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Variable Valve Timing is an engine that allows the lift, timing of the engine's valves to be adjusted for best engine performance. The engine has a lower efficiency at high speed. The intake valves are open a much shorter time so there is less time for the air-fuel mixture to enter the cylinders.  If the intake valves opened earlier at high speed, the air-fuel mixture would start entering earlier so more would get in. One way to open the intake valves earlier at high speed is to use a variable timing camshaft. It has a flexible connection between the camshaft sprocket and the camshaft, The connection includes a hydraulic piston operated by engine oil pressure and a solenoid operated oil control valve. When the electronic control module (ECM) signals the solenoid to close the valve, the oil pressure forces the piston forward. As the piston moves the internal teeth of the piston slides up the inclined teeth on the helical drive gear.