First you need to find the frequency at which the pipes resonate, which is where the drone will occur. Record the exhaust sound via mic through the rpm range. You can use a free program called Audacity to analyze the audio clip.

Here are some links for more information on the setups and how to calculate the pipe length:

Exhaust Drone is Completely Gone - Corral.net : Ford Mustang Forums

"In Audacity, you use Analyze>Plot Spectrum.
Select a very small clip first, look for the peaks in level and select around that.
Then do Analyze>Plot Spectrum...

Go To Edit>Preferences menu and play with settings for Spectrograms and you should be able to spread out the axis.

That huge peak you see is your drone.

The math is basically the speed of sound (1100ft per sec) divided by the drone frequency (you calculate it) then divided by 4. This will be the length needed in feet."

Hello!

I am very interested in your method of curing the exhaust drone. I have an S2000 with an HKS Hi Power exhaust that drones around 3-4k. I was reading about your side branch resonator or helmholtz resonator. I noticed that the stock S2000 exhaust has one of these side branch resonators. What I'm wondering is, how did you figure out the diameter and length of the resonator. You said you put a 27+ inch pipe on each muffler to eliminate the drone. How did you calculate this? I'm very interested in curing the drone.

Thank you