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12-01-2008, 07:58 PM #1
How To: Eliminate Drone / Cab Resonance!!!!
I fixed the drone in my truck with this. After reading some info. about drone on another site which I am a member, I decided to try this for myself. I cut about 15" off my tailpipe. (right before the bend to go out behind the wheel, so I cut out the last bend and the end) The drone magically was reduced. I later removed all the way back to the far rear hanger. There is no drone in the cab now.
For those interested in the reasoning, here is the idea behind it.
Drone in the exhaust is due to the engine driving the exhaust system at its resonant frequency. You can't get rid of it, any length of pipe has a natural frequency, but you can change the frequency (RPM) it happens at. Stock exhaust systems are tuned to put the resonant frequencies outside the normal RPM range the engine is run in. Sometimes they add resonators for this purpose. To raise the resonant frequency of a system, shorten its length. To do this, you can try tail pipes that exit by the wheel (like GM's have), or you can try a muffler with a shorter internal flow path. If you have a true dual system, you can change the natural frequency of the system dramatically by adding a balance tube. If you can't shorten the system, then you can add slightly less than one wavelength of pipe. This will also raise the resonant frequency of the system. The wavelength in a single converter system (not true dual) is a little less than three feet at 3000 RPM. Shortening the system by a foot or so will move a resonance at 2000 RPM to about 3000 RPM.
To move a resonance, calculate the wavelength at the RPM that gives the resonance you want to move. Then calculate the wavelength at the RPM you would rather it be at. The difference is the length of pipe you need to add or delete. Add to lower the RPM, delete to raise it. If you want to raise it but you can't cut any pipe out, then add one wavelength of pipe minus the amount you calculated. This will have the same affect. If you add or subtract a multiple of a wavelength exactly, you will not change the resonant RPM.
Wavelength = 1100 X 60 X 1/RPM X 1/4 X 1/2
Wavelength = 8250 / RPM
Wavelength = standing wavelength
1100 = speed of sound in air in feet per second
60 = convert RPM to Revs per second
RPM = RPM
1/4 = four cylender firings per revolution (make this 1/2 for "true dual")
1/2 = standing wavelength is half the wavelength of a "normal" wave
Most people know that placing the muffler closer to the rear reduces drone, but you can only move it so far back in a truck. The reason is not that the muffler is farther back though, it's because it is closer to the end of the pipe. Cutting off sections of pipe essentially does the same thing.1999 Sierra ECSB, 5.3L, 3.42 Locking Rear, True Flow Filter, Dynomax Super Turbo, Lund Genesis Tri-Fold, Bully Tailgate Lock
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