For purposes of OUR calculator, we define Duration as the number of degrees of crankshaft rotation that a port is open. Port timing is the degrees of crankshaft rotation after TDC to where port starts to open. Top Dead Center (TDC) is the top of the piston's stroke and Bottom Dead Center (BDC) is the bottom of the piston's stroke. A Port is an airway or duct that is cast or machined into the cylinder. The Crank Angle is the crankshaft rotation measured in degrees. There are 360 degrees total.
Obviously, this is a very close approximation due to small inaccuracies on variable data input. Also please see our Port Height Above BDC Timing Calculator for two cycle engines and our Port Timing Duration Calculator, also for two cycle engines.