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 angle gauges. The caliper is used to measure the port height, the divider is used to measure the chordal width of the port, and the angle gauges are used to measure the roof and side angles of the ports. Calipers and dividers are available from places like Sears or industrial supply stores. Angle gauges are fashioned from cardboard and specific to individual cylinders.
The most common grinding tools are electric powered. They consist of a motor, speed control, flexible drive shaft, tool handle, and tool bits. The power of these motors ranges from 1/5th to 1/4th HP with a maximum rpm of 15,000. Popular manufacturers include Foredom, Dremel, and Dumor. Each company sells a full compliment of accessories for all sorts of hobbyist activities. The most popular source for cylinder porting tools and accessories is CC Specialty in Tennessee (1-800-762-6995).
The tool handles and bits are the secret to porting. There are two types of tool handles; straight and right angle. The straight tool handles are used for machining the port ducts. The right angle tool handles are used to gain access to the port windows from the cylinder bore. Over the years I've tested hundreds of different tool bits and arrived at some simple materials and patterns for finishing the different surfaces of a cylinder. The materials of a cylinder range from aluminum as the base casting material, to a cast iron or steel liner, or nickel composite plated cylinder bores. Here are the basic tool bits used for porting; tungsten carbide works best for aluminum, steel, and cast iron, stones are best for grinding through nickel composite. The tungsten carbide tool bits are available in hundreds of different patterns and shapes. The diamond pattern is the best performing and the shape of the bit should match the corresponding shape of the port. Stones, or mounted points as they are termed in industrial supply catalogs, are available in different shapes and grits. The grits are graded by the color of the stones. Gray being the most course and red being the most