Reducing exhaust drone?

[miketyler 15]

I have a bit of drone around 1800-2000RPM. Was considering replacing the Pypes RacePro's with something different but came across an interesting thread on reducing drone. Some guys over at the Corral are reporting success reducing or eliminating drone by simply adding additional exhaust length. Its the way they are doing it that seems unusual.

 

Have any of you tried these type solutions?

 

http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone-16.html

Edited March 23, 2014 by miketyler

[LindenBruce 27]

Interesting. I too would like to know more. B.

[BuckeyeDemon 211]

i replaced the factory muffler and resonator with the largest maganflow on my truck. it had a horrible drone at around 1800 rpm (changes with heat/temp).

 

so, i made my own stub (not quite the same as a helmholtz). it is adjustable and i've altered the length and adjusted the diameter to alter the q-factor. there was a sweet spot in terms of length and the diameter definitely controls the bandwidth. it absolutely works.

 

 

 

 

just a few of the test stubs.

 

 

 

two simulations (comparing stub diameter)

 

[latoracing 256]

Interesting, especially the band width graph. Guess the expanding gasses "reflecting" off of each other helps with the overall frequency, or the canceling of certain sound waves. I wonder if this could be incorporated into a muffler for those system that are space constrained, ie. convertibles.

[miketyler 15]

Quoted from another internet source in that same thread

 

"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. "

 

Here's the math:

 

 

[BuckeyeDemon 211]

Interesting, especially the band width graph. Guess the expanding gasses "reflecting" off of each other helps with the overall frequency, or the canceling of certain sound waves. I wonder if this could be incorporated into a muffler for those system that are space constrained, ie. convertibles.

 

a lot of muffler designs incorporate this already. the factory muffler on my f150 has a helmholtz chamber (not the same as a stub, more wideband). Corsa's fundamental design concept is basically the stub, but packaged inside a muffler.

 

there are helmholtz resonators, stubs all over the intake track of my f150. you can also see them used on intake tubes (before hitting the throttle body) on camaros, mustangs, etc. if you've heard of a "CAI" or cold air intake for the late model vehicles, they are basically removing all of these factory sound attenuators. and of course you hear intake noise (which a lot of people mistake for believe more air is entering the engine)..

[BuckeyeDemon 211]

A table with the math already done based on what RPM you experience the drone.

 

temperature has a big impact...

[miketyler 15]

I would have to get a dB meter to see how my drone changes with temp. The assumption is that as exhaust gases and pipe walls get hot, the air travels faster increasing dB and drone? And yes, some integrated muffler applications:

 

SS Camaro:

Edited March 23, 2014 by miketyler

[miketyler 15]

And good idea on the interchangeable length test stubs. Placement along the length doesn't seem to be a big factor though some think the after-muffler location dampens more of the total resonance but tends to be more challenging for fit.

Edited March 23, 2014 by miketyler

[BuckeyeDemon 211]

I would have to get a dB meter to see how my drone changes with temp. The assumption is that as exhaust gases and pipe walls get hot, the air travels faster increasing dB and drone? And yes, some integrated muffler applications:

 

SS Camaro:

 

temperature changes the frequency where the resonance occurs (due to altering the velocity of propagation for sound as you mentioned). the drone seemed to peak around 1500 when i first started off down the road cold. after driving a bit, it shifted up a few hundred rpms.

[Fordrevhead 29]

Cool, thanks for sharing. I like the formula but testing to your ear is ultimately what counts. You could calculate then hit a pretty good range both ways with a telescoping snub pipe too.

[Rip Rock 13]

Very interesting topic. Wouldn't this kind of mod hinder performance?