Or you can use a Helmholtz resonator aka 1/4 wave resonator and tune out certain frequencies aka drone.
It's what I did with my Evo X. I used Decibel X Pro on my phone and recorded the drone sound and got the exact frequency from the app. Then it's a pretty basic math equation to figure out the length of the tube you need. Width should be roughly 1/2 the primary tube give or take.
-RPM that drone occurs at
-Number of cylinders
Y= RPM of your drone (ex- 3400rpm)
X= Frequency of drone (ex- 121.46 Hz)
You will also need to know the speed of sound. It varies with temperature, but is generally accepted to be 343m/s.
f = RPM * pulses/rev * (1/60)
f = Y rev/min * 4 pulses/rev * (1/60)
f = X pulses/second = X Hz
λ = v/f = speed of sound / frequency
λ = (343m/s)/(X Hz)
You want a wave 180° out of phase with your drone frequency so divide the wavelength by 4
If you have basic fabrication skills and equipment you can make one in your driveway (welder and hole saw) or take the measurements into an exhaust shop and have them weld it on for you.
