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."
Likes Received