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Want a smooth, quiet exhaust?

  #1  
Old 06-05-2013, 05:41 PM
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Wink Want a smooth, quiet exhaust?

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
 
Attached Thumbnails Want a smooth, quiet exhaust?-imag0024_zps042a479b.jpg   Want a smooth, quiet exhaust?-imag0027_zpsf472ee28.jpg  

Last edited by RobinsonRicer; 06-05-2013 at 11:01 PM.
  #2  
Old 06-05-2013, 06:34 PM
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Location: Olympia, WA
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Excellent! Gotta put the 94-95 lips on it to hide that huge pipe now.
 
  #3  
Old 06-05-2013, 10:54 PM
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Location: New York
Posts: 731
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Yeah, I was looking for something to hide it. I gotta strap it as high as I can to the spare tire well. It looks kinda funny now. That may be a cheap option. Thanks.
 
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