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These are some common words and terms associated
Passageways cast and machined into the cylinder.
The tube shape that comprises the ports.
The part of the port that interfaces the cylinder
The large port where the burnt gasses
exit the cylinder. Exhaust
The center divider used on triangular shaped
The minor exhaust ports positioned on each
side of the main exhaust port.
One main bridgeless exhaust port with one
sub exhaust port on each side.
Transfer ports link the crankcase to the cylinder
bore. The front set (2) of transfers is located
closest to the exhaust port.
The rear set of transfers is located closest
to the intake port.
Some cylinders have a minor set of transfers
located between the front and rear sets.
Transfer Port Area Ratio
The area of the crankcase side of the transfers
divided by the area of the port window.
The port or ports that are located opposite
of the exhaust port and in-line with the intake
port. These ports are usually by-pass ports
for the intake or piston and sharply angled
upwards to help direct the gas flow during
A mathematical computation of the area of
a port, divided by the displacement of the
cylinder, and multiplied by the time that
the port is open. The higher an engine revs
the more time-area the port needs. The higher
the piston speed the less time available for
the gas to flow through the port.
The number of crankshaft angle rotational
degrees that a port is open.
The crank angle degree when the piston uncovers
Measured in units of degrees of crankshaft
rotation. On a two-stroke engine there are
a total of 360 degrees of crankshaft rotation
in one power cycle.
Port Side angle
The side angle of a port measured at the window,
from the centerline of the bore with the exhaust
port being the starting point (0).
Port Roof angle
The angle of the top of the port at the window.
The distance from the top of the cylinder
to the opening point of the port.
Top Dead Center (TDC)
The top of the piston's stroke.
Bottom Dead Center (BDC)
The bottom of the piston's stroke.
The effective width of a port, measured from
the straightest point between sides.
Brake Mean Effective Pressure.
Scavenging is the process of purging the combustion
chamber of burnt gasses. Loop scavenging refers
to the flow pattern generated by the transfer
port duct shapes and port entry angles and
area. The gasses are directed to merge together
and travel up the intake side of the bore
into the head and loop around towards the
This is the time-area of the exhaust port
between the opening time of the exhaust and
the transfers. When the exhaust port opens
the pressure blows down, ideally to below
the rising pressure of the gasses in the transfer
ports. Blow-down is measured in degrees of
crank rotation and time-area.
The distance from TDC to the exhaust port
height. The longer the effective stroke the
better the low-end power.
Primary Compression Ratio
The compression ratio of the crankcase.
Secondary Compression Ratio
The compression ratio of the cylinder head.
Pressure waves that reflects from the end
of the intake or exhaust system and return
to the engine.
Pressure waves that travel from the engine
and out to the atmosphere.
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