Exhaust vibration absorbers - how much of a difference do they make?

In trying to track down a noise in my 128i, I noticed that the vibration absorber that's supposed to be bolted to the rear muffler is not present - in fact, it does not look to ever have been installed. I know that this car got a replacement muffler under warranty early in its life. It may be that the absorber was not installed on the replacement part.

For the record, this is the missing part.

In any case, yesterday I was next to the car checking the tire pressures and I noticed a slight buzz from the muffler. I put my hand on it and the buzz stopped. Whether this is the source of my mystery noise or not I don't know, but it did make me start to think about the missing vibration absorber. Anyone done any testing on this, or have any anecdotal evidence of just how much they do to attenuate vibrations in the exhaust system?

Millions of cars rolled off the assembly line without one of those absorbers.  BMW just likes to incorporate all kinds of engineering like that.  I would say it's likely a faulty muffler; busted seam weld?  Weld came loose on an internal baffle?

But, since 99% of the cars on the road don't have one of those dampers and don't rattle, I would say that you don't have a rattle because of a missing damper.  You have a rattle because you have a rattle.

In reply to Curtis73 (Forum Supporter) :

Agreed that there's likely something that's failed (I'm pretty sure some sort of noisy internal failure is why this car got its muffler replaced under warranty before I owned it). But I'm still curious about the effects of the vibration absorbers. They're heavy, and BMW isn't in the habit of just adding useless weight to their cars (for the most part). And there are a bunch of versions tuned to different frequencies; the one I linked to is 32Hz, while the one on the front downpipe is 62Hz. Seems like a lot of tuning effort went into them - I'd hope they actually do something.

I'm fairly sure that does something, and may impact the buzz you hear.  No OEM puts unnecessary weight or cost into their car.

I used to work in NVH - generally all of those dampers have value.  I haven't tested any personally, but I've seen enough NVH testing on other dampers (engine mounts, LCA bushings, etc.) that shows them working pretty well.

02Pilot said:

In reply to Curtis73 (Forum Supporter) :

BMW isn't in the habit of just adding useless weight to their cars (for the most part). 

 

You should weigh the stock exhaust on your 128.  If it is anything like the stock exhaust that was on my old 318ti, it weighs a ton 

In reply to mad_machine (Forum Supporter) :

I'm sure it does. But it's solid and sounds decent, so I'm not replacing it with something lighter and probably louder. It's my daily driver (well, it was, back when cars got driven daily) and I'm planning to keep it civilized.

not saying you should, I am just pointing out that BMW puts really heavy exhausts on their cars.  

In reply to mad_machine (Forum Supporter) :

I wonder if the sheer mass of BMW's systems has anything to do with the need(?) for vibration damping....

Cant say i've done any interesting experimentation with them but i save these damper weights whenever they're on a car that i part out and now have a small collection of them, so i assume interesting experimentation will commence any time now. 

mad_machine (Forum Supporter) said:

not saying you should, I am just pointing out that BMW puts really heavy exhausts on their cars.  

Yes, with good reason. Durability, noise level, and quiet without stifling the engine enough to hurt efficiency, probably some other things that the aftermarket doesn't have to care about either. Make your own exhaust that matches oe performance in all those categories and I'd bet it's either just as heavy or incredibly expensive.

That damper is there for a reason. It might not be to solve the particular noise issue you have, but some noise/vibration issue. That said, it wouldn't totally surprise me if not every car with that muffler got that damper. You confirmed that's for your exact model (ie not the 135 or something)?

In reply to dps214 :

Yes, I confirmed the p/n is for my specific car (VIN through realoem). That said, I did find a few people online mentioning that their E82s (mostly 128is) did not seem to have been fitted with vibration absorber at the muffler. Since my car had a warranty muffler replacement I have no way of knowing if it was there from the factory or not.

I also found that these seem to be becoming more common than I originally thought. Mustangs in particular seem to have them fitted, but I also found references to F-150s, Escalades, Lexus, and various Mopars. I can only guess that this is simply due to increased interest in reducing NVH in modern cars. There are also discussions online wherein people are experimenting with adding them to cancel frequencies in modified exhausts, which got me thinking about eventually adding one to my 2002....

