THE Exhaust Port is
satisfactory when used at engine speeds
BELOW 4000 RPM.
At engine speeds ABOVE 4000
RPM, the Exhaust Blowdown (the time needed to clear exhaust gases from
the cylinder before fresh mixture enters and measured from when the Exhaust Port
opens to when the Transfer Ports open to allow fresh mixture in)
TIME AREA (0.0033635 s-mm²/cc),
ANGLE AREA (0.505 deg-cm²/cm³) and
BLOWDOWN (18.6°) are INSUFFICIENT.
Changing the width of
an Exhaust Port is usually preferred to changing
its height. On the VeloSoleX S
both the width and height of the Exhaust Port should
thus increasing both the
TIME AREA and ANGLE AREA. It is important though
that the width of the Exhaust Port is not too wide otherwise Piston-Ring wear and
seizing are more likely to occur.
a 1 mm radius on the upper sides and
top edge of the Exhaust Port (but NOT
the lower edge)
is recommended which also allows the piston rings to
expand and contract more easily as they traverse
This gives a new Exhaust Blowdown
TIME AREA of 0.0039798 s-mm²/cc, ANGLE AREA of
0.597 deg-cm²/cm³ and BLOWDOWN of 22.0°.
This also changes
the Exhaust Port TIME AREA (0.0235901
s-mm²/cc), ANGLE AREA (3.539 deg-cm²/cm³) and OPEN DURATION
(130.7°) to a TIME AREA of
0.0248227 s-mm²/cc, ANGLE AREA of 3.723 deg-cm²/cm³
and OPEN DURATION of 137.5°.
Some engineers align the open edges
of the piston rings away from the ports but without stop-pins in the piston
grooves there is no real guarantee that the rings will not turn back
towards the ports after the engine has run for a
Changing the height of the Exhaust Port reduces the Swept
Swept Volume) from 37.01 cc to 35.78 cc. This also causes
the Swept, Trapped or True Compression
Ratio of 6.44 : 1 to change to 6.26 : 1 but this can be corrected
by using a Hotter,
As a result of radiusing the
better Exhaust Blowdown occurs with
power (and slightly less low-RPM torque).