An overhead cam, or overhead camshaft, is the part of a vehicle's engine responsible for opening and closing the valves which allow the air/fuel mixture to enter the engine's combustion chamber and exhaust to exit. The overhead camshaft is located in the cylinder head of the engine instead of in the cylinder block, as with an in-block camshaft. An overhead cam is popular due to the increase in the car engine's performance compared to other camshaft arrangements, such as cam-in-block. The two major types include single overhead camshaft and double overhead camshaft. When double camshafts are present, one controls the intake valves while the other controls the exhaust valves.
Internal combustion engines, the most common type found in cars, perform a four-stroke cycle. On the intake, the camshaft pushes on the intake valve to open it and allow air and fuel to enter the combustion chamber. The chamber then compresses the mixture and, once the spark plug fires, combustion occurs. In the final stroke, the camshaft opens a second valve to allow the exhaust to leave the chamber.
The single overhead cam controls both intake and exhaust valves. They still open at separate times, but only one camshaft is needed to control the process. In a V-shaped engine, which contains two columns of cylinders that meet to form a V, one camshaft is needed for each side. The arrangement is still a single overhead cam, but technically two camshafts are used for the engine.
In a double or dual overhead cam, one camshaft operates the intake valves and a second, separate camshaft operates the exhaust valves. The V-shaped engine will, once again, need two cams for each side to make it work. While the engine contains four camshafts in total, it's still considered a double overhead cam. This type is typically found in high-performance vehicles because it allows for more valves and thus more power.
Camshafts can activate the valves in a number of ways. Nodes, or bumps, on the camshaft push on a rocker arm which in turn pushes on the valve. When the camshaft spins so the node no longer pushes against the rocker arm, it falls back and a spring closes the valve. Rocker arms are smaller and allow the camshaft to act directly on them. In engines without overhead camshafts, the camshaft must act from farther away by using a long, straight rod known as a pushrod.