Overhead camshaft (OHC) valvetrain configurations place the camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters directly instead of using pushrods. When compared directly with cam-in-block (or OHV) systems with the same number of valves, the reciprocating components of the OHC system are fewer and in total will have less mass. Though the structures that support the system may become more complex, most engine manufacturers easily accept the added complexity in trade for better engine performance and greater design flexibility. The OHC system can be driven using the same methods as an OHV system, these methods may include using a timing belt, chain, or in less common cases, gears.
Many OHC engines today employ Variable Valve Timing and multiple valves to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs.
There are two overhead camshaft layouts:
- Single Overhead Camshaft (SOHC)
- Double Overhead Camshafts (DOHC)
Single Overhead Camshaft (SOHC)
Single overhead camshaft is a design in which one camshaft is placed within the cylinder head. In an inline engine this means there is one camshaft in the head, while in a V engine there are two camshafts: one per cylinder bank.
The SOHC design is inherently mechanically more efficient than a comparable cam-in-block design. This allows for higher engine speeds, which in turn will by definition increase power output for a given torque. The cams operate the valves directly or by a short rocker as opposed to overhead valve cam-in-block engines, which have tappets and long pushrods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.
SOHC designs offer reduced complexity compared to cam-in-block designs when used for multivalve heads, in which each cylinder has more than two valves.
Double Overhead Camshafts
A double overhead camshaft (also called double overhead cam, dual overhead cam or twincam) valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves. In engines with more than one cylinder bank (V engines) this designation means two camshafts per bank, for a total of four.
Double overhead camshafts are not required in order to have multiple inlet or exhaust valves, but are necessary for more than 2 valves that are directly actuated (though still usually via tappets). Not all DOHC engines are multivalve engines — DOHC was common in two valve per cylinder heads for decades before multivalve heads appeared, however today DOHC is synonymous with multivalve heads, since almost all DOHC engines have between three and five valves per cylinder.
[[Image:ohc3.jpg|right|frame|DOHC straight-8 in a 1933 Bugatti Type 59 Grand Prix racer
The first DOHC engines were two valve per cylinder designs from companies like Fiat (1912), Peugeot (1913), Alfa Romeo (6C- 1925, 512 - 1940), Maserati (Tipo 26, 1926), and Bugatti (Type 51, 1931). Most Ferraris used two valve per cylinder DOHC engines as well.
When DOHC technology was introduced in mainstream vehicles, it was common for the technology to be heavily advertised. While the technology was used at first in limited production and sports cars, the Fiat group is historically credited as the first car company to use a belt driven DOHC engine across their complete product line, comprised of coupes, sedans, convertables and station wagons, in the mid-1960's.