Here's more than you probably ever wanted to know about tuned pipes:
http://www.findarticles.com/p/articl...4/ai_n13498021
This is written for boaters, but the principles are the same for any 2 stroke motor. The pipe acts as a sort of supercharger. Tuned properly, it packs air/fuel mixture into the cylinder of the engine, increasing HP. They were used a lot on older pattern airplanes until the .60 engine restriction was lifted. Larger engines in a lower state of tuning seem to be the ticket today.

I'm not sure but that the Mac's mufflers aren't designed as a kind of 1/4 wave pipe. In the applications where I've seen them used, they do seem to boost power over a stock muffler, just not as much as a pipe.

Pipes can also serve as a kind of regulator of power output in certain applications, and can be very effective as mufflers. The long Hatori pipes of the late 80's helped quiet the noisy pattern planes down a lot by allowing lower RPM operation with bigger props.