Exhaust knock

Discussion in 'E46 M3 (2001-2006)' started by Plunky, Jul 11, 2018.

  1. 11 Jul 2018 at 11:44 PM #1
    Plunky

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    I've been having an issue where I am getting a bad vibration/knock from the exhaust hitting something. I'm fairly sure it is fowling on the 2 central cross members, one of which has the rubber mounts attached.

    I have since had the central rubber mounts replaced, as they were completely shot, and now I've also fitted Hack engineering rear mounts as 2 out for the 3 were torn.

    With all the new mounts fitted the noise has got even worse?!

    Any ideas?

    I have thought about adjusting the exhaust, as I would have thought with the new mounts it would probably want to pull itself back into place, however I cannot work out how I would do this as the joints are all flanges...
     
    Plunky, Jul 11, 2018
  2. 12 Jul 2018 at 11:37 AM #2
    crockett

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    The closest point my exhaust gets to anything is here
    [​IMG]
    It is very close!
    Loads of room between the support cross pieces. The pipes barely drop down if just the cross support with the rubber mounts is unbolted and I have two box hangers stuffed.
    Fitting the more rigid Hack hangers probably amplifies the vibration if there is contact somewhere.
     
    crockett, Jul 12, 2018
  3. 12 Jul 2018 at 12:32 PM #3
    Plunky

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    Thanks!
    Where am I going wrong then?
    Judging by what you’ve said, my exhaust is far too low!
     
    Plunky, Jul 12, 2018
  4. 12 Jul 2018 at 1:04 PM #4
    Jimbo25

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    Probably not likely with the hack mounts (I don't personally think they're a good idea, Exhaust system will grow over it's length as it heats up so restraining it more seems like a bad idea to me - and NVH worse) anyway

    But you can 'time' the exhaust centre section and backbox, I know because I've tightened up the clamps before with the centre/backbox join lower than it should be, and that puts it really close to the V brace under the car (and sounds bloody horrible) - maybe yours is a touch close and knocking?
     
    Jimbo25, Jul 12, 2018
  5. 12 Jul 2018 at 3:14 PM #5
    crockett

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    I don’t see much scope for adjustment in height without putting a strain on connections. The cross supports have elongated bolt holes for lateral movement of about 4mm but that’s just so you can let it find it’s natural position without straining anything.. You need to get under and inspect
    Not related to your problem but like Jimbo25, I’m comfortable with a bit of pipe wobble.:D
    For £83 for 3 OEM hangers, it’s not worth also changing the engine and/or gearbox mounts to polyurethane ones which is what Hack Engineering recommend and are not exactly cheap.
     
    crockett, Jul 12, 2018
  6. 15 Jul 2018 at 8:59 PM #6
    Plunky

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    The hack mounts do still allow a small amount of movement as they are built using a poly bush.
    What do you mean by time the exhaust?
     
    Plunky, Jul 15, 2018
  7. 16 Jul 2018 at 2:04 PM #7
    Jimbo25

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    If you imagine looking side on at the car, and the two exhaust pieces are joined in a straight line (centre to backbox) - it's possible to have this join above or below the centre line, effectively timing the two parts at an angle to each other. In theory impossible if you do the clamps up evenly, but in reality very possible, as I found out. My join was below the centre line of the pipes, and therefore the centre rested on the V brace. I've got a supersprint centre pipe so could be closer to the V brace than a standard centre, but I can confirm it's quite easy when you're rushing to tighten it up with the centre section resting on the V brace :eek:
     
    Jimbo25, Jul 16, 2018
  8. 17 Jul 2018 at 9:04 AM #8
    Plunky

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    So is the only real adjustment where the backbox joins?
     
    Plunky, Jul 17, 2018

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