Problem - Exhaust drone: Solution side branch, aka 1/4 wave resonator, or Helmholtz resonator

Problem: engine drone 3500-4000, it is quite bad through that zone, so lets say 3750 rpm + or - 250 rpm.

I've been doing some research into side branch or 1/4 wave resonators - closed "J" shaped pipe of SPECIFIC length added to the system.

Some people also use Helmholtz resonators (pipe leading into closed muffler)

Each of these has their own formula (no, I have not researched each's formula yet) The reason I have not checked into both thoroughly yet is packaging of a Helmholtz resonator, and placing which ever device in as optimal a location** as possible - close to the source/engine.

So based on optimal placement, I was thinking of tapping into the very beginning of my Brave exhaust(similar to system below) with a 1.50-1.75" diameter pipe, and running it the prescribed/calculated length back....

I was told I'd need "30 inches or so" I don't trust this - I'd prefer some help with the calculations...

So what length pipe do I need to target 3750 rpm??

My Honda S2000 has one on the factory exhaust. Its just a 6" or so pipe welded at 90 degrees. Its oval and about 2"x1" in cross section.

Read this:

http://www.mh-audio.nl/acalculators.asp#showcalc

Just the first I found.

The calc is easy.

The location along the pipe is not critical.

However, you need to know the frequency. It may not simply be 3750 RPM (62.5 Hz) but some associated frequency being excited by that driving frequency.

If you can borrow a spectrum analyzer that would be ideal, I've done this to an intake and it works great, but you need to know the frequency otherwise you're just guessing.

There may be an App which can get you close, not sure.

Here's the math to calculate the length:

https://www.performancetrucks.net/forums/gm-engine-exhaust-performance-21/how-build-exhaust-resonator-tube-eliminate-drone-489463/

tuna55 wrote: Read this: http://www.mh-audio.nl/acalculators.asp#showcalc Just the first I found. The calc is easy. The location along the pipe is not critical. However, you need to know the frequency. It may not simply be 3750 RPM (62.5 Hz) but some associated frequency being excited by that driving frequency. If you can borrow a spectrum analyzer that would be ideal, I've done this to an intake and it works great, but you need to know the frequency otherwise you're just guessing. There may be an App which can get you close, not sure.

If you're looking for the frequency of the buzzing: Briggs and Stratton Tachometer

Someone on the forum here posted an in depth article on how to use one, and I've been fascinated ever since. The B&S tach is actually a Treysit Sirometer. http://www.treysit.com/12.html

I just ordered one! lol

tuna55 wrote: However, you need to know the frequency. It may not simply be 3750 RPM (62.5 Hz) but some associated frequency being excited by that driving frequency.

Most likely it'll be primary exhaust pulse frequency:

(rpm / 60) * (number of cylinders / 2)

Would a set of capped dumps do the trick?

I'm building a 5.0 powered MGA and have heard that the 5.0 likes to drone at certain speeds/rpm. I was planning on running a set of side dump coming off of the exhaust pipe just before the mufflers and exiting just behind the doors.

Thinking that the pipes might be a reasonable version of the j-pipe and the caps in place would work the same. When they're un-capped at the strip the droning would be the least of the noise . . . maybe?

I know nothing—

This is all you'll ever need to know

This thread is relevant to my interests. Thank you to everyone who has posted so far.

Do some math to get close-then build it with an overlapping section that you can slide and clamp to find the perfect length.

I like the idea of a slip fit section and tune as needed.

A bigger picture question, is this anything like header pulse tuning where there is a best length, but multiples of that length will work, but not as well? In other words, if there isn't room for 30", would 15" cut some of the drone?

Like she said, length is critical. A half length pipe would target the wrong frequency.

I will be adding one of these to my Mustang some day. Last time I calculated it 28" was what I needed. Plan was to run them side by side parallel to the rear axle and just in front of it, that is the widest point of the exhaust routing that doesn't have a gas tank in the way.

Sent all of this to my co-driver. Thank you so very much.

TeamEvil wrote: I'm building a 5.0 powered MGA

And the build thread is where? You can't just say E36 M3 like that and not post a picture of the project.

ae86andkp61 wrote: I like the idea of a slip fit section and tune as needed. A bigger picture question, is this anything like header pulse tuning where there is a best length, but multiples of that length will work, but not as well? In other words, if there isn't room for 30", would 15" cut some of the drone?

