For the past couple years my K75RT(which has 100,000 miles) has been annoying me with the volume of the exhaust.  Even with good earplugs I am finding it to be too damn loud, especially around town at low speeds where there is no wind noise to drown it out, and even the wind noise doesn't totally cover the exhaust note at highway speeds anymore.

I have tightened up the exhaust connections and replaced the copper gaskets to no avail, and even went so far as to replace the original muffler with a lower mileage(~61,000 miles) unit.  But, alas, there is still a lot of exhaust noise coming out the hole in end of the muffler. 

My question, is there a life expectancy for a brick muffler after which it's ability to tame the exhaust noise decreases allowing the sound level to increase?  Has anyone ever dissected one to see if the internal baffles are burnt away over time?  Is it worth $700+ to install a new one?  I can't believe it was this noisy when it was new.  Has anyone installed a new muffler and achieved a reduction in sound level?

Also, looking at the parts lists at MaxBMW I see that there is a muffler for the K75 that has a catalytic converter noted in it's description.  How do I know if that is the one I might need?

Gryph I've got 180.000 K's up on mine and it's still quite.  It would be interesting to find what's inside them and whether they are repackable.
Regards Martin.

Dunno about the insides of mufflers but I'll guess converters were required by law sometime during k75 manufacturing. Maybe compare it's production date against your brick, if it's stamped somewhere.

I'm thinking the exhaust is another example of the brick being even better quality than cars, how many cars use stainless? My k75 lived on the coast most of its life before I got it and it's fine. A neighbour of mine with a similar vintage Japanese cruiser with a rusted out exhaust can't even get a replacement, new or second hand.

Some mufflers relied on fibreglass packing, I don't believe BM went down that track. It probably relied on metal baffles and perforations. It may be that over time one of the baffles or perforated tubes has come adrift causing less muffling. With the muffler removed can you detect rattling when shaking the muffler? Some of the café modifiers  have shortened the 75 muffler and could probably shine a light on their construction.
Regards Martin.

Most U.S. auto exhaust systems are stainless these days.  Corroded exhaust allows air leaks that mess up the emission control.  I've had exhaust systems on autos go over 15 years and 350,000 miles.  I've never had the noise increase on a car like my K75RT.

As far as packing, I don't think OEM's use it to quiet the exhaust.  To many warranty issues with that approach.  Packing rarely lasts more than a year or so. 

I suspect that the interior of these units consists of baffles, perforated tubes and two or more chambers.  What I am wondering is how long it takes for the exhaust gases to burn through the baffles and enlarge the perforations in the interior. 

All I know is that my bike wasn't as loud as it is now when it left the dealer.  It is noticeably louder than my 16 valve K100RS that only has 44,000 miles.  Since I have a spare muffler that has the same noise level I will probably have to cut it up to see what is going on.  I have to get this fixed, I am not enjoying my time in the saddle on this bike anymore.  I suppose I could just drop the $768 on a new muffler, but what if it doesn't fix the problem?

I don't spose any support bracket are broken, like some on mine, but would also make it louder at those spots, but might cause burning the gizzards out there.

No external damage to either muffler.  Also, the gasket is in excellent condition and I have tightened and replaced all the copper O-rings.  The noise is all coming out the end.

Point-Seven-five wrote:Has anyone ever dissected one to see if the internal baffles are burnt away over time?  Is it worth $700+ to install a new one?  

This customizer posted some images as he modified the oem exhaust during his work.

Thats' what I expected the internals to look like.  I'm thinking that the perforations are burned out and not putting enough back pressure on the exhaust.

Can anyone say how to tell if my bike can use a catalytic muffler, the one with the part number 1 464 505?   There is one in Germany, brand new for about $200 shipped.  It doesn't seem to have ports for a lambda sensor and I don't think any K75's came with the catalytic Motronic controls.  If I use it would the engine need retuning?  The MaxBMW parts list shows no special part numbers for engine control components for a catalytic muffler or cop bike.

MaxBMW makes reference to an RP model associated with the catalytic muffler.  Was this used on cop bikes?  Real OEM lists four different part numbers for mufflers for my VIN.  Confusing!

AFAIK none had Cats and none would have had O2 sensors as they all ran L-Jetronic.

That's right, but how to explain the part number for a catalytic muffler, and a Reference I found that said the '95 K75 was the first motorcycle to have a catalytic converter.

https://www.bmw-tech.org/wiki/BMW_K75

Gryph a members of our group of K Kronies lust after Staintune mufflers to get a lustier note. It may be possible to swap your noisier OEM muffler for a quieter one. Maybe as encouragement to do a swap post what sounds your muffler is producing. I only occasionally lust after more better note, mainly happy with the stealth approach. 
Regards Martin.

I'd say the reference about the first catalytic converter refers to the K, not the '95 k75, cause the K1 had it earlier. 

I'll guess it's a brick with an early primitive cat without a closed loop control. 

I'm trying to remember if your K75 is one of the many with a backfire issue on overrun, I'm thinking it is. If it is, maybe that's blown or melted a hole in the cat, especially if the muffler has the later part number. Then an earlier muffler would be better, or reducing the number of holes in the cat version. The cat version probably has more holes in the tube.

Daveyson, my mufflers are not the catalytic version, the part number ends in 291.  The catalytic muffler I referred to was posted on eBay by a German seller.  It's brand new and with shipping is under $200, so it could be a nice deal if it works. 

My bike has not had the backfire problem, just the occasional burbling when I don't close the throttle completely when engine braking.

I think that the next step will be to check sound levels with the db meter on my phone.  I need to do some research on the test procedure which I think involves drive-by acceleration in 3rd gear at some speed at a distance of 30 meters or some such. 

Where is TWB?  I'm sure he could shed some light on the subject.

With all that's been posted it seems pretty clear that your muffler needs to be replaced. Maybe you can find a less expensive after-market muffler or a used one from a bike that has been scrapped/modified. I'm scratching my head at why you would want to add a catalytic convertor on a bile that didn't come with one. I believe in keeping it simple - or you'll probably just be giving yourself new problems since the engine wasn't designed/tuned for a cat.

I have a good used one kor sale if you are interested. Johnnymac