hb4

Just had a local exhaust shop, Dan Fast Muffler and Exhaust, add a magnaflow muffler in place of RMBish installation done under a previous owner's watch. The goal was to eliminate the typical x-pipe drone at 2200 RPM or so (Car has x-pipes, flo-thru cats, and dynomaxes, and now a Magnaflow for a rear muffler). I think we got about 80% there in that it's now tolerable at 55-60 where before it was not, but it's not all gone.

The exhaust tone was and is still pretty good at all other RPM and load conditions to my ears; Tom M listened to it this morning at the local Cars and Coffee and concurred.

The shop did a great job of routing and placing the muffler and pipes. The muffler is tucked up and really hidden and the tips are placed much better in the bumper opening. They also corrected a tilted muffler and spots where the system was almost touching underbody parts. For some reason, the previous pipes were stacked and that was adjusted as well.

Before, then After
Attachment 547360

Attachment 547359

nuc

Looks Good!



'84 Euro 5 spd.

Stromius

Nice! Is there a weekly/monthly coffee in Redmond Town Center?

hb4

Sure is, weather permitting. The announcements are generally here:

http://www.6speedonline.com/forums/northwest-87/

Yesterday, there was a '62 Ferrari GTO that was stunning. Last changed hands at 3.4M USD. Driven by Hill and other members of the Ferrari team in those years.

hb4

Well, after a couple highway trips and a reaction from someone who heard the car before the muffler add, I'm not really happy with the results. The 2200 +/- RMP drone is softened but still there - enough that I found myself still avoiding some speeds up a grade on the highway. And the great sound of the exhaust is not really there anymore.

So I'm thinking that the 'J-pipe' solution (or what some of the Mustang and Camaro boards call a 'Helmholtz pipe') is worth a try, along with removing the new muffler. They have had great success for just this issue.

It isn't really a Helmholtz cavity, but that's what they are calling it. Basically, it's a 22" blind pipe welded onto each tailpipe that reflects a sound wave back to the main exhaust wave at 180 deg out of phase. It can have easy bends, etc., and still work.

I'm interested in any and all comments, suggestions for placement, opinions, etc. I figure I'll do this early next week and then report back on this thread.

Here's a photo of one on a Camaro


A Helmholtz cavity would do the same thing and can be shorter (but wider) and is an option if there's no room for the pipes. One possible size would be:
Port pipe: 1-1.2" dia x 4" long
Cavity: 5" dia x 12" long

Here's a Helmholtz cavity on a Mustang

Bill Ball

For me the solution was 87 (large) resonator section. That reduced the 2200 RPM resonance enough. Still there but not prominent. I couldn't stand X-pipe + RMB with my stock 89 resonator section. Stock rear muffler quiets the exhaust note down completely if you can live without the RMB burble.

hb4

Thanks Bill,

From reading various 'drone' threads here, I figured that the large resonator approach might work. I saw a picture of them cut open and they didn't appear to have much 'muffling' capacity; pretty much a tube with elongated perforations into an empty cavity. No internal baffles or stuffing, just turbulence space.

I liked the volume (and burble) of the system I inherited (which has Dynomaxes welded in) except for the drone and since I don't have any large resonators (or small ones for that matter), I'm going to try the 'J-pipes' and report back. The cost should be reasonable if the exhaust system can remain in the car.

I downloaded a spectrum analyzer for the Android and I'll take a ride and pinpoint the frequency, then have the J-pipes made at 1/4 wavelength. I'm guessing there will be a band of frequencies due in part to the different lengths of each run, and I will make the one for the shorter pipe a little longer than the bottom of the band and the other one a little shorter than the top of the band. I understand that if the J-pipes have some sound absorption placed in them it will broaden out the effective correction band or 'Q' so I might add some if it sounds like it's needed.

jcorenman

Bill,

Good stuff, watching with interest. On our S4, with x-pipe, hi-flow cats, small factory resonators and an RMB the resonance at 2200 rpm was annoying. The problem is that the x-pipe crossover basically doubles the acoustic frequency in each pipe, so the 1100 rpm resonance becomes 2200... which is highway speed with a 2.20 rear end.

I swapped the small resonators for the larger ones with only a small improvement, and finally put the stock rear muffler back on sans stuffing. Better, but still not right.

