Scavenging
The process of using the momentum of exhaust gas pulses to draw fresh air-fuel mixture into the cylinder during the valve overlap period.
Exhaust scavenging is a fluid dynamics phenomenon where the kinetic energy and inertia of moving exhaust gases are used to assist in clearing the cylinder of spent combustion gases and pulling in a fresh charge of air and fuel. As exhaust gas rushes out of the cylinder and down the exhaust pipe, it creates a high-velocity pulse that leaves a low-pressure vacuum behind it.
This scavenging effect is optimized during the 'valve overlap' period, when both the intake and exhaust valves are open slightly at the same time. The vacuum created by the outgoing exhaust pulse literally pulls the fresh intake charge into the cylinder, improving engine volumetric efficiency. Scavenging is highly dependent on exhaust pipe diameter, header design, and engine RPM.
If the exhaust system is modified improperly (for example, installing pipes that are too large in diameter), the exhaust gas velocity will drop, weakening the scavenging effect and reducing low-end engine torque. Conversely, a restricted exhaust system will build backpressure, preventing scavenging and trapping hot exhaust gases inside the cylinder, which can lead to misfires and detonation.