Selecting the optimal headers and exhaust system for your ride involves tube diameter, length, collector diameter, collector length, pipe diameter, and more. The goal is to have a system that produces maximum efficiency at a sound level acceptable for your application.
And to produce maximum power, the exhaust scavenging effect is key.
What is Exhaust Scavenging & How Does it Work?
Scavenging occurs when the rush of exhaust gas leaving the cylinder helps to pull in a fresh air/fuel mixture.
During the exhaust stroke, the exhaust gases escape the cylinder very quickly. This creates a high-speed pulse moving down the header tube. The low pressure behind the exhaust pulse creates a vacuum effect.
During valve overlap (the period when both the intake and exhaust valves are open simultaneously, expressed in degrees of crankshaft rotation), the vacuum pulls out burnt exhaust gases. It also helps to pull in more fresh air on the intake side—the more valve overlap you have, the more critical scavenging becomes.
As rpm increases, the air flows faster and the scavenging effect becomes more powerful.
How Exhaust Scavenging Affects Performance
The cylinder is full of exhaust gases at the bottom of the power stroke. During the exhaust stroke, the piston pushes the exhaust out of the engine. Any backpressure or restriction in the exhaust system requires the piston to push harder, which is lost energy. The engine is more efficient when less energy is used to move air in and out of the cylinder. This results in more power and better fuel economy.
Scavenging also has a mini “supercharging” effect. This increase in volumetric efficiency improves cylinder filling, which means more torque and horsepower.
Yet, exhaust scavenging is a delicate process to control.
The intake system, cylinder heads, and exhaust system must be matched to optimize airflow. It also requires the right camshaft duration, lobe separation, and valve timing for the desired rpm range.