I know there are a few threads related to exhaust resonance, but I'd like to dedicate a thread to the more general topic of dealing with resonances in after-market systems for the Mini. Let me start with a few parameters:

1. We're not talking loose mounts, leaks, mechanical interference here. We're talking acoustic resonance.
2. The resonances are due to the relatively short length of the Mini exhaust, and made "worse" when the battery is removed and a straight system is installed as the system is at least 2ft shorter.
3. Resonances manifest as a drone--most annoyingly at 3000 rpm which is cruising speed on the freeway (!)

I note that the thread on the RMW header has ventured into this territory at times, and other threads that started on topic turned to questions about rattles and broken mounts.

As many of you know, I've done the battery relocation mod, and have tried 2 different straight exhausts so far and suffered from extremely bad drones around 3-4k rpm. I've done a lot of reading about exhaust resonances only to come up with more questions than answers. I'm hoping to gather some data with this thread and see if there are some common problems that might lead to a solution or some solution principles.

I am about to try system III: an in-line resonator/muffler at around 5ft into the system. This is located just behind the central mounts.

I have tried to figure out the Helmholtz resonator solution used by the Honda S2000 and apparently some Lexus systems. There's a big thread on a Mustang forum about them, but nothing on 4 cylinder motors and short systems like ours other than the Honda one--and there's no data provided about that. The Mustang solution is a 26" closed end resonant chamber designed to damp resonances at 2000rpm. The Honda solution looks to be much shorter, about 9" from photos. I presume the resonance is more in the 3000rpm range for the Honda.

Generally the solution involves moving the natural system resonance to a frequency that is rarely seen in normal driving. For me that means 1000rpm or 6000rpm !! Lengthening the exhaust is not a solution as it defeats the purpose of a short, light weight system. Shortening it is impractical unless you're Onasled.

Here's my data:

CMC header
race cat
2.5" system
Hushpower II resonator after cat
2.5" inline Sweet Thunder chambered muffler
2x 2" Sweet Thunder chambered mufflers in split to tips.

==> bad resonance at 3200rpm covering range 3000 to 4000.

Phil

 

Short of getting into a full engineering analysis, there are a few basics that might help. I do think that many aftermarket exhausts are prone to the 3K drone, which leads me to believe is partially from the nature of the engine itself. That being said you could try:

1) Heavier or lighter wall tubing in certain sections of the piping
2) Flex joints in certain locations as well.
3) Making sure the tips are angle cut and not straight promoting a megaphone effect
4) Making sure an actual megaphone(expanding tip) isn't on the exhaust.
5) Keep trying different mufflers

I know most of these aren't very helpful, but might make a difference. I've actually put a cut-out valve in one leg of my rear section and changed the tone and loudness considerably by making it a dead leg at lower RPM. I have the valve opening automatically at a predetermined RPM, or manually.

Having the straight through design is hard. Properly done piping after the mufflers usually cost no HP but can help tones with the design of the bends.

I was just posting in the crankshaft dampener thread, vibrations and harmonics is usually one of the lesser understood disciplines, and not easy either. I'm no expert myself, just enough to be dangerous.

 

It's not too hard to do use helmholtz calculations to figure out the resonances. There are some issues though. First, lower frequencies need long resonators. I doubt the Honda resonator is tuned to 3k rpm if it is only 9" long. Where are you thinking of putting this? Last time I checked, there wasn't much room under the MINI. Piston airplanes use this quite a bit to almost completely eliminate exhaust noise at cruising rpm. Those chambers are several feet long. Second, it's temperature dependent. The speed of sound changes with temperature and exhaust cools down dramatically as it goes through the exhaust. This means you will need to make it adjustable for fine tuning. That's not hard to do, just make a slip-fit end but it comes back to space under the car. Calculations will get you within 4 or 5 inches. Here's an image from a formula SAE team that used resonators to dampen exhaust noise. I assume those are tuned for 10-11k rpm as that's where our noise levels are checked.

 

Funny thing about that picture is the can they're using is as big as the entire block of the engine.

 

Me too! I have built speaker systems, and a listening room --- learning just enough to understand what I was doing and learning what worked and didn't mainly by experimenting. Unfortunately experimenting with exhausts is both expensive and requires welding equipment and skills which I don't have. I'd like to get it right, but so far I'm 2 for 2 in the drone department

Thanks for the ideas!

