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Exhaust Drone Damper Weights? Anybody?

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#1
Has anyone used damper weights for cancelling drone in their exhaust? It seems like there is enough questions about drone in aftermarket 3" exhausts (e.g. MBRP), and the only good way of getting rid of it is a Helmholtz resonator or replacing the muffler/resonator. But it also sounds like dampers might be another good option, and thus my question.
 
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San Diego
#2
I've been thinking about this lately... not the damper weights, but how to get rid of some of the drone. I have a MBRP and it does drone.

I was looking at getting a resonator but would be open to hear other options.

How would the damper weight work?
 
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#4
Some of the drone has to come from the poly mounts folks put on. Stiffer mount is gonna transmit vibration into the vehicle floor. I might drop down to just 2 polys and see if it gets better.
 

LilPartyBox

1000 Post Club
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#7
I think it's weather related too. My MAP drones waaay worse in the rain and not at all when it's blazing hot. I'm hoping that adding the down pipe will add a bit more rasp and mitigate the drone some. Now a damper would reduce exhaust flow to reduce sound. I know the consensus around here is that catbacks do nothing for power but this still seems counterproductive - but effective at reducing drone while on long road trips. The struggles will be finding one that fits and having it properly welded into place. I can lose the 1/2 HP but i don't know about going through the trouble of having it fitted.
 
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Laguna beach
#9
I think it's weather related too. My MAP drones waaay worse in the rain and not at all when it's blazing hot. I'm hoping that adding the down pipe will add a bit more rasp and mitigate the drone some. Now a damper would reduce exhaust flow to reduce sound. I know the consensus around here is that catbacks do nothing for power but this still seems counterproductive - but effective at reducing drone while on long road trips. The struggles will be finding one that fits and having it properly welded into place. I can lose the 1/2 HP but i don't know about going through the trouble of having it fitted.
I'm going to geek out below because I am a big nerd.

The drone changes in the rain because the speed of sound changes based on temperature and humidity. I wrote up an entire thread about identifying and mitigating drone using acoustic theory. The weights aren't going to mitigate the drone for anything below ~200hz, and your target frequency, if you fire up a spectrum analyzer, should be 106.67hz. At 3" (unnecessary for stock turbo) the pipe is wide enough to be actually making the existing noise louder; no muffler or traditional resonator is going to touch those frequencies because they'd have to be roughly 54 inches long. The weights work because they stop exhaust systems themselves from vibrating; that's not the issue with 3" exhaust on the Fist, the issue is that the pipe is wide enough to propagate the waveform in the ~107hz range making it louder. What you need is something to generate an inverse waveform to actively cancel it out (i.e. a "j pipe" resonator) or a helmholtz chamber (they are NOT the same thing!!) or a 54" long (at 150 degrees f). In many stock exhausts, the straight sections, pinches, and some of the bends are engineered to disrupt the propagation of those waves. When you go to a 3" exhaust with mandrel bends, a lot of those stock "disruptive surfaces" get straightened out to better fit the pipe under the car, allowing wave propagation, which is why 3" exhaust drones on the FiST.

I'm going to guess you all drone worst at ~3200 rpm under load with 3" mandrel bent exhaust.

(3200 rpm) times (2 cycles per revolution) divided by (60) = 106.6666667hz. You can validate this with a spectrum analyzer app on your phone.

Now, 106.67 hz is our target frequency to cancel. What can we do?

For the sake of simplifying things, I'll call the speed of sound 1130 feet per second at 72 degrees f.
Now, 107(rounding up) / 1130 = 10.5607476
Wave length = 10.5607476

That means a traditional muffler at 72 degrees f targeting 107hz needs to be 10.5607476 feet long. That means you can either run semi truck smokestack style mufflers, sidepipe mufflers 10 and a half feet long, or utilize a helmholtz chamber or a quarter wave resonator. The helmholtz covers a broader frequency range but is *much* harder to make and fit under the back of the car, so a quarter wave is usually preferred, as it targets a narrower range, but with greater attenuation.

To figure out how long a quarter wave resonator you need, take your wavelength (10.5607476 ) and divide it by 4 (2.640186916) then multiply by 12 to get inches (31.68224299).

If you need to cancel 107hz drone at 72f you need a side branch resonator 31.68224299 inches long, preferably 2.5 or 3 inches diameter, but 2" (mandrel bent!!) is the biggest you'll likely fit.
 
Last edited:
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542
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#10
I've been thinking about this lately... not the damper weights, but how to get rid of some of the drone. I have a MBRP and it does drone.

I was looking at getting a resonator but would be open to hear other options.

How would the damper weight work?
If that was intentional (open to HEAR other options) then big props :p
 
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542
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Laguna beach
#11
Some of the drone has to come from the poly mounts folks put on. Stiffer mount is gonna transmit vibration into the vehicle floor. I might drop down to just 2 polys and see if it gets better.
The poly mounts create an oscillation at like 60hz, which you feel more than hear - the second mount back from the front changed back to a stock rubber one is likely to mitigate that 60hz due to where it mounts to the car. Vibration there oscillates the care like the body of a guitar - have someone get in the car and get underneath and tap at each of the exhaust hangers on the car body and that one in particular will sound REALLY loud.
 
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Menasha
#13
Would there be any benefit of replacing the front resonator with a y pipe and 2 pipes running right next to each other? Just spitballing random ideas.
 

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