I had put a thread up asking for some advice but I went ahead after doing some research and installed this to quiet my exhaust. 1/4 Wave Branch Resonator. It worked amazingly. The exhaust sounds smooth and theres no more drone at highway RPMs.

There are a lot of threads on it. Nothing actually "official" I could find. But I risked it since I couldn't find anyone who said it didn't work. Making it was pretty simple. I had my welding mechanic put it together and it was dirt cheap to do. So here are the instructions for anyone who's tired of a loud, droning exhaust.


Background:
You'll find when doing this that the fewer the cylinders the deeper exhaust frequency at the same RPM. The concept is to take the frequency that you want to remove and to turn the sound waves around to create destructive interference. You can think of a blowing over a bottle (Helmholtz Resonance), this is the same concept that an Intake Air Resonator uses- the box and tube thing you toss when you put in your shiny new cold air intake.

So this works the same way. Exhaust waves pulse over it, producing a frequency. The longer the tube, the lower the frequency. There is no negative affect on performance since it is closed off. I'd even say it improves performance. You get a smoother exhaust flow.


Process:
First you have to do some math. Take the RPM you want to quiet (typically highway speeds) and multiply it by the amount of times your engine's exhaust valves open per rev. So for a typical 4 cylinder its 2 times. For an 8 its 4. Then divide by 60 (to convert from rpm to rps)

So for my car I want to get rid of the highway drone at 2700 rpm. Its a 4 cylinder.

(2700*2)/60=90hz I decided to go with 95hz

This is your frequency. You can verify this by finding an app for your phone or computer that measures frequency.

Now you need to find the speed of sound. The speed of sound varies with temp. I measured 100˚F at the point where I put the resonator. That was about 343 m/s.

You take the speed of sound and divide it by the frequency. This gives you the standing wave length. For me is was about 3.1 meters.

You then divide this length by 4 to get the length needed to 180˚ the wave.

Mine was 37". Its a bit long but fortunately you can bend the resonator. It just needs to first be extended at a 90˚*angle from the exhaust. After that you can bend it to be parallel with the exhaust or however it will fit.

Cap off the end with a flat cap and angle it upward to keep water and deposits from building up. Ideally you want it after the muffler. I have a glasspack and a flowmaster so I put it before the flowmaster but after the glasspack.

Results were about a 20db drop and a significantly improved cabin at highway RPM. Taking off is a smooth, sporty sound and after 3000 the roar comes back. Its nice. I just wish mine didn't show as much. But its not bad. I can get a hanger and move it up.

Enjoy and let me know if you try this.

BEFORE


AFTER

 

What the ****? That has to be an eye sore looking at the car from behind.

 

I have never seen that before.

 

Someone's never seen a stock S2000 exhaust. The nub before the mid muffler.



Muscle car guys commonly call them "J pipes"

 

that is retarded.... like wearing a helmet 24/7

 

haha, it does a beautiful job. If you can hide it, it does a great job. Whos gonna be sticking their head under your car? I dont care if you can see if even, its worth the exhaust sound. It did that well.

I knew s2000's had it as well as a few others. I think the nissan 350/70z's have it. In the s2000 exhaust thats more for high frequency. This is a long one for low frequencies.

 

Shoot I may have to try this, I'm so tired of mine being crazy loud, as long as you can hide it under the car who cares what it looks like. Do you have any videos of it running?

 

Not yet. I wish I got a video of the before too. I just got the frequency and decibel readings. But Ill make a video tomorrow.

It did great because starting off I get a slightly quieter exhaust but still a great sound then as I climb to highway RPMs it starts to go silent. So cruising from 2750 to 3150 its so quiet. Thats where I tuned the resonator to. And I have a flowmaster 40 on the end of my 2.5" exhaust. After 3000 the car roars again and I get a very aggressive but not overwhelming sound.

I think if you make it right it can actually look pretty good. Stainless steel, clean welds, parallel to the pipe and itll look good.

If you do a google search you can find a lot. Some are Helmholtz resonators which are slightly different. This is a 1/4 wave so its a long pipe rather than bottle necked.

 

So it only works through a range? Not sure I like that at all.

 

You can install a few tuned to different frequencies if you really wanted to. But it does help with the overall exhaust as well. I saw anywhere from 5-20db drop throughout the entire RPM range. The idle is ridiculously silent. But the biggest is at the highway rpms where I tuned it for.

If you want any more than that, go to a smaller exhaust or invest in sound deadening- from all the solutions Ive found. You can also put an IAR on the intake pipe if you have a CAI or RAI and removed the stock intake pipe. That will quiet the engine and Ive found thus quiets the exhaust a bit actually.


On a side note, looking at the design of that last dual exhaust photo, thats contrary to the design I specified as it has to be 90 degrees from the exhaust pipe- perpendicular. Otherwise you'll get pulses that interfere with the flow a bit.

 

What the hell Bobby!?