20.The area in the rotation of a cam where the intake valve and exhaust valve are opened simultaneously is know as overlap. The reason for this is so spent fuel leaving the head will actually create a vacuum that will enhance cylinder efficiency by pulling the fuel charge into the cylinder, which is known as scavenging. Superior used a D-port on the exhaust runner to increase the efficiency of the scavenging. This helps because as the valves open and close, it's possible for the piston to pull from both the intake and exhaust. The flat area of the D-port of the exhaust will create a vortex that provides pressure in the exhaust, preventing the spent charge from being pulled back into the combustion chamber.