Exhaust Drone


        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
GN'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

                or

      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

Submitted by: Richard Kwarciany kwar at FNAL.FNAL.GOV

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