referring to the angle the crankshaft rotates after passing the BDC position) to 14°BTDC (Before Top Dead Centre, referring to the angle the crankshaft needs to rotate before reaching the TDC position) respectively, giving a total of 474° of crankshaft rotation during which the valve is closed. Further calculations factor in the effect of the engine speed, such that various RPMs will create different time durations within which the intake valves remain closed.