 

Phil, when we talked in Dallas I told you this would be a massive undertaking to make it quiet at those RPMs. Now you know why I have spent so much doing R&D on trying to make the exhaust quiet. There just isn't enough room under there going the straight thru route to make it quiet.

I do wish you luck though, it's a rough road ahead

 

Kapps: I've been looking at resonator formula, but they all require the volume of the chamber in which the resonant frequency is being generated. For example, the idea of a speaker box is simplest to understand: The box has a frequency dip at say 45Hz, so you can add an open tube to reinforce just that frequency and compensate for the dip. The Mustang guys were not into formula, but they talked about designing a tube (closed end) that resonated at 130Hz. This was apparently 26" long. I have learned from the Honda groups that the offending resonances are around 90-130Hz, but I don't know what length closed end 1/4 wave tube I need to dampen that. Do you? The diameter of the tube is not important, just the length. It can also be bent, so I can run it parallel to the center main exhaust tube, or between the split chambered mufflers. I could get about 18" before it became a problem with fit.

Jan: At least you see that I'm a determined idiot! I did hear everything you said to me, and have mulled it over. I think the Helmholtz resonator is the solution, but I'll try some other ideas first. A strategically placed muffler or resonator might just do the trick, and is the next step because it's relatively easy and low cost. The Helmholtz will require some work with my muffler guy and will cause clearance problems for sure. I won't give up though!

thanks.
phil

 

this is what i did , it sound great at all rpm.
btw thats not me

 

Sorry adrianr, are you showing that you did a one-ball mod? If the stock exhaust doesn't drone neither will a one-ball, as it's the same length. You still have a battery box--I don't, hence my dilemma with a shorter straight pipe system.

 

Phil, I'm not sure. I'll have to do some research and think about things. I've only used Helmholtz in combination with pipe organ tuning to figure lengths of intake and exhaust runner lengths; nothing that deals with damping sound.

 

It would be helpful if you could post a picture of the current system to see how much room/clearance you have to work with using the existing tubing.

For all the calculations and best-laid plans, outcomes can’t always be predicted, so experimentation may be required.

In your investigation you’ve discovered longer is better, but with our packaging limitations you may need more volume to compensate; your mufflers seem to be small by the dimensions given. A good muffler design that makes the most of a given space is a double pass/twin loop.
The down side is availability and cost. A larger volume baffled muffler will give greater attenuation with a slight backpressure penalty. There are many large oval straight through cans you could try in that location. Why not try a R56 take-off exhaust can? There should be a good supply of them at reasonable cost.

So given the space and products readily available, a resonator for post center bracket can’t be very large in diameter or length. A 3.5 X 18 inch will provide a slight reduction in high tones, while leaving adequate clearance from the heat shield in the channel beneath the gas tanks. Without the battery box, there may be room for a longer one.


While the HushpowerII is ok, think about letting that one go, a change in tonal quality can be had with a perforated core straight through. Here is a 5-inch diameter that will leave plenty of room for the heat shield.


With the header you’re using the flange & neck will have to be skillfully welded to leave enough clearance for the center bracket on the backside. A 6-inch will fit in this location also and is very effective, however, hanger bracket placement is critical to keep one that large from striking the upper section of heat shield.

You may need to approach the issue from another angle, resonance of the cabin itself and barriers to the objectionable sound. Start with the boot door skin from the inside, the area directly above the rear can, areas in the exhaust channel beneath the chassis, and the area behind the header on the fire wall. Here’s an application I just did on a club member’s ride using an acoustic thermal barrier product.

 

Thanks k-huevo!! All useful and on-point information. You didn't mention Helmholtz resonators--do you think them unhelpful or are you just unfamiliar with them? There's no doubt that volume helps, and transitions from pipe to muffler and back can sometimes work to dampen noise, but system resonance is heavily dependent on system length (among the many other parameters of course.) The first straight system I installed had a Flowmaster Super 44 muffler where the battery normally is, much like the R56 setup. Of course the 44 is an unpacked muffler and sounded very hollow and metallic -- it also backfired like a *****!
I installed 28sqft of de-amplifier sound deadening in the boot area, including all the surfaces you mentioned, and the Milltek that I am currently running is silent from inside the car, so it has had some effect. However an annoying resonance will penetrate almost anything, so eliminating the source is still the preferred option for me.
I was thinking of replacing the inline chambered muffler with a can as you show in your second photo, which can only be 14" given the split for the two Thunders and the tips. Still it's where I will start, then work on the Helmholtz chamber. I have a design already, I just have to talk with the muffler guy...

thanks again!

