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Old 06-01-2015, 01:06 AM
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Side Branch Resonator

OP:

I recently got straight pipes and there's a bad drone at 2k RPM. I've been looking online on ways to get rid of exhaust drone and I came across this "branch resonator" method numerous times and it seems to have good results. Many muscle car guys used it and its basically a dead end pipe that's closed off at the end extending from the exhaust pipe.

Not very knowledgeable about cars and I was wondering if this would work? I don't want to put out the money to do this and have it not work. Any input on this? Seems legit lol the pipe's purpose is to bounce sound back and hit itself, essentially cancelling the drone out altogether at the given RPM.

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UPDATE 6/2/15:

So after like 5 hours of research I figured out the steps on how to calculate the length you need for your branch resonator.

STEP 1: Use this to find your frequency of drone in hertz:

1. you will need two variables: the drone RPM, and # of cylinders your engine has.
2. divide the RPM by 60 to get the RPS, revolutions per second. You divide by 60 because RPM is revolutions per minute. 60 seconds in a minute so you divide by 60 to get RPS.
3. divide the # of cylinders by 2 to get # fired per revolution.
4. multiply RPS by # fired per rev to get the drone frequency in Hz.

1. 2,000 RPM and 6 cylinders
2. 2,000 RPM / 60 = 33.3333.... RPS
3. 6 cylinders / 2 = 3 fires per revolution
4. 33.3333..... RPS * 3 fires per revolution = 100 Hertz

STEP 2: The formula to get the length of your resonator is:

Wavelength = Speed of sound / Frequency
Resonator length = Wavelength / 4

We divide by 4 because we need a quarter of the wavelength for this to work. These are called side branch resonators or 1/4 wave resonators. I've seen both while doing research on this.

STEP 3: To get the speed of sound you need to find the temperature of your exhaust where you will install your resonator. To do this you will need to buy a temperature gun.

Etekcity Lasergrip 1080 IR Thermometer Etekcity Lasergrip 1080 IR Thermometer
$15 and I'll never use it again but oh well lol once you get the temperature use this calculator to get the speed of sound.

Speed of sound calculator

STEP 4: Once you get your frequency and speed of sound, this is the easy part. Just plug your data into the formula!

So for example, my exhaust temp was measured at ~240 degrees Fahrenheit and the speed of sound at that temp is 1296.637 feet per second. My above calculations gave me 100 Hz for my drone frequency.

Wavelength: 1296.637 ft per sec / 100 Hz = 12.96637 ft
Resonator length: 12.96637 ft / 4 = 3.2415925 ft

If I round that off its pretty close 3'3" long or 39" from the bend back. So the whole thing will look kind of like a J so from that bend to where it starts to extend straight out, have that be your length.



STEP 5: Go to an exhaust shop and get that resonator fabricated onto your exhaust

I read that it is important you have the pipe bent at a 90 degree angle and then straight just like the picture above. Also have the resonator piping size the same as the rest of your piping. After that you should be good to go! No more drone I'm not responsible for this working or not so do this at your own risk. These are just calculations and information I've found while researching and it has good results so I'm going to have to try this out haha

Here is a little diagram of my exhaust I made just to be extra clear. I was recently told that closer to the engine the better so if you have space behind your cat you should put it there



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UPDATE 6/4/2015:

Ok so I went to the exhaust shop today and got the side branch resonator done. It came out to be $150 so not too bad. The results..... Outstanding. It did exactly what it was suppose to do and it didn't change the exhaust note at all. Amazing grumble of a straight pipe with no drone. Perfect and I love it haha highly recommend doing this if you have an annoying drone but you dont want to give up your loud exhaust Here's a picture of the setup

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Last edited by phi1542; 06-11-2016 at 01:14 AM.
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Old 06-01-2015, 06:47 AM
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Re: Getting rid of exhaust drone?

huh...changing resonate frequency...
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Old 06-01-2015, 11:10 AM
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Re: Getting rid of exhaust drone?

Originally Posted by xci.ed6 View Post
huh...changing resonate frequency...
Do you think it will work?
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Old 06-01-2015, 11:15 AM
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Re: Getting rid of exhaust drone?

