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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