DOHC
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 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). 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 Fiat (1912), 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 for the technology to be heavily advertised. Fiat, Lancia and Alfa Romeo can be credited with placing mass produced twincam (DOHC) engines in coupes, sedans, convertables, and station wagons beginning in the mid 1960's. Later, in the 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, though by comparison, Toyota debuted two production DOHC engines in 1967: the inline-4 9R engine (Corona 1600GT) and the Yamaha-designed inline-6 3M engine (2000GT).