mixture gasses to flow out the transfer ports during the scavenging phase.
2. 2.Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The gasses travel up the backside of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke. It is critical that the burnt gasses are scavenged from the combustion chamber, to make room for as much unburned gasses as possible. That is the key to making more power in a two-stroke engine. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. Most of the gasses aren't lost because a compression pressure wave has reflected from the baffle cone of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the exhaust port.
3. Now the crankshaft has rotated past bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The compression wave reflected from the exhaust pipe is packing the unburned gasses back in through the exhaust port as the piston closes off the port the start the compression phase. In the crankcase the pressure is below atmospheric producing a vacuum and a fresh charge of unburned mixture gasses is flowing through the reed valve into the crankcase.
4. The unburned mixture gasses are compresses and just before the piston reaches TDC, the ignition system discharges a spark causing the gasses to ignite and start the process all over again.
What is Porting?
Porting is a metal finishing process performed to the passageways of a two-stroke cylinder and crankcases, that serves to match the surface texture, shapes and sizes of port ducts, and the timing and angle aspects of the port windows that interface with the cylinder bore. The port windows determine the opening 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 