I've read the procedure for tuning a pipe. 

Whose procedure is it ?  Can you post a copy or link ?

Tuning procedure is explained at
www.macspro.com under "Tuning Info"

This is the Mac's Products web site.

  This has no relationship to stunt, and is almost certainly not going to yield good results. We are not attempting to improve the power, if anything, it reduces the power for a stunt engine. The intent is to tune it well off-peak in normal level flight, so the pipe operates as a regulator.

    Brett

Not close to flying yet!  This is the extent of my progress so far.  If it looks like it might work, I'll think about building a model for it.

Floyd, since you're an engineer and a guy who at least knows how to hold a violin, here's the basics as I know them.  Randy or Brett can point out where I'm wrong:

A tuned pipe works pretty much the way the air column on any wind instrument works -- pressure pulses go down the column, reflect off the end, come back, and then interfere either constructively or destructively with the pulses being produced at the head of the column.

In the case of a clarinet, flute, or whatever, the pressure pulses affect the sound being produced.  In the case of a tuned pipe that's tuned for maximum power delivery, the pulses are timed to (hopefully) supercharge the engine both by making a vacuum when the exhaust opens to enhance scavenging, and by making a positive pressure right before the exhaust closes to provide a bit of supercharging.  Adjusted correctly, the pipe is aiding the engine at the intended "best" RPM.

You don't want maximum power delivery.  You want speed regulation, or better yet, you want the engine to speed up under load.  In our case, then, you want the pipe to be adjusted longer than is "correct" for the jittering, excited racer standing next to you.  You want the pipe to be adjusted so that at your desired RPM it's not really helping much at all, but such that if the engine speeds up a bit it actually interferes with the correct engine operation (by delivering pressure on exhaust open and/or vacuum on exhaust close); of course if you do that then if the engine slows down a bit it'll start helping.  Moreover (and I don't understand this bit at all) if you run the right sized (smallish, compared to maximum power output) venturi and run the engine a bit rich, then when the engine gets loaded down the pipe will make it run faster.

Overall, the effect is that your airplane doesn't necessarily go faster than it would have, but rather that it flies at a more constant speed than it would have otherwise.

I've seen Brett Buck say that the first thing you do is set up the pipe the way Randy says, then never touch it.

The basic, over-simplified notion, which I'm just regurgitating from things I've read, is that the motor has a "happy speed" and the pipe has a "happy speed".  If they match, then the pipe will help keep the motor at that speed.  But the pipe doesn't have enough control authority to keep the motor at the pipe's happy speed if that's different from the motor's happy speed -- so if they don't match, then it'll be the motor that's in charge, and the pipe will just be adding weight.

 no one has mentioned that a pipe for stunt has a much diff defuser cone than a pipe for speed ,for stunt u need a pipe like Randy sells with a short defuser cone  i forget the degree angle  . a speed pipe comes on it just wants to go faster  Randy's pipe acts like a brake by killing the power past a certain RPM to keep it from running away.

When I flew my Humongus/.46LA/Mac's Muffler, when it was running just right, it sounded "like the wheels were going to fall off". Even with the stock .46LA muffler, they can make the same noise, often mistaken for pre-ignition. But also often running too lean. MUST have some "head room" when toward the top of the hemisphere...meaning, rich enough to not sag lean overhead.

I've heard the same thing with my piped .46VF. Can't say if I recall hearing that with my PA .51 or not. But Dirt wrote about that "knocking" sound  a few times with the dreaded .21FP, if I remember right.  Steve