Rear silencer modification question

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alex020869
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Rear silencer modification question

Post by alex020869 » Wed Dec 20, 2017 1:14 pm

After noticing a few guys on here having had a rear silencer opened up and modified, can anyone shed any light on what exactly gets modified?
Any links or photos to find out if it can be done by myself?
Any feedback from those who already have it.

Thanks in advance.
M235i Melbourne Red.

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Barney McGrew
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Re: Rear silencer modification question

Post by Barney McGrew » Wed Dec 20, 2017 1:31 pm

I've ordered one (hopefully arriving this week) to do this too, so did some research and will attempt in the new year.

I should say, you need at the minimum a decent angle grinder and welder.

When you open the silencer up there are 3 chambers. The entry pipe is perforated to allow the gas velocity to slow and the gases disperse a bit and deaden sound etc.

The pipe to the left (in the picture) exits straight out of the back, whereas the one on the right is sent through the length of pipe and can exit the pipe on the right (also perforated) and then through the flap which is opened by the ECU. The perforated chambers should be full of sound dampening material (the stuff he's pulled out on the picture). Essentially all that's happening is the perforations allow the sound waves/pulses to dampen somewhat as the material absorbs the sound - motorcycle silencers are similar to this, even when straight through they have a perforated pipe with material in to absorb sound. Blocking this (or taking the material out) means the sound is allowed out more. The pipe coming off the entry pipe into the middle chamber looks like a Helmholtz chamber, as it doesn't appear to go anywhere and so is about creating or 'tuning' pulses.

Picture courtesy of f30 bimmerpost forums:
Image

What people do is weld the perforations up and wire the control flap open, so that the pipe is effectively straight through and emulates the MPE (to a point). The pipe into the middle chamber could also be blanked off to see how it affects the sound.

I need to check when I get it, but I think to fully weld the perforations you need to also cut further round so you can expose the underside to get access to both sides of the pipe for welding.

Some have said you can use exhaust 'bandage' and wrap to seal these perforations. This will probably work and will certainly be the easiest approach, but I'm not sure on how long this will last. Will most likely be a good length of time, but I think it will slowly break down overtime, but the worse that can happen is it ends up going back to stock. I'm going to try and find a way to avoid this and weld the perforations or at least weld a cover over them to 'seal' them.

Once all is done you can re-fit. The easiest approach is to cut the silencer off as you would to fit an MPE and then reattach with a sleeve.

The BMW ones cost a fair bit, but I've ordered one of these for just over a tenner that someone posted in another thread and saves a decent bit of cash.

http://www.ebay.co.uk/itm/76MM-3-O-D-TU ... 3182653456

I didn't want to do this to my stock silencer as you never know quite how it will sound, so using a second hand one is a nice option and allows you to go back if it sounds like a bag of nails or falls apart from dodgy welding ;)

alex020869
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Re: Rear silencer modification question

Post by alex020869 » Wed Dec 20, 2017 9:17 pm

Thanks for the concise write up. Great explanations too.
Where did you purchase the silencer that you’re going to modify, was it off here? I’m going to put an ad up in the ‘Wanted’ section.
Would be interesting to hear some opinions from folk that already have the modification though....
M235i Melbourne Red.

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Barney McGrew
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Re: Rear silencer modification question

Post by Barney McGrew » Thu Dec 21, 2017 3:00 pm

alex020869 wrote:
Wed Dec 20, 2017 9:17 pm
Thanks for the concise write up. Great explanations too.
Where did you purchase the silencer that you’re going to modify, was it off here? I’m going to put an ad up in the ‘Wanted’ section.
Would be interesting to hear some opinions from folk that already have the modification though....
I got mine from this guy on ebay - https://www.ebay.co.uk/itm/BMW-M1-M2-ge ... 2781609213

He has a few available, not sure if it's someone on here, but looks like it's 'nearly new' stock where people have had a MPE fitted and these are the factory silencers taken off, so not many miles on them.

I put in an offer and got accepted, just don't take the mickey and he's ok with sensible offers. You might be able to get one off someone on here for cheaper but this has free delivery included too so no need to haggle of collection or sorting couriers etc.

