bcdavis

Looking at the other examples of these type of resonance tubes,
it seems pretty clear that they need to reflect the sound of the
exhaust back towards the source. It's simple wave cancellation,
just like noise canceling headphones. You can't cancel the
bothersome noise, unless you project the inverse sound back
towards the source. Think of it like bouncing a rubber ball off
a wall. Unless it goes directly backwards, it won't come
back to where it came.

The other examples of these resonance tubes make it
clear that the sound is going to go directly back towards
where it came from. Whereas your design bounces it
back at a 90 degree angle to the main exhaust flow.
Part of the drone is vibration of the metal tubes, but
part of it is a specific sound frequency of the actual
exhaust and engine sounds. So you need to bounce
it directly back towards the source, to cancel it.

SMTCapeCod

Some additional discussion.
http://www.planet-9.com/cayman-boxst...struction.html

Different application, but extensive flogging of the same horse.

There would be so much refraction and reflection from sound traveling down the pipe, hitting the wall, and bouncing back toward the main flow...I'm dubious about wave cancellation.

On the other hand, seems like that spur tube would be pressurized and would moderate/dampen the gas pressure pulses.

antlee928

Hey Stefan, that Black Sea R & D prototype looks great. Maybe this is something that could work for me. How/Where do I get more info on this rear muffler set up?? Cheer

Speedtoys

A 20+ DB drop -at- the target frequency..including all of the 'matching' frequencies up & down from there (70, 140, 280, etc)...

Id stop being dubious about it..I couldn't THINK to myself before, I can now have calm chats with the wife in the car in the same conditions.

The drone-free 928 single 3" exhaust will be in Seattle this coming weekend!

hb4

See this ASHRAE paper on engine noise control, where it shows the connection of a resonance tube at 90 deg. The cause of the resonating sound (drone) is a half-wave section of the exhaust system that acts as a closed chamber and causes sound to bounce back and forth, adding each time, i.e., resonating.

I kind of like how Speedtoy's tube points to the engine side of the 'chamber', however. It just feels better. Maybe it adds bandwidth, which I think is a good thing. More skill required to make that oval shape, compared to a circle.

Wonder what's inside Constantine's muffler?

911tracker85

I recently installed headers and a 3" back on my 84. two Magnaflow mufflers used to get the db level down. but do have droning I would love to reduce.

I need to read these links before I have the exhaust welded up.

as always, this is a great forum.

Stromius

It's a custom design (and Constatine's IP ) but I am pretty sure he's able to reproduce, even if I have to ship it back...I am happy to do so if I can find a loaner S4 rear. Contact him! (PM 'Constantine' or via blackseard.com .

Black Sea RD

Hi Stefan,

Glad you like the BSRD muffler protoype we had made years ago! You pushed it's performance envelope and great to hear it doesn't have any drone in your application.

We initally protoyped this muffler to be a slotted in between the stock mufflers and the RMB style mufflers to be used with a stock exhaust system. We didn't like the stock muffler since it is very heavy and a bit too quiet. The RMB was a bit too loud and seemed to reduce felt torque a bit.

We came up with this design and we had good reviews on a 1989 928 S4 and a 1995 928 GTS. Another Rennlister then tried it and said he found droning but he had other modifications to his exhaust system we had not intended for this muffler.

If there is enough interest we can contact our manufacturer and try to get a run made. They are all hand made and of 100% stainless construction.

Cheers,

Bill Ball

I would be interested. Any estimate on price?

Oli928

Meee tooooo....

Black Sea RD

Well it seems there is some interest in making a small run of our muffler.

We will contact our manufacturer and see if they are willing to do more of them and what pricing looks like.

We will post what we find on Rennlist.

Cheers,

hb4

After a short run with Jeff and Jim on Tuesday it was obvious that the drone is not dependent on engine speed. We had a consistent drone at 300Hz, with peaks where the engine speed base and harmonic frequencies aligned. For the first time, I held it at 4500RPM and the drone was extreme. At 4500, the engine firing rate is 300Hz.

(Note that Jeff's resonance was measured at ~150Hz, 2200RPM, so his drone-causing geometry is different than mine. His was corrected by a ~30" resonance tube. I heard Jeff's car, and he heard mine and the conclusion was that his exhaust at 2200 sounded like mine before the correction, and that his sounds much reduced afterwards.)

How is a constant 300Hz tone produced through an engine firing rage of ~100 to 400 Hz? The working theory is that the X-pipe acts like a coke bottle when the exhaust gasses pass over the junction. A resonance is created in the opposite side pipe. According to the numbers, the resonating half-wavelength is 30", which means there needs to be a section of the exhaust system that acts like a closed end pipe to acoustic sound pressure somewhere after the X-pipe. Next time I'm up on the lift I'll see if I can find what that could be.

SMTCapeCod

I dunno. Seems like in a metal/alloy xpipe a brace or plate would mitigate resonance that could potentially rise to audbile/annoying levels.

Or even some high heat packing/insulation on short term basis. Wrap it...appreciable change or no?

Speedtoys

You cant wrap the hoot away, hes gotta flute in his exhaust somewhere..

Any/All RPMs, 301Hz, and again...when engine tone matches it, it goes up at least 10db..and is excruciatingly painful in the car to listen to..thankfully we dont cruise at 4500rpm.

terry gt

Drone is gone !!!!!! at hiway speeds 0 noise just a very slight purr when you step on it 3000 and up its a very nice sound louder than a RMB . So the system is MSDS headers / X pipe / twin hi flow cats /duel dynomax VT mufflers / gibson X muffler rear . All in 2.5" stainless . Pictures to follow.