High Overlap Valve Timing & Reversion

[Barnsnake]

There are a couple of different aspects of reversion. Both are related to the momentum of the comparatively dense intake charge.

With a camshaft that is designed to extract peak power at high RPM, it leverages the momentum of the intake charge through the port to continue cylinder filling ABDC. At low speed (idle) there is insufficient momentum to prevent reverse flow at the intake valve.

At higher RPM the intake charge has higher velocity and therefore more momentum. When the intake valve closes the column of fuel/air compresses against the closed valve. When the momentum is gone, the column pushes(springs) back upwards through the intake port. This phenomenon is subject to harmonics, so it will behave differently at different engine speeds. Intake manifold designers can use this pressure wave to increase power, or implement designs to minimize the effect. With individual runner systems there is often an engine speed at which you can hold your hand an inch above the air inlet and get wet with fuel. This is the most obvious and visible (with a timing light) example of reversion.