The Black Art of Cylinder Scavenging

What’s The Big Idea?

The basic principle behind cylinder scavenging is that once an air-fuel mixture has been burned the exhaust gases need to evacuate the cylinder to make room for new fresh air to come in. As the exhaust valve opens there is a time period—called valve overlap—where the intake valve will begin to open as the exhaust valve is still closing. This means both valves are open at the same time and is the fundamental principle behind scavenging. 

Why It’s Important

Moving burned gases out helps create room for fresh air which means more horsepower can be made. A secondary function of moving the gases out is that a venturi, or low-pressure zone, is created that helps suck in more air to the cylinder. It’s like having a tiny supercharger inside your combustion chamber that draws excess air in! 

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How Is It Done?

Since engines are just air pumps, the way this is done is by matching components like the camshaft, exhaust header length & width as well as the cylinder head itself. It’s a complex subject that varies greatly from engine to engine and is truly custom in every sense of the word. What works for one engine is highly subject to the individual modifications made to that engine. 

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Tuning to Win

Of course, as you increase the engine’s revolutions per minute the effect of cylinder scavenging becomes more and more important. Volumetric efficiency can be greatly affected by cylinder scavenging as well which can lead to one engine “just feeling better” than another one. It’s a lot more complex than simply changing a camshaft out or buying long tube headers. Perfecting the process involves a lot of research and equipment like flow benches that don’t come cheap. 

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Final Thoughts

Cylinder scavenging is one of the black arts of engine building that consists of other concepts like valve overlap, header design and port design alike. All of it goes into making an engine that flows air correctly and simply feels like a winning package. It’s one of the main difference between a highly tuned race motor and a well-built street motor that should be understood by anyone who is into engines. Hope you enjoyed learning about cylinder scavenging. Until next time. 

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