How is the pressure of an exhaust port measured?
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.
Where are transfer ports located on a cylinder?
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.
How big should the exhaust and inlet ports be?
There should always be sufficient cylinder wall on the sides of both exhaust and inlet ports for a 2mm width of piston skirt to bear against and affect a seal. To ensure that you don’t go too far in widening the exhaust port, you will have to carefully scribe the outline of the port windows on the piston skirt with the crankshaft rotated to TDC.
What does the transfer port do on a moped?
These passages transport fresh fuel/air mixture supplied by the intake from the crankcase to the area of the cylinder currently above the piston. The transfer ports also play a vital role in cooling the cylinder and scavenging gases.
Why do transfer ports have a steeper angle?
Another factor is the angle of the transfer ports roofs. That affects the angle of mixture entry. A steeper angle gets the mixture up to the spark plug faster which is good for high rpm power but bad for low rpm power. A good example is two cylinders I have for the same engine. One had transfers with only 114 degrees and 45 degree roofs.
Which is shorter transfer port or exhaust port?
Usually the transfer port will have a shorter timing than the exhaust port. Some Tips: It is helpful to point a flashlight or a halogen lamp through the plug hole or to remove the cylinder head. If there is play in the conrod bushings, you should take all readings with the piston pushed downwards.
Why is the exhaust port duration so important?
The exhaust port duration has no significance apart from the duration of the transfers because that relationship sets the amount of time available for the exhaust gases to leave the cylinder before the low pressure intake mixture is transferred into the cylinder.
What is the timing of the transfer port?
The port timing angle (shown in blue) is then 260° – 100° = 160°. This means, that port is open while the the crankshaft turns 160°. Usually the transfer port will have a shorter timing than the exhaust port. Some Tips: