DOHC

DOHC, standing for double overhead camshaft and sometimes written as twin-cam, describes a valvetrain layout in which each cylinder head carries two camshafts mounted above the valves: one dedicated to opening the intake valves and the other to the exhaust valves. It is the arrangement that dominates modern petrol and diesel engines, having displaced simpler designs as the demand for higher revs, more power, and cleaner combustion has grown. On a V-configuration engine, which has two separate heads, a DOHC layout therefore uses four camshafts in total, a fact reflected in descriptions such as 'quad-cam'.

The camshaft itself is a rotating shaft whose lobes push the valves open in time with the pistons, driven from the crankshaft by a timing belt or chain at exactly half engine speed in a four-stroke. Placing the camshafts in the head, directly over the valves, shortens and stiffens the path between lobe and valve, often acting straight onto a bucket tappet or a short rocker. By splitting that duty across two shafts, the intake and exhaust valves can be operated entirely independently, each optimised without compromise from the other.

The most immediate practical advantage is that separating the valve banks makes a four-valves-per-cylinder layout, with two intake and two exhaust valves, easy to arrange in a compact pent-roof combustion chamber, with the spark plug placed centrally between them. More valves mean greater total port area and better breathing, so the engine can ingest and expel gas more freely at high speed. The light, direct-acting valvetrain also keeps reciprocating mass low, allowing the engine to rev higher safely and to make more power from a given capacity.

A further benefit, increasingly important, is control over valve timing. Because intake and exhaust are driven by separate shafts, manufacturers can fit independent variable valve timing to each, advancing or retarding the cams individually to suit different speeds and loads. This dual-independent control broadens the torque curve, improves idle quality and emissions, and supports techniques such as internal exhaust gas recirculation, none of which is possible with a single shared shaft.

The alternative is the SOHC layout, with one camshaft per head operating both intake and exhaust valves through a more elaborate rocker arrangement; it is simpler, narrower, and cheaper, but more constrained. The trade-offs of DOHC are a wider, heavier, and more complex head and a longer, more demanding timing drive. