ptuomov

I've been trying to figure out how to modify my true dual 3" exhaust to be much quieter without creating additional back pressure. Since this is a turbo car, the "scavenging" or any sort of other pulse tuning is not terribly relevant to performance. It's very relevant for exhaust noise and note, though.

One thing in particular that I've noted is that some 928 systems have UNEQUAL-length tailpipes and some others have EQUAL-length tailpipes. Has anyone noted any pulse canceling effects when making the swap? That is, one could in theory cancel the pulses perfectly at some frequency by making one tailpipe longer by just the right amount and then combining the pulses at the end. Better yet, has anyone fabricating custom exhausts made the computations of what frequencies their unequal-length exhausts cancel at the end? Care to share so we can compare notes?

Landseer

The magnaflow-containing system for the 84 USA was designed with two things in mind: 1) pass inspection 2) stealth.

I was "led" into duals because the only aftermarket cat I could find reasonably was for an 85+. If dual, why not split the run -- that was my weak logic.

No consideration was made for power. Car had enough for its use.

I'm not proud that I can't add anything of value towards your question, though. Hopefully others will show other systems and comment with better reasons.

Landseer

This system is about the most awesome-sounding thing I've ever had the pleasure to drive.
Again, off your point, but worth considering.
I think John V. and others, long ago, touted the big, single pipe system for power and performance.

ptuomov

From the naturally aspirated performance perspective, I agree with you about a big single pipe being a good option. It combines the pulses at the Y, with the cross-sectional area about doubling from say dual 3" to single 4". For a normally aspirated engine, the Y-has to be at the right place, though, because the increase in CSA will reflect back an inverted pulse. So the Y-has to be in the right place. From the sound perspective, it's very hard to muffle low frequencies in a large single pipe.

In my case, I am not too concerned about pulse tuning, since I have the turbos. I want an agreeable exhaust note.

To get that agreeable and not too loud exhaust soudn, we're exploring the following options:

Merge the flow for a section after the cats using a large single pipe or muffler. Then, divorce them again. This combines pulses and gets rid of the uneven exhaust note. It also creates a low-pass filter and gets rid of high frequencies.

The divorced flow in the duals now has the same pulse pattern. We'll put them thru Borla mufflers, one on each side. I am thinking about those multi-core mufflers, which divide the pipe into many small sections with smaller cross-sectional areas. This will allow the mufflers to muffle some low frequencies, as well.

After the mufflers there are the unequal exhaust pipe sections, much like in the current system, and in many others. The flows are then merged again, but with the pulses now cancelling for some frequencies after the phase shift of the longer pipe. [The math was all f'ed up.]

Finally, the merged flow needs to go thru one additional big *** muffler in the end. We haven't picked anything yet, it would be nice to known what is out there with a single 4" inlet.

Imo000

I think you are just over comlicating things for no reason at all. An X or an H pipe is all you need for balancing.

Mike Simard

Here's a big-*** 4" Magnaflow that pokes out of my car. It's a straight thru design and has little restriction to flow.
My car sounds just fine with that and doesn't have a droning spot. I just wish my dailey driver truck was like that, it has a stupid Flowmaster and is annoying to me.

The same muffler on your car can't hurt. Maybe it will work well to have a completely different design there? Anything to dampen low frequencies would be ideal, you're already basically quiet from the turbos.

ptuomov

Now, where's the fun in not overcomplicating things?

The stock S4 dual pipe exhaust has a built-in Herschel-Quincke resonator (i.e., trombone) with the unequal length pipes from resonators to the muffler. I think it's really cool and very effective in getting ride of what is commonly described as "the drone" at certain rpms.

http://www.enoisecontrol.com/related...rrier_wall.pdf

Is your system a Y-pipe into a single or duals all the way back to the rear muffler? It's good if you don't have drone, on the other hand do you really cruise with it a lot at 2200 rpm? ;-) In any case, if you have a link to photos of your complete system, I'd be interested in seeing it.

What's the muffler part number and how does it work, by the way?

I think it does indeed help to have different silencer designs in series. Two similar resonators in line, for example, wouldn't help. The first one already got rid of the frequencies that it is designed to get rid off. The second would simply create a restriction and turn the pressure loss into sound and heat. Adding multiple similar resonators in line may at some point start increasing the noise level.

Right now, here's what I have planned for the new exhaust's silencing:
- Turbos
- Cats
- Combined cross-over and low-pass filter (like a long x-pipe)
- Borla XR-1 mufflers
- Trombone resonator (from unequal length pipes)
- BAM in the rear with a lot of volume, specifics to be determined
- Rear-pipe resonator (short pipe from the BAM, aka the tail pipe)

Anything obvious missing from there?

