Building Expansion Chambers

Two weeks ago I started learning how to form 18 guage steel into the Motion Left RevRun expansion chamber. Hand forming the steel cones with a sand-filled hammer is labour intensive and takes a deep understanding of how the metal stretches as it is bent. Even with a special turned rock maple form it is necessary to constantly make adjustments to account for the gradual stretching of the metal.

An expansion chamber operates on the principle of acoustic reverberation. Partially combusted gases are usually wasted as they are forced out of the cylinder and into the standard exhaust. An expansion chamber takes advantage of this partially combusted gas as it is forced out of the cylinder and bounces it back into the cylinder in what can best be described as an "echo." This principle only works with the two-stroke engine since every stroke is powered as opposed to the four stroke cycle where every other stroke is powered. Through the use of variously composed cones this echo can be tuned to take advantage of a specific cylinder, power range or speed. Since the cones are fixed it means that top performance is achieved only at specific reverberations. This means that an engine running an expansion chamber will have a "power band," or a specific rpm at which there is a sudden increase in power. Through the use of multiple cones the power band can be concentrated at a specific rpm, or spread out over a wider range. Where this power band falls and how broad it is is determined by the geometry of the cone angles. On the other end of the process, un-combusted gases forced back out of the carburator can be retained using a velocity stack.

It is generally acknowledged that while many computer programs and formulas exist explaining it, the principle remains a black art only perfected through experiment, which is exactly what Motion Left is doing. The RevRun is the first expansion chamber from ML and is designed for 64cc and smaller engines, specifically the Polini. 

 

By the end of the day I had beaten my thumb to a pulp with the sand-filled hammer, my arm ached and I had sliced open my hand, and learned the basics of tack welding light-guage metal.

I have been told that a member of Seattle's Mosquito Fleet recently reported 62mph with a RevRun he had just purchased and ran on his 64cc Polini with stock 15mm Bing carburator. Other reports describe jumps in speed in excess of 10 mph at speeds above 50 mph. Not bad... 

For more information on how expansion chambers work and an amazing animated GIF click here.