Crash, I know that there are alot of opinions on pipe tuning and they vary from time to time . The Pattern guys were basically using a long pipe to gibe lots of torque and swinging props designed to give them a good constant speed in all directions, with the torque range for pulling veritical manuevers. In pylon , we are wanting speed so the long pipe isnt quite what we are going for, we want RPM and speed. what you need to do is figure out what diameter and pitch is going to be correct for the airplane you are flying. When you finally get that magic size, buy a prop that is smaller in diameter for tuning purposes.
Mount the engine and pipe combo on a good test stand and run the engine at WOT and get a good RPM reading at peak. Now comes the fun part, start cutting the header length 1/4 inch at a time and re check the RPM after each cut! you should see and increase in RPM .the RPM risesas the header gets shorter. When the RPM peaks or doesnt improve any, I would add 1/2 " back to the rubber coupler and race it at that length. You need to remember that the pressure tap needs to be located at the largest diameter of the pipe. You can NOT locate it at the header because it will be a negative pressure at that location and cause the engine to go lean when it comes on the pipe. My experience is from the mid 80's when we were running tuned pipes in Q500 before the rossi became the standard engine. we also ran crank case pressure but pipe pressure will work just fine.
I hope this helps some but I would like to see if anyone else has a different method , I am sure there are others and maybe they can give a better idea.
Good luck