fit but doable...
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  28. Mackers
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    Mackers Member

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    [Oct 8, 2011]
    cheers for the advice, i think i might see where i could buy a decat, as i don't fancy knocking mine out, as i've not had the car 3 wks yet. :)
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  29. adamss24
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    adamss24 Well-Known Member

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    [Oct 8, 2011]
    Diesels pass well without cats/particulate filters in place, they are there just for smell really ! My a4 has passed 4 years Mot with both cats removed and it does not smokes excesively either.
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  30. Mackers
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    Mackers Member

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    [Oct 9, 2011]
    i've looked under the car and OMG, the pipe splits into 2 just under the "gearknob" then rejoins "under the backseat" and to the back box... but after it splits, there is 2 cats :O
    Google Images
    the top image is what i've seen, the bottom image is a side on pic, looks like fun eh :S
    #29
  31. Final 3Green Land
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    Final 3Green Land Member

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    [Nov 17, 2013]

    Thats two sentences!!
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