[sugarwood]

During the compression upstroke, as the piston starts to move towards the spark plug, the cam can vary the intake valve closing. If the cam closes the intake valve early, it will maximize compression, since more fuel/air is trapped in the cylinder. If it closes it late, you will reduce compression, since there is less fuel/air to compress. So, why would you close the intake valve later in the compression cycle? What is the upside? I am having a hard time linking this to the idea of leaving the intake open longer to allow more time for air during higher RPMs. By the time the piston is moving upwards, no more air/fuel is entering the cylinder anyway. Varying the closing time of the intake valve during upstroke is only a matter of how much you compress. Whatever was taken in, was done during the downstroke of the piston, not the upstroke.