the variator adjusts the valve timing on the inlet. why is that good? variable valve timing gives you the best of both worlds in terms of low rpm torque and high rpm power.
the variator works by moving the cam pulley when oil pressure (rpm) gets high enough to force the variator against its spring. as the oil pressure (rpm) drops the variator is pushed back into the low rpm position by its spring.
how does a moving cam pulley adjust valve timing? when the pulley moves it effectively changes the position of the pulley on the timing belt, closer or further away from the crank pulley, that will advance and retard the time at which the valves open and close in relation to the crankshaft.
the default position is set for low rpm torque, as most engines are, however this means at high rpm the intake valves close before the total requirement of air is able to enter the cylinder, that means a power loss, but the variator makes sure the inlet valves close later, allowing more air to enter.
why dont they just have it so they close later all the time? at lower rpm this would cause scavenging where unburnt fuel and air enter the exhaust, killing torque & reducing mpg.
when the variator fails it is almost always its spring that snaps (it can be replaced seperately to save a few quid
). this means the variator will stay in the high rpm position, which leads to scavenging at low rpm.