In reply to 02Pilot :

My 90 Q45 has them.  From what I can tell, Infiniti loves them

Try sticking a heavy magnet on there temporarily to see if it changes the harmonics and eliminates the vibration. 

Woody has an excellent idea.

Newer cars are engineered with a ton of noise things.  Ford advertised their "quiet steel" for a long time, which was more or less a method of stamping shapes into things like the firewall that were geometrically and mathematically designed to prevent sympathetic vibration, thereby reducing the amount of sound they were able to transmit.

Exhaust dampers, guibos, viscous rubber mounts for things... all in an effort to keep things quiet.

But if you think BMW wouldn't add weight if it isn't necessary, just compare a new 3-series' weight to an E30.  Heck, your wee 128 weighs more than a 3 series from that era.  If BMW didn't add extraneous weight for attenuating NVH, the only people buying them would be motorsports people.  The competition goes quiet, Bimmer goes quiet.

I'm simply saying; you have a vibration that is apparently not caused by the lack of that bushing, so adding the bushing likely won't help it.  BMW engineers probably found a 2 db spike in exhaust noise at a certain RPM, so they added that damper to counteract it.  The average consumer likely doesn't care that the car weighs 9 oz more, but they would complain if there was a tiny noise.

The question then becomes, which is more important to you?  I say, fix the muffler rattle and drive it as-is.  If there is some NVH happening because the damper is missing, then add it.  But right now, I feel like we're saying "my tire has a nail and keeps going flat, and I noticed that the car doesn't have factory wheels.  Maybe I should replace the wheels."  Replace the wheels if you want, but it won't fix the flat tire.

In reply to Curtis73 (Forum Supporter) :

To be clear, it's not a rattle - if it were, I'd certainly not be trying to resolve it with a vibration damper. It's a buzz, and not a terribly loud one, at least at idle when I discovered it. It's still very possible that something has failed internally in the muffler, but then again if it had, would simply have putting my hand on the muffler caused it to stop? This is not an obvious case of something broken loose and bouncing around in the muffler case - I've dealt with that before. And if it is the beginning of an internal weld splitting or some such, it raises the obvious question of whether the lack of the vibration absorber contributed to the failure.

Agreed that Woody's idea is excellent. Unfortunately, I already ordered the replacement part last night. I'll bolt it up when it gets here and see what happens.

These are not just a generic part.  They are tuned to dampen a specific frequency and typically have to attach at a specific location on the exhaust.   There are technical bulletins some mfrs have put out with a few different frequencies Tuned Mass Dampers that you can try at different locations.  You can get the "Vibration" app for your phone and measure the frequency with it.

Curtis73 (Forum Supporter) said:

But if you think BMW wouldn't add weight if it isn't necessary, just compare a new 3-series' weight to an E30.  Heck, your wee 128 weighs more than a 3 series from that era.  If BMW didn't add extraneous weight for attenuating NVH, the only people buying them would be motorsports people.  The competition goes quiet, Bimmer goes quiet.

I'm simply saying; you have a vibration that is apparently not caused by the lack of that bushing, so adding the bushing likely won't help it.  BMW engineers probably found a 2 db spike in exhaust noise at a certain RPM, so they added that damper to counteract it.  The average consumer likely doesn't care that the car weighs 9 oz more, but they would complain if there was a tiny noise.

...so, proving that weight isn't unnecessary.

Or... weight is sometimes an engineering choice when not hearing customers complain is priceless.

So I got the vibration absorber and bolted it up. It's a heavy sucker, probably 2+ pounds. The sound I was chasing (as discussed in this thread) seems significantly attenuated, so it seems clear that the source was the exhaust. Whether it appeared because the hangars are getting soft and old, or the packing has become less effective over time, or because something it failing internally, I don't know. But adding the vibration absorber seems to have made a difference.