Valve overlap is a term used in engines to describe the period during engine operation when both intake and exhaust valves are open at the same time. This occurs when the piston nears top dead center (TDC) between the exhaust event and the intake event. The duration of valve overlap is between 10° - 20° of crankshaft rotation, depending on the engine design. Valve overlap is designed into the engine and is most useful at higher speeds. At higher speeds, the extra amount of intake charge brought into the combustion chamber provides a substantial increase in available power. The amount of time that both valves are open is directly related to engine rpm. The higher the engine rpm, the shorter the amount of time that both valves are open. The objective of overlap is for the exhaust gas which is already running down the exhaust pipe to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. At idle and low rpm, overlap will produce low engine vacuum and a rough idle.