Kris,
These little nitro engines are two stroke pumps with ports in the sleeve and crank. As the piston goes up & down it changes the pressure in the crank case and in the combustion chamber. The action of the piston sliding up and down past the ports regulates the flow of fresh fuel and burnt fuel. The TIMING (duration a port is open) is one component of the larger equasion of how the engine was intended to produce it's peak power.
The exhaust system we use is actually a combination of a header and a tuned resonance chamber (Pipe as we like to call it). The header aids in removing burnt gasses and pullining in extra fresh charge; this is called scavanging. The PIPE reflects the impuse after it has travelled through the header, back into the header & ultimately the exhaust port; this is called packing. these two actions allow the engine to burn more fuel than its actual displacement. Think of it as super charging.
An engine that effectively uses the tuned exhaust produces more power, gets better fuel efficiency and runs cooler.
Just PM me if you want to know more specifics.
chuck