How Camshafts Work

By: Karim Nice  | 

The camshaft (click on image to see animation). See pictures of car engines.

If you have read the article How Car Engines Work, you know about the valves that let the air/fuel mixture into the engine and the exhaust out of the engine. The camshaft uses lobes (called cams) that push against the valves to open them as the camshaft rotates; springs on the valves return them to their closed position. This is a critical job, and can have a great impact on an engine's performance at different speeds. On the next page of this article you can see the animation we built to really show you the difference between a performance camshaft and a standard one.

In this article, you will learn how the camshaft affects engine performance. We've got some great animations that show you how different engine layouts, like single overhead cam (SOHC) and double overhead cam (DOHC), really work. And then we'll go over a few of the neat ways that some cars adjust the camshaft so that it can handle different engine speeds more efficiently.


Let's start with the basics.

Camshaft Basics


To understand why this is the case, imagine that we are running an engine extremely slowly -- at just 10 or 20 revolutions per minute (RPM) -- so that it takes the piston a couple of seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but let's imagine that we could. At this slow speed, we would want cam lobes shaped so that:


This setup would work really well for the engine as long as it ran at this very slow speed. But what happens if you increase the RPM? Let's find out.



The animation below shows how a regular cam and a performance cam have different valve timing. Notice that the exhaust (red circle) and intake (blue circle) cycles overlap a lot more on the performance cam. Because of this, cars with this type of cam tend to run very roughly at idle.

Two different cam profiles: Click the button under the play button to toggle between cams. The circles show how long the valves stay open, blue for intake, red for exhaust. The valve overlap (when both the intake and exhaust valves are open at the same time) is highlighted at the beginning of each animation.

Any given camshaft will be perfect only at one engine speed. At every other engine speed, the engine won't perform to its full potential. A fixed camshaft is, therefore, always a compromise. This is why carmakers have developed schemes to vary the cam profile as the engine speed changes.

There are several different arrangements of camshafts on engines. We'll talk about some of the most common ones. You've probably heard the terminology:

  • Single overhead cam (SOHC)
  • Double overhead cam (DOHC)
  • Pushrod

In the next section, we'll look at each of these configurations.


Camshaft Configurations

Damage from a piston striking a valve
Damage from a piston striking a valve

Single Overhead Cam




A single overhead cam



Double Overhead Cam




A pushrod engine
A pushrod engine

Pushrod Engines



A pushrod engine




Variable Valve Timing

The variable cam system used on some Ferraris
The variable cam system used on some Ferraris

­There are a couple of novel ways by which carmakers vary the valve timing. One­ system used on some Honda engines is called VTEC.






For more information on camshafts, valve timing and related topics, check out the links on the next page.


Originally Published: Dec 13, 2000

Camshaft FAQ

What does a camshaft sensor do?
A camshaft sensor feeds information to the engine about the components and conditions of the air, fuel and spark ignition.
Do camshafts increase horsepower?
Yes, camshafts can increase the horsepower of an engine as much 50 HP.
What are the signs of a bad camshaft?
Signs of a failing camshaft include the check engine light coming on, the vehicle doesn't drive like it used to (idles roughly, accelerates slowly, stalls or gets reduced gas mileage), or the vehicle simply won't start.
What is the function of a camshaft in a diesel engine?
A camshaft does the same thing in both a gasoline engine and a diesel, however because diesels require a lot of compression, the camshaft duration tends to be shorter.