 

Do you still want a loud exhaust or are you trying to acheive a quiet exhaust system?

I did toy a long time ago off using a Dynomax 9 inch round stainless muffler built for an RV , very quiet but free flowing

http://www.dynomax.com/ecat/pdfs/mufflersandspecs.pdf

Unfortunatley it looks like they dont do a 2.5 inch one any more.

I would personally use a straight pipe from the cat to the largest and quietest muffler I could get out back.

Good luck

 

Perhaps you should have tried the 60 series?

I’m familiar with Helmholtz principles as they relate to sensory perception and Psychology. I’m also familiar with the applied practice in NA intake plenum design, but, I read a white paper that stated a Helmholtz “resonator” used as a tuning tool in an exhaust system was useless above 3,600 rpm and best left to diesels, so I’ve dismissed it, valid or not, in our application. When you say “Helmholtz chamber”, do you mean termination box?

 

could you go with a more conventional exhaust for noise compliance and just add cut outs on servos for h.p. ?

 

As you know, I've had my system for a few years now, and I swear that I don't have this issue. Heck, maybe I've grown immune to it though... If you are ever in the Bay Area, let me know so you can listen for yourself.

You might know this already, but if you want to stiffle the internal cabin noise more, go into air re-circulate mode. It's become the wife button for me...

The whole catback is like 18 lbs. The additional loss in weight with ditching the battery box (about 8 lbs) and a lighter battery (approx 20 lbs less) makes for a noteworthy difference.

Hope you can figure something out...

 

Helmholtz tuning works at high rpm's, it's just that the tuning length can't be accurately calculated at high rpm's. The higher you go, the farther off the calculations will be. In this case, testing is the only way to go. Luckily, Phil wants to tune for 3-4k rpm so the calculations will be fairly accurate.

Also, helmholtz can be used for both intake and exhaust tuning. It works the same between intake valve and plenum as it does between exhaust valve and collector.

 

Yes, the weight saving is significant. Honestly, I've become obsessed with finding a solution to the droning problem. I think the Helmholtz resonator is an elegant solution, hence my inclination toward it--but it's really nothing more than that.
btw, I keep forgetting to confirm that as the exhaust gases heat up, the drone gets considerably worse, which is what everyone says. I'd say that the fundamental frequency lowers with higher temps--am I right all you theorists out there?

 

Temperature variables disrupt the Helmholtz calculation. Let’s say the “chamber” is constructed to cancel a specific pitch, it might end up being too narrow to match real world temperature and load condition variables. A few resonators may be required to be effective.

Since the Milltek has proven itself to have an unobtrusive note with your header, how about keeping the forward resonator as a core component.

 

Well, I'm only using the rear section of the Milltek--the part that normally goes around the battery box. The center section is new, made by Juston of CMC for me, and has the Hushpower II in it.

The "secret" to the Milltek is the length (almost 3ft longer on each side compared to the straight exhaust) and the little packed cans.
I contacted the dual-loop people, and will see if they can offer anything.

 

milltek worked well for me, with the janspeed header,
but not so with RMW Shorty, looking at the back end is only half the question, its the combination which gives the sound, and i guess boost and exhaust valve size are also big factors, think bugle...
http://en.wikipedia.org/wiki/Bugle_(instrument)

will the extra fuel dumped by face lift r53 for more "bangs and pops" also be a additional noise factor, many tracks in the uk have a limit which modified minis often fail, always on the drive by, but never the static test

for those with cone shaped tail pipes, could it be a worthwhile exercise drilling holes in the cone?, weight saving being another bonus

 

Re. reducing the volume by damping the tail-end of the car, in addition to the hatch itself and the bottom of the boot, another major source of sound coming in is through the plastic tail-lights.

I used carpet scraps to pretty much fill the half-cubic-foot volume in front of the tail-lights, and it helped quite a bit.

 

There's not doubt about this, OldRick! I stuffed a large part of my weight savings back into the rear end of the car to dampen offending road and exhaust noise. As I said, the Milltek is effectively silent inside my rear-seatless car nowdays. However, a drone is a drone, and it seems to penetrate everything...

 

Perhaps Active Noise-Canceling could be used to minimize the resonance?
Maybe this could be adapted to the car sound system?
http://www.headwize.com/projects/noise_prj.htm

Worth a try.

 

Here's a sketch of my current idea:



The resonator will be adjustable until I find the best length. It could be as long as 18".