The GT has only mid-mufflers, "Bullet" brand which are no longer available. It is quite restrained with light throttle which is nice around town, but rather entertaining when the throttle is opened. There is a resonance around 2200 (I think it is the "organ pipe" between the crossover and the mufflers) but much more subdued than the S4.

It sounds like you have found a great muffler shop, please let keep us posted on how it goes!

911tracker85

a little OT as mine is a single exhaust '84. had previously repl cat with Y pipe due to damage/leak.

finally finished my fabricated 'cat back' system. started with a aftermarket muffler to repl the pumpkin. incl cutting the heat shield off the pumpkin. sound much better but too loud. then cut in a 4" magnaflow to repl the resonator.

now great, mellow sound and not too loud.

good luck.

SeanR

Best solution for me was to add a GT/GTS muffler to mine. I have the smaller resonators so it was annoying at 2100-2200. With the stock muff, the sound is great and I have no, repeat no, resonance or drone.

hb4

Sean and Jim, I assume you both have X-pipes and cats before the resonators? Also, in some of the posts on this topic there's an opinion that the GT cams add to the drone issue, but Jim's experience is the opposite.

According to what I've read, 'organ pipe' resonance happens because the length of an exhaust section equals - or is a multiple of - the wavelength of the exhaust sound and the exhaust system geometry acts as reflectors at the ends of that section. The open end of the exhaust section acts as a sound reflector as well as any large volume in relation to the pipe itself. It seems like the resonator can be an 'end', but a straight through glasspac does not - or at least not much. A resonance at 2200 RPM works out to a section length of approx. 4' (or 8, 12, 16).

So, from this perspective, the possible reflecting elements and my guess at their reflectivity are:

Valves: full reflection.
X-pipe: ??
H-pipe: ??
Flow-thru cats: minimal
Stock cats: high reflection? (don't know their construction)
Stock muffler: high reflection
Empty stock muffler: high reflection
Bullet: minimal reflection
Oval/straight thru muffler with filling: medium to high reflection
RMB/end of exhaust: high reflection.

It sure seems like the resonance is right under me, so if the x-pipe can act as one end of the reflecting section then the x-pipe to center muffler/resonator is about the right length. I think Jim's bullet muffler only reflects a little sound and mostly extends the resonance length all the way to the rear muffler - which lowers the resonance RPM down to a point where it doesn't matter because you pass it pretty quickly.

That being said, I have no idea why Sean's 'small resonator-GT/GTS' system works so well. And the placement of the J-pipes needs to be in the section where the resonance is created. But where is that?

PorKen

One word: balance pipe(s).

Simple but surprisingly effective.

Put this SS H-RMB together over the 4th weekend from old S4/GT piping. The exhaust note becomes more and more mellifluous mit RPM. It provokes you to rev it out.

HeX-pipe™, big resonators, and H-RMB. Even w/o an interior, it's easy on the ears at highway speeds.

hb4

Ken,
Looks like a simple solution and from your report it really works. I have an appointment today at the muffler shop and now have to decide what to tell him to do!

I ran an android frequency app this morning and it said that the resonant frequency I'm hearing is at 294Hz. That works out to a 1' resonance tube to control resonance generated by a 2' section of exhaust. I'd need one for each pipe. A 1' pipe is much easier to fit than a 2' pipe, so the cost of the resonance tube experiment would not be much more than that for a balance tube.

We'll see what the shop says; - maybe I should install a 1' balance tube!

PorKen

Attach a J-pipe to the H-pipe.

jpitman2

What about a K pipe? My 83 S has a BIG centre muffler , 2 in, 2 out, and the 2 in - 2 out pumpkin - all stock stuff. It has a drone around 1600rpm, which in stop/start traffic can make your head hurt after a while. I have had similar issues with other cars in the past, and have success curing/reducing a drone by insulating pipes with asbestos lagging - I know that material is a big no-no now, but there are other things. Friend of mine recently wrapped his Miata headers in some sort of tape to reduce heat in the engine bay, after adding a supercharger. One side benfit is you can work around that side of the engine with very much reduced chances or burns, as you can actually touch the pipes briefly without damage. Anyway, adding some mass to the pipe will a) reduce the radiated noise, and b) change the resonant frequency of the pipe itself, and reduce heat radiated up into the body. Based on some of the pics above there seem to be some pretty long pipes without mufflers in them, which are ideal for radiating sound.
jp 83 EuroS AT 53k