A double overhead cam (also called a dual overhead cam, DOHC, or "twincam") engine is a type of internal combustion engine where the camshafts that operate the intake and exhaust valves are mounted above the cylinders, and where there are separate camshafts for inlet and exhaust valves. In engines with more than one cylinder bank, such as the V engine, this designation means two camshafts per bank.
Usually the cams operate the valves directly or by a short rocker, as opposed to pushrod engines which have long rods to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.
Double camshafts are required in order to have multiple inlet or exhaust valves that are directly actuated. However, not all DOHC engines are multivalve engines - DOHC was common in 2-valve engines for decades before multivalve heads appeared. Today, DOHC is normally synonymous with multivalve, since almost all DOHC engines also have between 3 and 5 valves per cylinder.
History
The first DOHC engines were 2-valve designs from companies like Peugeot (1913), Alfa Romeo (6C , 1925), Maserati (Tipo 26 , 1926), and Bugatti (Type 51 , 1931). Most Ferraris used 2-valve DOHC engines as well.
When DOHC technology was introduced in mainstream vehicles, it was common the technology to be heavily advertised. Many mid-1980s Honda products featured "DOHC" plaques, and automakers often used "DOHC" as the engine's name. Most early mainstream DOHCs were 4-cylinder engines as well: Ford's first DOHC motor was the Lotus-reworked Kent (1962), and General Motors' first was the 1975 Cosworth-tweaked 2300.
See also: