Scavenging occurs when the rush of exhaust gas leaving the cylinder helps to pull in more fresh air (and fuel).
How does it work?
During the exhaust stroke, the exhaust gases escape the cylinder very quickly. This creates a high-speed pulse of exhaust flowing down the Header Primary Tube. The fast moving pulse creates an area of high pressure in front of it and an area of low pressure behind it.
The low pressure behind the exhaust creates a sucking effect (vacuum). During Valve Overlap, the vacuum pulls out more burnt exhaust gasses. It also helps to pull in more fresh air.
As the process continues, and rpm increases, the air flows faster and the effects of scavenging become stronger.
How does it affect performance?
Effective scavenging is how you make more power! The more valve overlap you have, the more important scavenging becomes.
At the bottom of the power stroke, the cylinder is full of exhaust gases. During the exhaust stroke, the piston has to push the exhaust out. Any backpressure (restriction in the exhaust system) requires the piston to push harder. This is lost energy that could have been used to turn the wheels.
When less power is used to push and pull air in and out of the cylinder, the engine is more efficient. This results in more power and better fuel economy.
Scavenging also has a "mini supercharging effect". It speeds up the incoming air. This draws more fresh air into the cylinder.
More clean air in the cylinder, mixed with the right amount of fuel, and proper Ignition Timing, means more Torque and Horsepower!
Scavenging is a delicate process to control inside the engine. It requires that the intake system, cylinder heads, and exhaust system all be matched to optimize airflow. It also requires the right Duration and Lobe Separation on the camshaft to optimize the Valve Timing.