This mod has been done for years on many different BMW silencers, as the process is basically the same - cut open the main body and seal up the perforated holes.

The nice thing with this mod is you are not making any changes to the gas flow, so the power is not affected, neither in a positive way or more importantly in a negative way. All you're doing is bypassing the sound deadening properties of the perforated chambers with the glass fibre material.

alex020869
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Re: Rear silencer modification question

Post by alex020869 » Fri Dec 22, 2017 10:54 am

I’ve just sent him a messge so hope he gets back to me with one.
Thanks again for the help. It’s very much appreciated.
M235i Melbourne Red.

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AndyBish135i
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Re: Rear silencer modification question

Post by AndyBish135i » Fri Dec 22, 2017 5:22 pm

This one just been posted for sale...
viewtopic.php?f=60&t=106204
:wink:

alex020869
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Re: Rear silencer modification question

Post by alex020869 » Fri Dec 22, 2017 5:41 pm

AndyBish135i wrote:
Fri Dec 22, 2017 5:22 pm
This one just been posted for sale...
viewtopic.php?f=60&t=106204
:wink:
Unfortunately it’s for collection in Fareham and I’m in Scotland but I’ve PM’d him anyway 😉 Thanks though.
M235i Melbourne Red.

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Barney McGrew
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Re: Rear silencer modification question

Post by Barney McGrew » Tue Jan 02, 2018 9:19 am

I've been working on the silencer I received over the Christmas break and it appears BMW have changed the latest versions to the one pictured above.

They have now used a sealed pipe in the entire system except for the last small section in "chamber 3" in the picture. This has about a 3 inch long perforated pipe with a bag of glass fibre wadding wrapped around it. There is no other perforation or damping wadding anywhere else in the silencer.

Therefore if you get one and want to do this mod, all you need to do is open up about 6 inches of the left side of the silencer (looking from the back of the car - the side with the valve) and then remove the bag of wadding and seal up the holes. I ditched the idea of any welding as it's a tight space and found the easiest way is to wrap a metal sleeve around it and then secure with a couple of large heavy duty jubilee clips. The clips themselves almost seal it up anyway, but I cut a length of thin steel to wrap around and act as a sleeve which works well. I feel this will be a much better way of sealing it long term than using exhaust bandage and wrap and was easier to fit, although it's a tight space and it easy to cut your hands if you've left sharp edges from angle grinding it open so I had my welding gloves on.

It then needs welding back up to seal the silencer section. While at it I looked at the valve and noticed the problem with the rattle is quite simple.

There is about 1mm or so freeplay in the valve when the spring is removed. This means if the spring tension is not maintained the valve will easily rattle in its mount. I presume the freeplay is to allow for expansion as the exhaust heats up and when cold the spring is meant to push against this freeplay and prevent it rattling. In theory it's quite a nice design, but it appears the spring isn't up to the job and probably suffers over a number of hear cycles and so eventually allows the freeplay to cause the valve to rattle until it warms up and the freeplay is reduced, which is why the rattle is so noticeable on cold starts. A much sturdier spring might help overcome this and I reckon BMW could actually come up with a simply solution if they actually tool the time. I've removed the spring and welded the valve in the open position and will see how it sounds on the car.

I haven't got it on the car yet, but my thoughts having opened up the exhaust and played around is that I doubt it will make much difference anyway now that they have used a sealed pipe in chamber 1 and I'm not sure the effort is going to be worth it, certainly not from sound inside the car, but i'll report back when I finally get it on the car - I ran out of CO2 so need another cylinder before I can weld the silencer back up properly.

It's good to be able to view and play with the valve on an expendable silencer though and useful to see what effect it will have when on the car.

alex020869
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Re: Rear silencer modification question

Post by alex020869 » Wed Jan 03, 2018 9:53 pm

Great write up once again 👍
Do you know how old the silencer is that you purchased?
I’m waiting on one coming but will be interesting to know if mine is like your one and could potentially make no difference.
Look forward to knowing the outcome of your work.
What’s your plan if it makes no difference at all??
M235i Melbourne Red.

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