What Are Dual Overhead Cams?

A cutaway dual overhead cam engineSee more pictures of car engines.

­­Generally, engines with dual overhead camshafts (DOHC) are higher performance engines, they produce more power, and can run at higher spe­eds.

The camshafts have the job of opening the valves that let air into and exhaust out of the engine. The camshaft uses rotating lobes, called cams, that push against the valves to open them. 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.

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The main benefit of dual overhead cams is that they allow an engine to have four valves per cylinder. Each camshaft operates two of the valves, one camshaft handles the intake valves, and one handles the exhaust valves.

Having four valves per cylinder gives an engine several advantages.

  • By having four valves in a cylinder instead of two, a larger portion of the area can be used to let air in and exhaust out. The engine can make more power if more air enters the cylinder, and it wastes less power if it is easier to pump the exhaust out of the cylinder. At higher engine speeds, the engine pumps a lot of air though the cylinders. Having four valves per cylinder allows the engine to pump enough air to run and make useful power at these higher speeds.
  • Another interesting thing that some car makers do is have a separate intake runner for each of the two intake cylinders. One of the intake runners is wide and short for maximum airflow, the other is a tuned intake runner. When the intake valve is open on the engine, air is being sucked into the engine, so the air in the intake runner is moving rapidly toward the cylinder. When the intake valve closes suddenly, this air slams to a stop and stacks up on itself, forming an area of high pressure. This high-pressure wave makes its way up the intake runner away from the cylinder. When it reaches the end of the intake runner, where the runner connects to the intake manifold, the pressure wave bounces back down the intake runner. If the intake runner is just the right length, that pressure wave will arrive back at the intake valve just as it opens for the next cycle. This extra pressure helps cram more air-fuel mix into the cylinder -- effectively acting like a turbocharger.

For more articles about car engines and related topics, check out the links on the next page.

Originally Published: Jan 31, 2001

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Dual Overhead Cams FAQ

What is a DOHC engine?
A DOHC engine is a dual overhead camshaft engine, or an engine with two camshafts.
What is the advantage of a DOHC engine?
The biggest advantages of a DOHC engine are increased power and increased speed. This type of engine can particularly increase your vehicle's engine power at high speeds.
What are dual overhead camshafts?
A dual overhead camshaft is an engine that features two camshafts per bank of the cylinder head. One camshaft is for the intake valves and the other is for the exhaust valves.
How does a camshaft work?
Camshafts push against valves, which opens the valves. The camshafts rotate while this happens, and this in turn affects engine performance and speed.
Why is dual overhead cam better?
Dual overhead cams can be better if you're looking for increased performance and high speeds. The unique designed of a DOHC engine can result in both of these benefits.