Gundo Hack - Vibration or Drone Issues?

Did a search but do not see any issues reported by doing a gundo hack on the mufflers. Does the computer need a reset? After installing modified mufflers there is a terrible vibration/drone at idle. Anything off idle and it goes away. Mufflers are installed properly and there is no rubbing of any sort that can be seen, they are not hitting the chassis. Anyone experienced this?

There is no need for a computer reset or adjustment . It's very odd you have the issue only at idle . Any chance you have loose internal baffles ?

It's a head scratcher. I bought the mufflers used to do the gundo hack, so could be they are defective. I absolutely love the result minus the idle vibration. Nice bark when you start it up and stock quiet when cruising. Really nice sound when you get on it, not loud but sporty. Don't know where to go from here as the vibration/drone only occurs at idle when the car is in gear (tiptronic). That may be a clue as the idle drops when in gear, put it in neutral and the sound goes away. It's not a subtle vibration/drone, it is loud and quite annoying. The seats vibrate. Put the stock mufflers back on and problem goes away. Anyone have any ideas?

I think you solved the issue by saying going back to stock mufflers made the issue go away . Sounds like the mufflers you bought to modify have broken internal welds on the baffles or chamber walls . The OEM mufflers are a 3 chamber design , so there are two vertical dividers inside . Those dividers are perforated and have pipes going g through them . Any of those pieces can vibrate if welds have broken . You might be able to look inside with a borescope . If not you will have to cut open to inspect/repair . Do an internet search for " pics of inside a Porsche 996 muffler " and you will see the internals . I have stock mufflers modified by Fister Motorsports and there are no vibrations or drone . My point being if the internals are OK the exterior mods won't cause any issues .

Final thought. M&K used to make 996 mufflers can't you have a set made ? Aren't you the distributor for them ?

Your set of components/installation, specifically, has "induced" a resonant frequency that you're hearing. Some component is vibrating vigorously after the Gundo hack. Maybe just vibrating. Maybe vibrating and hitting another component like the frame. You will have to search for this vibration under the car. While there drone is evident, you'll inspect the muffler/areas and using a "stick", you'll carefully press on components or wedge them to try to influence the sound. The Gundo hack itself may need to be redone to increase or decrease the hole size or change the mass of a component to change the resonant frequency. A baffle might be loose. The issue is something specific to your vehicle. A methodical search will be needed. Change one muffler at a time.

I'm the owner of M&K Exhaust. We discontinued the 996 mufflers many years ago as the market wouldn't support non-Asian imported exhaust products for the water cooled 911 at that time. The used mufflers were bought here on Pelican and apparently misrepresented. The seams were cracked and had to be repaired, stands to reason that the internals may be garbage as well. The mufflers are not vibrating on the chassis, we went over all that. Very disappointing. Appreciate the insight, may dissect these and just modify them internally as an R&D exercise.

There is a company out there that disects the OEM mufflers and completely gut the interior . Everything is removed then the shell is welded back together . Removes a bunch of weight and supposedly sound great with no drone or resinonce . Sorry I can't remember the company name but you might consider doing the same .

You sure no clamps, bends, pipes, bolts are touching anything around the chassis?

I had a pesky heat shield flapping around once

Removed the mufflers and the vibration went away. Looks like the mufflers I bought were misrepresented. Seems to be a lot of that going around these days ...

Sorry to sound like a broken record , but as the owner of M&K can't you have a " one off " set of mufflers made ? Have the welding jigs been thrown away ? Computer build data gone ? I would think your car could be a rolling advertisement for the product and could peak interest for it . I had M&K mufflers on several air cooled Porsche's in the past and they sounded great , were lightweight and very well made . Maybe you should reconsider the 996/997 as a product line . Or maybe not you know better than I .

Appreciate the support! Our focus has always been the air cooled Porsches. When the 996 came out we looked at the future but decided to stay the course. Time passed and we took another look, I bought this 996 as an R&D car. Then covid hit. The market remains strong for the air cooled cars so for now they stay the focus. The gundo hack was done to compare that sound to the mufflers we built back in 2000, yes ours sounded a whole lot better than these, ha!

Did production 996 mufflers ever get made ? Or only R&D ? I am always watching for a used set to magically appear for sale .

We made a few pre-production sets for testing purpose. Installed them on a few local cars and did some dyno testing. The advantage was a significant weight savings and they were louder than stock. The dyno test showed no improvement in power which was expected. The sound quality was not what we were after. Further testing showed that the cats made a significant impact in that sound quality. Fast forward to today and it seems deleting cats is more popular than it was back then. Ultimately cost killed the project as at that time overseas aftermarket mufflers were dirt cheap. To directly answer your question a few of these mufflers were sold at a discount for testing purposes but I have no idea where they are now. I've considered selling our 996 but folks today are willing to spend more money on these systems so we might take a look at that.

My 996 C4 has a 3.8 FSI with aftermarket headers , a catless x-pipe and Fister Motorsports modified 3.6 OEM mufflers . Sound is excellent if maybe a pinch too loud but my only real complaint is the weight . Having said that good quality OEM mufflers have zero resonance or rattles . So having them modified may be the best bang for buck solution . If you decide to go into production on the 996 mufflers I would be interested .