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CYLINDER PORTING
An Overview of Basic Principles and Processes
By Eric Gorr

[ Continues From Page 1 ]

Terminology
These are some common words and terms associated with porting.

Ports
Passageways cast and machined into the cylinder.
Ducts
The tube shape that comprises the ports.
Windows
The part of the port that interfaces the cylinder bore.
Exhaust Port
The large port where the burnt gasses exit the cylinder. Exhaust
Bridge
The center divider used on triangular shaped exhaust ports.
Sub-Exhaust Ports
The minor exhaust ports positioned on each side of the main exhaust port.
Triple Ports
One main bridgeless exhaust port with one sub exhaust port on each side.
Front Transfers
Transfer ports link the crankcase to the cylinder bore. The front set (2) of transfers is located closest to the exhaust port.
Rear Transfers
The rear set of transfers is located closest to the intake port.
Auxiliary Transfers
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.
Boost Ports
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 scavenging.
Port-Time-Area
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.
Duration
The number of crankshaft angle rotational degrees that a port is open.
Opening Timing
The crank angle degree when the piston uncovers the port.
Crank Angle
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.
Port Height
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.
Chordal Width
The effective width of a port, measured from the straightest point between sides.
BMEP
Brake Mean Effective Pressure.
Loop Scavenging
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 exhaust port.
Blow-Down
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.
Effective Stroke
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.
Compression Waves
Pressure waves that reflects from the end of the intake or exhaust system and return to the engine.
Expansion Waves
Pressure waves that travel from the engine and out to the atmosphere.

Continues with [ Tools of the Trade ]

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