Exhaust systems are extremely complicated. What looks like just a bunch of metal pipes merged together is actually an engineering masterpiece, every fold, bend, and pipe diameter is carefully selected to achieve a specific effect.

This “effect” has a lot to do with balancing out the three most important characteristics of the exhaust system:

• Exhaust Scavenging,
• Backpressure, and
• Exhaust Velocity.

This is true for both OEM and aftermarket exhaust systems, with the exception that OEM exhaust systems are more geared towards compliance with emissions standards over outright performance.

Before you upgrade your exhaust system, it’ll help to familiarize yourself with concepts such as scavenging and backpressure.
We’ve touched on these concepts in one of our articles that explains how exhaust systems work and what components are involved.
In this article, we’ll briefly discuss the scavenging effect what it is, what it does, and why you should understand it.

What Is Exhaust Scavenging?

Put simply, scavenging refers to the process of replacing spent gases in the engine cylinder with a fresh charge of air and fuel.
Your engine is basically a large air pump that makes power by rotating pistons through a crankshaft, powered by multiple explosions of air and fuel sprayed in a fine mist.

Every modern four-stroke engine follows the same cycle:

• Intake: Air-fuel mixture enters the combustion chamber.
• Compression: Piston travels up to top dead center and compresses the air-fuel mixture.
• Combustion: The spark plug ignites the air fuel mixture, and the piston gets pushed to bottom dead center.
• Exhaust: Piston travels back up to top dead center, but this time it pushes the spent exhaust fumes out through the exhaust valve.
The whole point of the exhaust stroke is to expel the spent gases that remain in the engine cylinder (better known as the combustion chamber) after the combustion cycle is completed.

When the spent gases are evacuated, it frees up the much-needed space for the next combustion cycle.
But have you ever wondered how exactly the exhaust system draws the spent gases out of the engine cylinder?

Sure, the piston does some of the work by pushing the gases out when the exhaust valve is open, but there’s a lot more at play, to enter the scavenging effect.

How Does Exhaust Scavenging Technology Work?

There are two main forces involved in scavenging:

1. The piston pushes out exhaust fumes while travelling back up to top dead center, and
2. The pressure differential between the combustion chamber and the exhaust manifold causes the exhaust gases to get pulled out.

Allow us to explain, the pressure within the combustion chamber is 6 to 7 times higher than atmospheric pressure, and the pressure outside the combustion chamber (in the exhaust manifold) is equal to atmospheric pressure.

Naturally, the exhaust gases are going to want to move towards the low-pressure area, that is, out of the engine cylinder, and into the exhaust manifold.
This happens because gases tend to travel from high pressure to low pressure, that’s why wind exists.

And it is this pressure differential that causes exhaust gases to leave in the first place.