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.
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 GM's first was the 1975 Cosworth-tweaked 2300.
You can actuate four valves per cylinder (or three, like Mercedes-Benz) with one camshaft by using rocker arms to open one set or both, depends on how you want to do it.
As for DOHC, again, there is an advantage in weight, at an extra cost, both financially and in complexity.
Remember, Roger Penske's car won the Indy 500 a few years ago with a pushrod engine, everybody else had DOHC.
Not all DOHC engines are multivalve engines â 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.
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).
When DOHC technology was introduced in mainstream vehicles, it was common for the technology to be heavily advertised.
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