First we take a look at the cam pulley itself. 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.
WHY ADJUSTABLE CAM PULLEY?
Different timing is needed for different speed of engine (rpm) to get the most efficient combustion.The cam pulley can be adjusted to suit the driving style of the driver. The powerband of the engine can be shifted by varying the timing of the valve.
Advance timing means the valve start to come down before the piston reaches the tdc. The advance timing will enhance the lower and mid range of the engine but peak power will be less. This setup is suitable for daly driving in the city where the engine revs at low to mid rpm levels.However if the timing is too advance, it can cause detonation/knocking/pinging
Retard timing in the other hands means the valve starts to come down after the piston has reach its tdc.Retard timing will enhance the high the peak power but sacrificing the mid and low range power. This setting is suitable for drag or racing purposes.
The valve/cam timing is usually altered together with the ignition timing
COMMON CONFUSION OF VALVE TIMING, LIFT AND DURATION
Note that the adjustable cam pulley only changes th time when the valve activity start. The duration the valve will remain open is still the same and the lift/depthness of the valve being pushed down is also the same.