RE: Tuned Pipe throttle question
Tommy,
Setting up the pipe requires that you use a tachometer.
It is best done with the engine mounted in a test-stand!
You cannot trust your ears, to determine, if the frequency of the sound produced by an engine (hence the RPM), has increased or decreased.
Always write the RPM and also the conditions for every reading (how much was cut from the header), on a sheet of paper, or in a spreadsheet on your PC.
You will also need several tie-wraps (or reusable pipe-clamps), a metal tubing-cutter and something to de-burr the inside of the aluminium tubing, every time you cut it.
Select the prop; size model and make, with which you want to fly your model.
Take note that if you want to use a different prop, you will need to go through everything, all over again.
Mount this prop (for a .46 engine on a sport/pattern model, you would typically want one in the 10x6 - 11x7 range) on the engine.
You first must take the reading peaked (backed a bit rich), with the standard muffler installed.
Write it down!
Install the header, full length; and the tuned-pipe. The rear-end of the header must be pretty snug with the front-end of the pipe (leave just ~1 mm/.04" between them within the coupler).
Batten everything down with tie-wraps (or pipe-clamps). You will not need to undo the end that's on the tuned-pipe's front; only the one on the header.
Now, 'play with' the high-speed needle and take an RPM reading (as always, peaked and backed up a bit); and write it down. Repeat this after every cut that you make from the header.
Use the tubing-cutter to cut short lengths from the header. Don't forget to de-burr the inside of the rear-end of the header.
Make the cuts 1/4"(6 mm) at a time, taking RPM readings again and again; as long as RPM is increased noticeably. When the rise becomes marginal, make the last cuts only 1/8" (3 mm) at a time.
And now, this is very critical! Every time; after measuring max-RPM, close the throttle to idle; open it to full and make sure the engine reverts to the same, 'on the pipe' RPM!
With the setup at its original, full-length, it will probably not be a problem, but as you continue to cut and test; you will eventually reach a point where you would need to momentarily lean the mixture, or to pinch the fuel-line, for the engine to come back to maximum RPM.
Once this happens, the setup has become too short to fly with! Insert the last piece of aluminium tubing, that you cut from the header into the middle of the silicone coupler and reconnect the whole installation.
Check RPM and 'after idle' recovery to full', to make sure it is as it was.