Ramona

My '82 Euro has a straight thru 3" system. Ceramic coated cast iron original headers. Y section then straight ceramic coated section to an SS Edlebrocke muffler under the rear seat then a straight thru Magnaflow muffler at the rear where the original was. Car is fairly noisy but an excellent sound. When I first built the system I used one of those straight thru mufflers with the megaphone 4" tips angled up slightly. Sounded superb but was attracting too much attention. I changed the rear to a Magnaflow which has the curved perforated pipe instead of a straight section and is mounted so the tail pipe is horizontal. The Edlebrocke is the split in two "straight" thru type. Even though mufflers can be straight thru type, by varying the internal layout and size you may achieve what your after.

ptuomov

Do you think this will fit in?

Ramona

"Quote"
The stock S4 dual pipe exhaust has a built-in Herschel-Quincke resonator (i.e., trombone) with the unequal length pipes from resonators to the muffler. I think it's really cool and very effective in getting ride of what is commonly described as "the drone" at certain rpms.

ptuomov

Thanks for that link. Few years ago I cut open the front muffler from my 928 and it was similar to the reactive mufflers in your link. It had a double skin with some sort of ceramic filler and was very heavy. From that article it seems you have to have one reactive muffler and straight through fibreglass packed mufflers if you want a quiet exhaust. Now to find a 3" reactive muffler that does not rob too much performance.
http://www.enoisecontrol.com/related...rrier_wall.pdf

Mike Simard

Just for fun you could add auger type mufflers into your research. I've used them in sidepipes and they are good at damping low frequencies, they left a "crisp" sound with no 'rumble'. Maybe a 4" in/out at the back. You could try different BAMs if you stick with a common size and had easy to change mounting.

I don't know why certain exhausts have an unpleasant drone but it does seem to be common with guys who add an aftermarket muffler to a pickup. That usually winds up being: bunch of equal pipe after the cats---aftermarket muffler---bunch of equal pipe to the tail.

My setup is worlds away from yours but since you asked:
Single 4" into a short manaflow 14162 and a long 14163 at the tail which you saw above. These are the straight thru race mufflers.
It's not quiet but it's not unpleasant. People come outside when I make test drives like a parade but I don't think anyone would complain.
The pic below doesn't show the BAM.

Why not make some recordings of your car before and after?

ptuomov

I don't think one should use an auger-type muffler if building a system from scratch.



As far as I understand, the auger-type muffler operating principle is separating the flow into two paths, then making half the flow take a longer path, and merging the flows later. Frequencies that end up being 180 degrees offset cancel.

This to me is exactly the same principle as in the trombone resonator, i.e., unequal length pipes. Unequal length smooth pipes are going to cause much less flow restriction than the auger spirals. I can't see why the benefit would be any higher. Given that it's very easy (often necessary) to make dual pipes unequal length in 928, I think auger muffler is not something we should be using.

Update: http://speedtalk.com/forum/viewtopic.php?t=9997


As you say, muffling the low frequencies is the difficult part. Other than the trombone resonator, I haven't really seen any ways of muffling low frequencies without a trash-can size BAM.

I'll try to record some sounds, but I don't have the right equipment to do so. There's a couple here:

http://picasaweb.google.com/lh/photo...eat=directlink

Ramona

I would suggest Magnaflow XL turbo mufflers. This gets around having similar style mufflers in series and employs mufflers that get rid of low frequency noise. In my case I am considering installing the longest 3" one they have where the the original front muffler was.
Having twin pipes makes this more difficult unless you were prepared to fit smaller ones staggered.

Mike Frye

Tuomo,

You really know how to get a good technical discussion going. I know when I see a thread started by you I'm definitely going to learn something. I mean look at this thread. Everyone has stayed on topic and had something to contribute (until I got here).

This kind of thing is why the 928 technical forum is the best anywhere.

ptuomov

Magnaflow XL would surely quiet the exhaust, but it has a design that I think will cause too much back pressure. It's a baffle/compartment muffler without a straight-thru flow path:



In case my resonance experiments fail, I may have to use the magnaflow 12594 di 3 / so 4 muffler as the rear muffler:



My understanding is that this muffler has simply a perforated y-pipe inside it.


My posts are a little bit too much out there in the half-bakery department for some people, but I am happy to hear that my speculations entertain you! ;-)