<P> A dual overhead camshaft (DOHC) valvetrain layout is characterised by two camshafts located within the cylinder head, one operating the intake valves and the other one operating the exhaust valves . This design reduces valvetrain inertia more than is the case with an SOHC engine, since the rocker arms are reduced in size or eliminated . A DOHC design permits a wider angle between intake and exhaust valves than in SOHC engines . This can give a less restricted airflow at higher engine speeds . DOHC with a multivalve design also allows for the optimum placement of the spark plug, which in turn improves combustion efficiency . Engines having more than one bank of cylinders (e.g. V6, V8--where two cylinder banks meet to form a "V") with two camshafts in total remain SOHC unless each cylinder bank has two camshafts; the latter are DOHC, and are often known as "quad cam". </P> <P> Although the term "twin cam" is often used to refer to DOHC engines, it is imprecise, as it includes designs with two block - mounted camshafts . Examples include the Harley - Davidson Twin Cam engine, Riley car engines from 1926 to the mid 1950s, Triumph motorcycle parallel - twins from the 1930s to the 1980s, and Indian Chief and Scout V - twins from 1920 to the 1950s . </P> <P> The terms "multivalve" and "DOHC" do not refer to the same thing: not all multivalve engines are DOHC and not all DOHC engines are multivalve . Examples of DOHC engines with two valves per cylinder include the Alfa Romeo Twin Cam engine and the engine of the Jaguar XK6 . Most recent DOHC engines are multivalve, with between three and five valves per cylinder . </P> <P> More than two overhead camshafts are not known to have been tried in a production engine . However, MotoCzysz has designed a motorcycle engine with a triple overhead camshaft configuration, with the intake ports descending through the cylinder head to two central intake ports between two outside exhaust camshafts actuating one of two exhaust valves per cylinder each . </P>

How does a single overhead cam engine work