Exhaust Resonance

Exhaust drone is due to the engine RPMs matching the exhaust systems resonant frequency. The resonant frequency can be pushed outside of the engines useable RPM range which would eliminate the drone which is often associated with improperly setup exhaust systems. Manufacturers typically build their vehicles this way from the factory to avoid the annoying drone during cruising.

Shortening the exhaust piping raises the RPM with which resonance occurs at, lengthing lowers the RPM. It is possible to move the drone by first calculating the current RPM resonance happens at, then the RPM desired. The difference between the two is the amount to add or subtract. Add if the desired RPM is lower, subtract if the desired RPM is higher. Multiples of the current wavelength do not modify the exhaust resonance. Thus, to raise the RPM and still add piping it is possible to first add one wavelength of the current resonanting pipe minus the desired wavelength. This achieves the same goal. In addition, adding an H or X pipe to a dual exhaust system changes the resonant frequency greatly.

NOTE: For a true dual exhaust, input half the total number of cylinders. The "cylinders per pipe", not the total engine count.