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Stroke:		Enter the stroke
Rod Length:		Enter the Conrod Length
Height:		This is the Port Height Above BDC
Duration:		This is open duration of the port in degrees
Inlet Port is measured by setting a mark (0 degrees on timing disc) for the point the port starts to open and then rotate the crank until the port is closed. The duration in degrees is your port timing. Inlet timing is measured by rotating the crank over TDC Exhaust & Transfer Ports are measured by setting a mark (0 degrees on timing disc) for the point the ports start to open and then rotate the crank until the ports are closed. The duration in degrees are your port timings. Exhaust & Transfer ports are measured by rotating the crank over BDC.
After entering the stroke and rod length, enter: (a) the port height and click "Calculate Port Duration" to get the duration or (b) enter duration and click "Calculate Port Height" to get the piston height. Further Information: Altering the port timings of the cylinder will determine the characteristics of a two-stroke engine. In general, these figures state the time in which gas can flow through the ports to fill or empty the combustion chamber. A cylinder will produce peak performance when this system works at it's best: the burned gases are swept out of the cylinder completely while only a minimum of fresh fuel-air mixture gets lost through the exhaust port with them. Remember: a two-stroke engine has no valves, so that the exhaust port is open at the same time the fresh gases enter the combustion chamber. When the revs are rising, the time the crank needs for one revolution becomes shorter. As the gases always travel at the same speed however, this means that a cylinder needs larger ports to work properly at high revs. At the same time such an engine will lose power at lower revs and only produce a narrow power-band. So the relation of all ports is always a compromise, especially with piston-ported engines, where also the inlet port timing is controlled by the piston simultaneously. Finally, the shape and width of ports also play an important role Blow-down: The blow-down timing is the part of the exhaust timing which is open before the transfers start to open as well. It is calculated by subtracting the transfer timing from the exhaust timing and divided by half. The longer the blow-down timing, the further up the rev range peak power is produced. This again is relative to the overall timing: even if 120 degree transfer / 170 degree exhaust & 130 degree transfer / 180 degree exhaust give the same blow-down timing of 25 degrees, the peak power will be produced at higher rpm in the second case as the overall timing is longer. As a rule of thumb, any exhaust timing longer than 180 degrees will cause a loss of bottom end power and is of little use on a road going scooter. Longer transfer timings will in general support a wide rev range. Transfer timings of Lambretta engines are normally between 115-130, while exhaust timings may vary from 155-195. The inlet timing of piston ported engines can be set between 140-170 degrees. Engines with a reed valve do not have a fixed inlet timing as this will be determined by crankcase pressure and revs. The inlet timing in general follows the rule: Maximum inlet timing is 350 degrees minus the exhaust timing. Any inlet timing beyond that figure will cause the engine to splutter badly at low revs and this is unacceptable characteristic at least for a road engine.

Calculate Port Timing from Port Height and Vice Versa