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Have you failed MOT due to secondary cat delete?

Discussion in 'MBClub Polls' started by stereophoney, Jan 18, 2015.

Have you failed your MOT because of a secondary cat delete?

  1. Yes

    6 vote(s)
    10.9%
  2. No, passed without question

    30 vote(s)
    54.5%
  3. No, i've got a 'friendly' tester

    5 vote(s)
    9.1%
  4. Yet to find out

    14 vote(s)
    25.5%
  1. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    Change with respect to what?

    As mentioned, the small (primary) cat is near the engine and reaches light off early. The larger main (secondary) cat is further away and lights off a bit later.
     
  2. flowrider99

    flowrider99 Hardcore MB Enthusiast

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    If removing the secondary CAT's is possibly going to cause an MOT issue then surely its better to remove the rear silencers if you want more exhaust noise, or does removing the CAT's v's the silencers a better option for power, exhaust note etc.
     
  3. JohnEBoy

    JohnEBoy Hardcore MB Enthusiast

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    You are stating the smaller CATs are nearer the engine and the larger further away.

    It's the opposite on the E55K - the physically-larger CATs are on the downpipes and the physically-smaller CATs are further away from the engine.

    Does this change what happens?

    I'm not an expert but my understanding from what I've seen is that having no silencers or CATs yields the most performance increase if the exhaust size (diameter) remains the same.

    But if you start messing around with back boxes, you run the risk of hitting drone and it is a lot louder.

    I didn't touch my standard rear boxes for those reasons - despite the fact I may have got more power output as a result.

    Removing CATs on the other hand just increases the volume of the exhaust.[/QUOTE]
     
  4. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    Interesting, I've not seen an installation where the main catalyst is nearer to the engine than the starter catalyst. There are some good reasons to avoid having the main cat close to the engine. For example with a ceramic substrate, engine vibration can cause problems with long-term durability. It's also 'interesting' from a heat management and packaging viewpoint. The smaller catalyst may still light off first due to reduced thermal inertia, but for sure putting the larger brick next to the engine will help that to light off sooner than if it were further away.

    It may be a strategy that reduces light off time for the main cat giving them some headroom on the emissions tests. Without knowing the details of what MB have in the cans, it's not easy to speculate as to why they've chosen this configuration, though I'd be confident in saying that both cats would need to be present to pass a regulated emissions test.
     
    JohnEBoy and PobodY like this.
  5. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    Just found an illustration of the E55K exhaust system which shows the cats nearest to the engine to be circular in section which suggests (to me) that they have metal foil substrates. These are more resistant to damage from vibration and warm up quickly.
     
    JohnEBoy likes this.
  6. Petrol Pete

    Petrol Pete Hardcore MB Enthusiast

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    ^^^ Same question as above ??
     
  7. Petrol Pete

    Petrol Pete Hardcore MB Enthusiast

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    ^^^ Got jumped. The question about the secondary cats, do the filter the gasses further even though they are cooler than the primary or are they there to give back pressure. /

    Cheers
     
  8. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    Ideally, there would be just one catalyst that warms up quickly and has enough internal surface area to process the exhaust gases produced over the full operating range of the engine. This used to be the normal arrangement, but as the emissions regulations tightened up, the single catalyst didn't light up quickly enough to manage the initial cold start emissions well enough to pass the regulated test.

    Solution; put another catalyst nearer to the engine that gets going more quickly hence giving time for the main catalyst to get up to working temperature. Typically though, this catalyst can't cope with the emissions over the full operating range if the engine. The additional (main) catalyst provides more (usually a lot more) surface area that can then process the emissions from when the engine is working hard.

    There is a question as to whether the exhaust emissions aftertreatment systems are optimally sized to just cope with emissions made by the engine when working at the speeds and loads defined by the standard test cycles or whether they can manage the emissions over the whole operating range of the engine.

    For sure the catalysts, silencers, DPF, turbochargers, EGR valves etc. introduce restrictions to the exhaust gas flow. When looking for performance, ideally the intake and exhaust systems would be optimised. But do you optimise for best power, best torque, best driveability, best sound etc.? In the end there are compromises to make. The catalysts and other after treatment devices are installed by mandate to minimise exhaust pollutants (CO, HC, NOx, PM) leaving just CO2 and water. Taking some of the devices out of the system does compromise the 'cleaning' performance and undermines the whole point of having them in the first place.
     
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  9. Allan AMG

    Allan AMG Hardcore MB Enthusiast

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    Move me from the *yet to find out, to a No.
    Passed first MOT today @mercedes dealership too :)
     
    Last edited: Sep 14, 2017
  10. AMGeed

    AMGeed Hardcore MB Enthusiast

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    That'll be a no then if you passed:D
     
  11. Allan AMG

    Allan AMG Hardcore MB Enthusiast

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    :wallbash: Edited :)
     
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  12. JohnEBoy

    JohnEBoy Hardcore MB Enthusiast

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    Mine passed the MOT every time without secondary CATs so the emissions must have still been within tolerance.

    After a full de-CAT, the first year it apparently still passed the emissions, the next time the different tester used another technique to get around stating it would not pass the emissions. So I am none the wiser on this one.
     
  13. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    This illustrates how useless the MoT 'Emissions Test' is as process for understanding tailpipe emissions from a vehicle.

    Without being under load, the engine doesn't need much fuel and air so doesn't make much by way of exhaust pollutants unless it is not runnng correctly. As long as the AFR or Lambda is in the right ball park, and the engine isn't knackered, the idle or fast idle engine-out emissions might be within or close to the limits specified in MoT test method even without a catalyst.

    https://www.gov.uk/government/publications/in-service-exhaust-emission-standards-for-road-vehicles (only 222 pages FFS...)

    These are not the same limits used for the 'official' Euro (x) emissions test.

    Out on the highway or during an emissions test where the engine is under load (especially during acceleration events), without an exhaust catalyst (and other emissions management systems), the tailpipe emissions will be the same as engine out. So there will be significant levels of CO, HC, NOx and PM coming out unchecked.
     
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  14. PobodY

    PobodY Hardcore MB Enthusiast

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    The fact that a tester will find a way around testing a car with no catalysts defeats the whole purpose of an emission test.

    I admit to being happy that my little Nissan doesn't fail on emissions, but it's also a grey import that has no official EU emissions data (despite being physically the same as a Sunny GTiR).
     
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  15. GeeJayW

    GeeJayW Hardcore MB Enthusiast

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    There is a conflict of interest for the MoT tester. The customer is really paying for the test and will of course be unhappy with a fail result and may well take their business elsewhere if that happens. Especially if they have their car serviced at the same garage that does the testing. It's a flawed system, but I guess it's better than no system.
     
  16. PobodY

    PobodY Hardcore MB Enthusiast

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    Technically whilst performing the MOT, the tester is working for VOSA rather than the garage. - Unfortunately it's hard to make that disconnection and blame the garage for the failure.

    I've always said that I want a "fair" test on my car; I don't want a dangerous car that might kill someone, but some of the failures I've had don't actually make the car un-roadworthy. - I guess the same applies to emissions; I don't want to fail on some technicality, but if it's blowing smoke I need to get that fixed.
     
    GeeJayW likes this.
  17. E55BOF

    E55BOF Hardcore MB Enthusiast

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    You obviously know your stuff, so I would believe you, but I'm not sure I would undestand the numbers...
     
    Last edited: Sep 20, 2017 at 9:16 PM
  18. Codger49

    Codger49 Hardcore MB Enthusiast

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    That last sentence could be interpreted as saying "I only care about emissions I can see".
     
  19. PobodY

    PobodY Hardcore MB Enthusiast

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    Well if a 2l petrol engine has and exhaust that looks like a diesel, that's a problem. - I would prefer that it's as clean as it can be, but after 27 years you can't expect "Euro 4" emissions from it.
     

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