Here, our original wave reaches that “cure”. Following the diffuser, and after perhaps a couple of inches of straight-walled chamber, the wave encounters a converging cone that effectively constitutes a closed end to the expansion chamber. A part of the wave energy will already have been inverted by the diffuser and sent back to the cylinder, but there is enough of its original strength left to rebound quite strongly from that closed end, and it reflects with its original, positive, sign. In due course of time, this wave arrives back at the exhaust port itself, stalling the outflow of the fresh charge. Indeed, it will momentarily reverse the flow there, stuffing what might otherwise have been lost back into the cylinder. The net result of all this activity on the part of the expansion chamber - first pulling and then pushing at the fresh charge to hold it in the cylinder - is a big boost in power. In fact, it is the only thing you can do to a two-stroke engine that will
As was mentioned before, the expansion chamber is not purely a sonic wave device: Back at the closed end of the chamber there is an outlet pipe, and it is too small to keep the pressures inside the chamber equalized with atmospheric pressure. Consequently, there is an abrupt pressure rise inside the chamber, toward the end of its operating cycle, which is felt at the engine's exhaust port and plays a very large part in preventing charge loss.
This entire process can work wonderfully well - and it also can fail miserably if the various elements of the expansion chamber are not properly dimensioned. All of the various waves and pressure sucking and surging about the exhaust port must operate in agreement with the engine's requirements. When they disagree, the result is worse than can be obtained at a much lower price paid in time and money with the stock muffler. As it happens, the motions of those waves are stubbornly tied to exhaust gas temperature, and supremely indifferent to what the engine would prefer in terms of their arrivals. The time intervals between the initial wave departure, and the return of its reflected components is a function of wave speed, and the system's lengths. Thus, as wave speed is subject only to the laws of physics and exists as something one must simply use without altering, the task of designing an expansion chamber for some particular application is to establish lengths, diameters and tapers that will use the pulsations within the exhaust system to the engine's benefit.
