With a converging-belly section-diverging type tune pipe the goal is to have the diverging section create a returning rarefaction wave and the converging section create a returning pressure wave. The belly section acts as an appropriate time delay between the returning waves such that the pressure wave arrives at the exhaust port after the transfer port has closed. This pressure wave pushes the excess fresh mixture in the exhaust pipe from a short circuit, back into the cylinder. Here the short circuited fresh mixture is actually desired since this allows the returning pressure wave to "super charge" the cylinder giving it more fresh mixture than if the cylinder were filled at ambient pressure. This is a similar result to turbo-charging or super-charging a four-stroke engine. If the mixture contained within the cylinder before combustion occurs were allowed to expand to ambient pressure, its volume would be larger than the displacement of the engine. This phenomenon is quantified as volumetric efficiency; it is calculated as the ratio of the ambient pressure volume of the fresh charge, divided by the displacement volume of the engine. The operation of a two-stroke engine equipped with a properly tuned pipe is shown in the animation below, for a step by step description of the process, please follow the link below the animation.There are exhaust manufacturers now that are mating up tuned pipes to tuned (ported) engines to get the best possible 'supercharging effect' at given rpm's. In the past a tuned pipe would have been tested on a stock engine but the length and shape of the pipe will differ on a 'tuned' engine because of the way it is able to rev higher .
