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.

----------------------------------------------------------------------------------------------------------------------------------------------

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.

$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

----------------------------------------------------------------------------------------------------------------------------------------------

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