A 1/4 wave length should also act as a 3/4 wave for a 3x frequency, and a 5/4 wave for a 5x frequency, etc. But to hit the drone frequency, it would work the opposite of headers, where you'd need to add length. So if your 1/4 wave is 30 inches, you would get less benefit from 90 inches, less yet from 150 inches, etc.

more research

That pic may answer my question - can the resonators be placed before the muffler? the truck forum posts state the resonator pipes must be added behind the muffler and the discussion peters out after a Corvette driver asks if it couldn't be before the mufflers instead. The No Drone diagram shows the resonators are more effective the closer to the exhaust manifolds they are.

If your a corner carvers member Blaine fab gave out the tech(formulas). In a exhaust/race muffler thread. Albeit a couple of years ago.

In reply to Driven5:

Does that mean that a shorter multiple would have some benefit?

This is the exhaust system off a 540i. As you can see there are multiple chambers for different frequencies and they're all before the muffler.

In reply to MrJoshua:

The sound wave travels up the pipe and back down to rejoin the exhaust stream. The reason a 1/4 wave length is used, is that up and back is subsequently 1/2 wave length. As also noted, 3/4 would come back at 1.5, and 5/4 would come back at 3.5. To cancel the sound the waves must be 1/2 wave length out of phase. So the up and back length must result in some multiple of the wave length, plus 1/2. The shortest you can go and have this work is 1/4 wave.

If you cut the length in half that becomes a 1/8 wave length, so you'll only end up 1/4 out of phase. Which I'm pretty sure won't really cancel much (any?) of the target frequency. It will however nicely cancel the frequency for which it does act as a 1/4 wave, which would be ~2x of your target.

.

This thread does have me thinking that an easy and clean way to implement this may be to get a common aftermarket reversible 2:1 muffler. Rather than 1 in and 2 out, set it up with 2 in and 1 out. Run the exhaust in one of the inlets, and out the single outlet, and run a 1/4 wave pipe up off the second 'inlet'. With a clamped connection, it would be easy to test multiple pipe lengths to tune it 'just right'...Or even have an 'quiet around the neighborhood/town' pipe and a 'highway drone killer' pipe, depending on what you're doing that day.

Driven5 wrote: In reply to MrJoshua: have me thinking that an easy and clean way to implement this may be to get a common aftermarket reversible 2:1 muffler. Rather than 1 in and 2 out, set it up with 2 in and 1 out. Run the exhaust in one of the inlets, and out the single outlet, and run a 1/4 wave pipe up off the second 'inlet'. With a clamped connection, it would be easy to test multiple pipe lengths to tune it 'just right'...Or even have an 'quiet around the neighborhood/town' pipe and a 'highway drone killer' pipe, depending on what you're doing that day.

I've not really looked at the inside of many mufflers, but the ones I have looked at wouldn't appear to like being run in reverse.

TeamEvil wrote: Would a set of capped dumps do the trick? I'm building a 5.0 powered MGA and have heard that the 5.0 likes to drone at certain speeds/rpm. I was planning on running a set of side dump coming off of the exhaust pipe just before the mufflers and exiting just behind the doors. Thinking that the pipes might be a reasonable version of the j-pipe and the caps in place would work the same. When they're un-capped at the strip the droning would be the least of the noise . . . maybe? I know nothing—

If the length is right I don't see why it wouldn't work.

These take the resonance out of my Z4M coupe and replace the rear mufflers completely. You can stick a small resonator in line with whatever else you have to cure resonances.

I've looked into this extensively for my '91 MR2, which drones badly between 3200 and 3800 or so RPMs. I've tried several different muffler and resonator combos, and some are more effective at masking the drone, but nothing has gotten rid of it. From what I've read, "drone" is the result of the noise made by the exhaust gas moving through the tubing, combined with the interior of the specific car - which is why MR2's have really bad drone problems, but it's less severe in other cars with the same engine.

From a couple sources (mostly Mustang, as the 5.0 seems to have some pretty bad drone problems in Mustangs) I found a few different formulas, and they all come out to around 30 or so inches for that RPM in a 4 cylinder.

Placement is important, because the further up the exhaust stream you go the stronger the sound waves are, and the better they bounce back and cancel each other out. There have been people who have reported success with placement further down the stream, including after the muffler.