ADJUSTABLE CAM PULLEY

Cam pulley is used to rotate the camshaft which in turn will push the valve down.The timing of the valve will depend on the camshaft and the cam pulley.The timing means  when the valve is pushed down. If the valve comes down before the piston reaches its highest point called the top dead center(tdc), the timing is considered ADVANCE.If the valve comes down after the piston reaches  the tdc, it is considered as RETARD. The advance/retard is measured by degrees for example advance 2°or retard 2°.
The adjustable cam pulley, as the name implies, it means the pulley is adjustable.This will allow the valve timing to be changed This is simillar to a vvti system except the timing on vvti engine is variable depending on engine speed and it it is changed while the engine is running.As for the adjustable cam pulley (acp), the timing can only be set at one position/angle and can only be changed when the engine is not running.