What Is an Intake Manifold?

What Is an Intake Manifold?

Last Updated September 6, 2023 | Alison Smith

Intake manifold by itself against white background

The intake manifold is the part of an engine that supplies fresh air to the cylinders. It works with the air intake, throttle body, air filter, and fuel delivery system to ensure the proper mixture of air and fuel is being burned by the engine. If your reusable air filter is clogged up with debris, it can mess up the air-fuel ratio. Check out our article on how to clean your reusable air filter and ensure your engine is breathing properly.

In a fuel injected engine, the intake manifold's two main purposes are maintaining a high volume of air and distributing it equally between each cylinder. In carbureted engines, the intake manifold distributes the fuel/air mixture from the carburetor to the cylinders.

Parts of an Intake Manifold

Intake Manifold Plenum

The plenum is the large cavity at the top of the manifold. It acts as a reservoir, holding the air until it’s ready to go into the cylinders. The plenum evenly distributes the air to the runners before it passes through the intake valve.

The size of the plenum impacts the engine’s performance. Aftermarket manifolds can have a split plenum that can be separated into two. This design makes it easier to clean the inside of the manifold.

Intake Manifold Runners

The runners are tubes that carry air from the plenum to the intake port on each cylinder head. For fuel-injected engines, there are ports for the fuel injectors in each runner. Fuel is injected just before the air goes into the intake port.

The size of the runners is a critical factor when it comes to engine performance. The width and the length of the runners largely determine where the peak horsepower of the engine is.

Intake manifold diagram showing the plenum, throttle body, and runners

How Does an Intake Manifold Work?

Air travels through the intake and the throttle body into the plenum of the intake manifold. The plenum then equally disperses the air to each of the runners. During the intake stroke, the piston moves down and creates low pressure within the cylinder. This sucks the air from the runner (an area of high pressure) into the cylinder.

When the intake valve closes, it creates pressure waves that send the air back up the runner. It then bounces off the plenum and comes back into the cylinder when the intake valve opens again.

This process happens over and over again for each of the cylinders until you shut off your engine.

Evolution of Intake Manifolds

Prior to 1990, many vehicles had carbureted engines. In these vehicles, fuel is dispersed inside the intake manifold from the carburetor. Therefore, the intake manifold is responsible for delivering the fuel and air mixture to each cylinder.

To prevent the fuel from condensing on the cold walls of the intake manifold, heating is required. This can come from electric heating in the manifold, exhaust gases passing underneath, or from coolant circulating around it. Most intake manifolds from this time are made of cast iron or cast aluminum.

Starting in the early ‘90s, the majority of engines began using fuel injection to deliver gas to the cylinders. In these engines, the intake manifold is only involved in air distribution. Since heat is no longer needed to prevent fuel condensation, other materials can be used. It’s common to see cast aluminum as well as plastic intake manifolds on modern vehicles.

Performance Intake Manifolds

The design of the intake manifold affects how much air is delivered and how quickly. Everything from the diameter of the openings to the size and shape of the plenum and runners can alter how and when air is delivered.

Performance intake manifolds are equipped with larger plenums and runners for better airflow. Manifolds with a split plenum allow for easier polishing and cleaning. Spacers can sometimes be added to adjust the plenum size, which can help you obtain certain engine performance curves.

A plenum that is tapered toward the final cylinder will ensure more even air distribution. Some manifolds also have an air gap that helps reduce heat buildup for more power. Performance intake manifolds pair well with a new exhaust, cold air intake, cylinder heads, and throttle bodies.

Underhood shot of a performance intake manifold installed on an engine

Variable Length Intake Manifolds

Variable length intake manifolds have tuned runners that are designed to give you more power across a wider range of RPM bands. Even if you have a naturally aspirated engine, tuning your intake manifold can give you a supercharger effect.

As we established, the intake manifold runners are full of dense waves of air. How fast the air bounces back through the runner depends on its length and width. With the correct timing, the dense waves will arrive at the intake valve the moment it opens. This extra pressure sends more air into the cylinder, which gives it a supercharger effect for more power.

The timing of the intake valve opening and closing depends on the engine speed. With a fixed length runner, you only get the performance boost within a narrow RPM range. That’s why performance manifolds often come with variable length runners. Variable length intake manifolds have a combination of different sized runners to give you power across a broader RPM range.

At lower RPMs, it’s better to have a longer runner as it will take longer for the air to hit the plenum and come back down. Narrow runners are also good for lower RPM ranges as they increase the velocity and turbulence of the air for better combustion. Shorter runners are better for higher RPMs as the air is traveling faster. Runners designed for higher RPMs are often wider, allowing for enhanced airflow and less restriction.

Some variable intake length manifolds have butterfly valves that open and close depending on the engine speed. During lower RPMs, the valve will close to send air through a longer or more narrow section. At higher RPMs, the valve opens to redirect the air on a shorter, more direct path. Where fixed intake manifolds only provide a boost within a small RPM range, having tuned runners gives you more power at a wider range of RPMs.

Porting and Polishing

Porting and polishing your intake manifold is all about enhancing airflow. Sometimes the opening at the end of the runners doesn’t match up to the port openings on the cylinder heads. This slows down the air as it makes its way toward the intake valve and negatively affects performance.

Porting the manifold requires grinding away excess material so the opening on the runner and the cylinder head port are a similar diameter. Some also choose to open up the throttle body bore within the intake manifold as well.

Once you have a ported intake manifold, polishing is the next step. Polishing involves using abrasives to remove roughness on the walls of the runners. By smoothing the walls, you can increase runner volume.

A ported and polished intake manifold will improve airflow, leading to more horsepower and better performance.

Sources: How Does the Intake Manifold Affect Your Engine, How Stuff Works | Symptoms of Bad or Failing Intake Manifold Gaskets, Autoblog | Intake Manifold-Explained, Engineering Explained | Beginner's Guide: What Is an Intake Manifold (and What Does It Do)?, Haynes | What Are the Signs of a Leaking Intake Manifold, ItStillRuns

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