RE: OS 140 RX pipe tuning procedure
We really don't have an FAQ or manual for tuning a pipe. Basically, though, tuning will be different for each airplane/engine/prop/fuel combination. Change the prop, retune the pipe.
You need to make sure the engine is well broken-in, first. If not, you'll waste your time working with the pipe.
To tune the pipe, you start out with it too long. Start the engine, run to full throttle, lean to peak RPM and record the RPM. Stop the engine. Shorten the header 1/4". Restart, retune, record. and stop. As long as the peaked RPM increases, you can shorten the pipe. When you get a decrease in RPM, or no increase, stop. Lengthen 1/4" and fly. Listen to the model. You'll definitely hear it when it's on the pipe versus not on the pipe.
A pipe that's a bit short for the RPM will make the engine trend lean. A long pipe will trend rich. Lower RPM means longer pipes, and vice-versa.
Please be aware that pipes are tuned for a rather narrow RPM range, so you need the pipe to be tuned for where you need it. Usually on uplines in Pattern aircraft, and level on racers.
Hope this helps.