ning and closing timing of the intake, exhaust, blowdown, and transfer phases that take place in the cylinder. These phases must be coordinated to work with other engine components such as the intake and exhaust system. The intake and exhaust systems are designed to take advantage of the finite amplitude waves that travel back and forth from the atmosphere. Porting coordinates the opening of the intake, exhaust, and transfer ports to maximize the tuning affect of the exhaust pipe and intake system. Generally speaking porting for more mid-range acceleration is intended for use with stock intake and exhaust systems.
These are some common words and terms associated with porting.
Passageways cast and machined into the cylinder.
The tube shape that comprises the ports.
The part of the port that interfaces the cylinder bore.
The large port where the burnt gasses exit the cylinder.
The center divider used on triangular shaped exhaust ports.
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 scavenging.
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 the port.
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 exhaust port.
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.
Tools of the Trade
There are two main types of tools used in porting, measuring and grinding. Here is an overview of how these tools are used.
The basic measuring tools include a dial caliper, an inside divider, and an assortment of 