Exhaust J-pipe to reduce drone

[John Larkin]

Ever seen anything like this? I think it is interesting and considering how much drone most of us have, this has promise. If I could figure out how it works that is.....

http://ohiorides.org/forum/showthread.php?1609-removing-exhaust-drone

 

[John Larkin]

And then I found this:
knock out a drone frequency

Simply put, the formula for determining the length of a Helmholtz chamber used to knock out a drone frequency is:

1100 feet/second (roughly the speed of sound) divided by the target frequency you're trying to eliminate (in cycles/second) gives a full wavelength in feet per cycle. Divide this number by 4 to get a quarter wavelength (a much more convenient length to work with). Finally, multiply this wavelength by 12 to get the chamber length in inches.

Tap this length of pipe into the exhaust tube, making sure the other end of the pipe is capped off, and you have a Helmholtz chamber that takes the annoying frequency and bounces it back into the flow 180 degrees out of phase, knocking it out. The results: no more drone.

 

[John Larkin]

Now the question becomes: how do you measure the frequency of the offending drone?

 

[xrunner123]

I've been wanting to try this for years. It's also called a 1/4 wave branch resonator- do a google search. You can calculate resonant frequency but if you must measure it you can record it and analyze it with FFT software- they have some free ones that audiophiles use.

 

[KEVINS]

There was another thread on this late last year and there may be more info in it..

ks

 

[Toady]

Now the question becomes: how do you measure the frequency of the offending drone?

Mr. Clark might know a way or two...

You could use an RTA app for iOS or Android devices. They're not lab-perfect, but I've used my wife's iTouch to ring out PA systems.

 

[Phoneguy]

I saw an article about this about a year and a half ago, one of the setups was adjustable so you could move it until you liked it, then cut and weld.

I went with Dynomax VT mufflers, very happy !! Can actually have a conversation in the car at normal levels when cruising 55-65 mph, dont NEED to crank up the stereo either, but I do like my music loud.

Bryan

 

[John Larkin]

Found more information, pretty interesting.

I was playing around and graphed the equations for the exhaust.

The frequency note of an engine = ( RPM's ) x ( number of cylinders ) / 120

Which looks like this: This graph shows the frequency note that the exhaust produces as RPM increases.

Then using that equation, set it equal to the frequency produced by the branch resonator.

You have to use a calculator to find the speed of sound as temperature changes. I used 150 degrees fahrenheit. Here is the link for the calculator

Resonator frequency = ( Speed of sound ) / ( Length of pipe ) / 4

Set them equal and solve for Length. RPM is then your explanatory variable.

Then you get a graph like this. RPM's is on the horizontal and the proper length of pipe for the branch resonator is on the verticle axis.

So pretty much you find out the RPM where the drone is loudest and use the second graph to find the proper pipe length for a resonator.

To find the exact length, Take 54,491.4 and divide your RPM into it. Say your drone is at 1900 RPM. Take a calculator and type 54,491.4 / 1900 and it will equal 28.679 inches.
_________________

 

[fitz3820]

I saw an article about this about a year and a half ago, one of the setups was adjustable so you could move it until you liked it, then cut and weld.

I went with Dynomax VT mufflers, very happy !! Can actually have a conversation in the car at normal levels when cruising 55-65 mph, dont NEED to crank up the stereo either, but I do like my music loud.

Bryan

Which system are do you have with the VTs? I have a 2.5" dual exhaust and am looking at VTs on my GN.

 

[DavzGN]

Wonder if it work connecting one to a dump pipe?

David

 

[fitz3820]

Which system are do you have with the VTs? I have a 2.5" dual exhaust and am looking at VTs on my GN.

Nevermind - did a search "Pypes 2.5" system with Dynomax VT mufflers instead of the Racepro mufflers that come with their system."

 

[mtocrs1]

I had a set of J-pipes on my G8 GT and they eliminated all drone. Different car, same problem. It works. The system I had on my G8 was from Solo.