Further question: for a given pipe, does varying the header length tune?
The tuning depends on the distance from the engine to the reflecting surface, how it gets there makes little or no difference. I have had a severe resonance issue (in my professional life) that involved waves whacking their way through what looks like a cat hairball of tubing, the traditional shape for models is not required.
A *very slight* difference is how much heat is lost, lowering the temperature, and thus lowering the speed of sound in the exhaust. That would tend to lower the tuned RPM. Probably, the aluminum conducts heat and radiates it faster than the carbon/epoxy, to it will be a little cooler overall.
Randy suggests using the header full length, although almost no one does that. The advantage to that is that the exhaust always cools as it goes down the pipe, so the further from the engine, the cooler it is, so it has less tendency to collapse the front of the pipe. He also supplies an aluminum insert for the front of the pipe, that keeps it from collapsing from the pressure of the coupler. Before that, we used various things, like a ring of 1/16 copper tubing JB-Welded around the outside of the pipe. That has the effect of keeping it round, gives a smooth sealing surface for the coupler, and a flare to keep it from blowing off. I still do that, insert or not, just because sealing on the rough surface of the pipe is iffy.
If it does it at all, it collapses under the coupler - the tension on the rubber squeezes it, and, the rubber insulates it so it gets much hotter under the coupler than anywhere else.
Note that depending on which pipe you have, it can have multiple places at which you can reference the tuning. Almost all stunt tuned pipes contain baffles across the interior, in addition to a "tail cone" of some sort. The waves of exhaust reflect from all the baffles, and the tail cone, and since they are pretty widely separated, you get multiple return waves at different times. Flat surfaces return sharp "Cracks" over a very narrow RPM range, the tail cone returns more of a "thud", not as high a peak, but wider. Point being, you can tune it to any of these surfaces, or a combination of surfaces. So you might get similar results at "17 inches to the first baffle" as you do at "19.5 inches to the first baffle" - one you are using the tail cone return wave, the other, one of the baffles. I have always measured the traditional way, "to the first baffle" but actually use the tail cone for most applications.
The fact that there are multiple baffles also means that there are always multiple tuned RPMs and multiple "valleys" where the pipe tuning hurts the breathing instead of helps it.
Randy's chart will get you a good starting point, it may not be optimum and you might want to deviate from it when you learn more, or for experiments sake.
Brett
p.s. while I encourage people to do careful experiments, one thing that tends to happen with beginning pipe users is that they figure that every engine run problem they have is caused and fixed by changing the pipe length. Changing the length over small ranges has completely predictable effects, but once you have it working, the *pipe length is the very last thing you need to be adjusting on a day-to-day basis*. All the other things that effect engine runs on non-piped engines still happen and are still a bigger source of issues.