A good discussion, Crist. Thanks for getting it going.
Tim, Crist's point about density altitude is right on. It's easy to see how it could be mis-interpreted but he's right. In aviation lingo high density altitude means less dense air (as in the mountains) and vice versa. Pretty simple language confusion.
Item 13 I suspect refers to the possibility of pushing the pipe in so far as that you start re-ingesting exhaust gases. If the pipe is too short the rebound wave will force the gas flow backwards through the engine pushing fresh charge out the venturi and re-charging the cylinder with hot exhaust. I think it is this extreme condition that Randy refered to. Your suggestion regarding the need to further lean the engine with longer pipe lengths (for the same RPM) is certainly valid as well but occurs, of course, at the opposite extreme.
A couple of comments regarding the effects of various tuning devices, specifically nitro, head clearance, pipe length and plug heat.
All of the above effect the "ignition timing" of the engine (as opposed to intake/exhaust timing). Glow engines can be thought of as compression ignition engines with a booster system. We've all probably experienced the occassional engine start WITHOUT a battery attached. This is proof positive that a large part of the flash point process of these engines occurs via the "Heat of Compression", which simply means that a volatile mixture can be squeezed to the point that it reaches its flash point simply through the heat generated by the compression of that mixture. Model diesel engines are, of course, the ultimate example of this phenomenom.
With our glow plug we have merely integrated an ignition expeditor into the equation which allows us to ignite the mixture at a lower level of compression. Pretty straight forward.
In our case the point at which ignition occurs is a function of the combination of heat of compression and the heat of the glow plug. Much like the ignition timing on a car, we can control the power delivery of the engine by altering at what point in the compression stroke ignition is initiated. If we lower the compresssion (raise the head) or use a colder plug or run a lower nitro content fuel ignition will occur later in the cycle and vice versa.
Once again, in extremis, if you go too far in the first instance the engine won't fire at all let alone run. You'll never get the temperature to the flash point. If, on the other hand, you increase compression excessively, run a very hot plug and/or throw in a bunch of nitro, ignition will occur prematurely and the engine will knock, overheat and pretty much beat itself to death.
Somewhere in between these extremes lies stunt heaven!
Now, when you throw a tuned pipe into the mixture you've done nothing more than add a more or less adjustable element into the compression equation (and, therefore, the ignition equation since compression=heat=ignition).
The principle of the pipe is based on the fact that the last bit of the exhaust pulse on a two stroke engine is expelling a part of the incoming fresh fuel/air mixture which would ordinarily be lost with the exhaust stream. The pipe is a pulsing wave producer which alternately pushes and pulls the exhaust wave. When the length and volume of pipe is correct for the RPM and air density the rebound wave will push those almost lost molecules of fuel/air mixture back into the combustion chamber just before the piston closes the exhaust port.
At the point this happens the intake port has already closed thus trapping not only the fuel/air mixture already in the cylinder but also that forced back in through the exhaust. The resulting charge is a greater mass of combustibles than would have existed without the rebound wave. Thus, the total amount of mixture compressed is greater and, therefore, the heat of compression is increased.
The net result of the pipe is, therefore, two-fold. First the larger charge produces a greater propulsive force (a big firecracker makes more "boom" than a little one") which thus increases torque, and, second, the ignition is advanced slightly since the heat of compression will make the mixture ignite earlier in the compression stroke.
The above is why a tuned pipe equipped engine can produce more useable power at a given RPM than does the same engine without a pipe...it's not unlike a turbo-charger on a four stroke engine.
Given some comfort with the above knowledge and a more or less infinite amount of time to "fiddle" with our toys, a truly refined run can be developed to provide just the right kind of power for a particular ship. You can juggle the variables of head clearance, plug heat range, nitro content and pipe length (as well as intake size to a certain degree) to achieve an engine run at the perfect RPM for a perfect prop with a perfect break...both amount of break and when it occurs...and precisely the right amount of power burst in high drag conditions to suit the weight and wing loading of your airplane.
Oh, you can get the plane and engine to fly competitively well pretty much time after time without a lot of juggling but, occassionally you'll stumble into the condition nirvana that I've mentioned and discover just how delightfully easy it is to fly as well as you're capable of with almost no effort. When it is right you'll be happy. When it is perfect you'll think the devil is playing with your soul 'cause it's that much better.
I think it might be safe to theorize that the ideal for setting up a piped stunt engine would be to find the pipe length that--for the desired RPM--re-ingests 100% of the unburned mixture expelled by the exhaust stroke, thus maximizing the amount of fuel/air mixture available for the subsequent compression and power strokes.
This is only, however, the first step. The second step would be to adjust the head clearance to precisely the point that the power "delivery" to the airplane is perfectly attuned to the needs of the airplane and the pilot. This might be with significant break under load (a heavy high drag ship or a pilot who likes to "hammer" the corners), a modest break which just give a "blip" of boost in the corners (my personal favorite), or a flat high torque run with no break to speak of or maybe just a touch at the top of the hourglass which make a very smooth and easy flying constant speed setup with lots of power but little change in speed.
These three steps in power delivery would likely be achieved by successively lower compression ratios with all else remaining the same.
Please note that if you change any of these variables the rest will change accordingly. Thus, you can achieve the high compression constant four stroke set-up Doug prefers by, perhaps, using a very free pipe (large volume with little back pressure) a very large venturi, low load (a low pitch, clean planform prop), and comparatively high nitro and rich mixture.
Like someone mentioned, however, I think a safer approach for the average or new pipe user would be a lower compression ratio set-up which places the engine in less jeopardy should there be a slip up in the operation.
Oi vey, as usual, way too long. sorry 'bout that.