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Expansion Chamber
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Tuning For Power

THE original VeloSoleX S 3800 has peak power at around 2500 RPM and there is little improvement to be gained by adding an Exhaust Expansion Chamber (The phenomenon of the Exhaust Expansion Chamber was first discovered in the 1950s by Walter Kaaden, Chief Development Engineer at the East German company MZ).

An Exhaust Expansion Chamber designed for a small 2-Stroke engine provides a power peak over a fairly narrow RPM range by reverse super-charging. A good design provides the correct wave polarity to pull out spent exhaust gases from the cylinder as well as drawing out some air-fuel mixture from the Transfer Ports via the cylinder and exhaust port into the Exhaust Expansion Chamber. This fresh mixture previously drawn into the Exhaust Expansion Chamber is then forced back into the cylinder after the Transfer Ports have closed but before the Exhaust Port has closed. The occurrence of these forward and backward pressure waves is directly dependant on the Engine Port Angles (Opening / Closing) and the Engine RPM.

One side effect of a modern Exhaust Expansion Chamber is that the Effective Compression Ratio will be increased, which will necessitate a reduction in the amount of Ignition Advance (to perhaps as little as 16 BTDC). On a really efficient exhaust system, the Compression Ratio of the engine may need to be reduced if detonation is to be avoided at the power peak.

For the VeloSoleX S 3800, an Exhaust Expansion Chamber designed to peak at around 2500 RPM will be about 1.3 metres long and only small power gains would be achieved. Even if it was tuned for a power peak at around 6000 RPM, where more significant gains could be achieved, the Exhaust Expansion Chamber would still need to be 0.87 metres long. These long chamber lengths will create problems when trying to mount the whole system to the VeloSoleX S 3800. It is therefore NOT RECOMMENDED to add an Exhaust Expansion Chamber to the VeloSoleX S 3800.

If you are keen to proceed with your own design remember that the formulae available will only give you a reasonable design starting point, that engine temperatures will rise seriously and that Walter Kaaden was fairly astute when he said: "You'll know when you have the design right, because the chamber will then be impossible to fit on the motorcycle without having it drag the ground, burn the rider's leg, or force the relocation of at least one major component."

 

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Copyright 2008 BRIAN'S VELOSOLEX. All rights reserved. This is a personal website containing information for and by VeloSoleX enthusiasts. It is not intended to replace any official information and whilst every effort is made to ensure the accuracy of all published information no guarantee is offered nor liability accepted for any loss or damage that may arise from any errors or omissions.