Always wondered what that was for, makes sense though. I don't have any experience with it, and I can't think of anyone else here who has done it....so you might have to be the pioneer.
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Old 06-01-2015, 02:13 PM
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Re: Getting rid of exhaust drone?

I've seen this work on muscle cars, but I've never done it personally.

I am running two resonators, after a long trial and error process. Ended up putting in the largest resonator we could find (about 2.5-3 feet) along with a 12" resonator behind that.

Front to rear pic.

Rear to front pic.

Dual Magnaflow.


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Old 06-01-2015, 06:07 PM
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Re: Getting rid of exhaust drone?

Originally Posted by RedRyder View Post
Always wondered what that was for, makes sense though. I don't have any experience with it, and I can't think of anyone else here who has done it....so you might have to be the pioneer.
Yeah I might have to be..... Shouldn't be too expensive anywho.

Originally Posted by spacemonkee23 View Post
I've seen this work on muscle cars, but I've never done it personally.

I am running two resonators, after a long trial and error process. Ended up putting in the largest resonator I could find (about 2.5-3 feet) along with a 12" resonator behind that.

Front to rear pic.

Rear to front pic.

Dual Magnaflow.


Mind the oil everywhere. Power steering rubber line has a pinhole leak. No time to fix it.
Yeah and the logic should be the same with any other car too. Even with yours it seems similar. I read for this branch resonator method longer resonators like yours and soon maybe mine, cancel out drone a low RPM and shorter ones cancel out at high RPM. Yours is about 4 feet so it would make sense that it canceled out yours. Does the exhaust tone still sound nice on yours? Like did it change at all or just no more drone?
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Old 06-01-2015, 07:29 PM
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Re: Getting rid of exhaust drone?

Originally Posted by phi1542 View Post
Yeah I might have to be..... Shouldn't be too expensive anywho.



Yeah and the logic should be the same with any other car too. Even with yours it seems similar. I read for this branch resonator method longer resonators like yours and soon maybe mine, cancel out drone a low RPM and shorter ones cancel out at high RPM. Yours is about 4 feet so it would make sense that it canceled out yours. Does the exhaust tone still sound nice on yours? Like did it change at all or just no more drone?
It quieted it down a little but I am not for loud exhaust any more. Here is a clip, sounds best with headphones or speakers with a sub.

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Old 06-01-2015, 10:16 PM
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Re: Getting rid of exhaust drone?

Originally Posted by spacemonkee23 View Post
It quieted it down a little but I am not for loud exhaust any more. Here is a clip, sounds best with headphones or speakers with a sub.

I have the same exhaust, except no resonators for me. Shocked at how much quieter it is. I have a bit of drone with mine sadly though
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Old 06-02-2015, 01:01 AM
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Re: Getting rid of exhaust drone?

Originally Posted by spacemonkee23 View Post
It quieted it down a little but I am not for loud exhaust any more. Here is a clip, sounds best with headphones or speakers with a sub.

I didn't know Magnaflow made N1 style mufflers lol wow that sounds nice. Really deep and no rasp! How's the drone on that after the big daddy resonator hahaha

hmm since no one seems to have it I guess I'm going to have to be the first to try it on here ._. everyone says it seems to make sense and it has good results. I'm going to call for a quote soon and try and save up some money for this! I'l report back once I get it done
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Old 06-02-2015, 02:05 AM
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Re: Getting rid of exhaust drone?

Does anyone know how I can find the temperature of my exhaust? I need to find the speed of sound to do this but idk how to check the exhaust temperature since sounds travels at different speeds at different temperatures. This formula was off of clubrsx so since they have a 4 cyl I'm not too sure on the accuracy of that for my v6.... I'm trying to search up a universal formula and have come across one. This has become a little more complicated lol I need to find the speed of sound and Hz it drones at o.o I would ignore the formula I have above. The formula i've been seeing around on google is: (speed of sound/frequency of drone Hz)/4 and you should get your length in feet i think? Not even sure if SOS is in m/s or ft/s... People have a problem with labeling measurements...........
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Last edited by phi1542; 06-02-2015 at 03:10 AM.
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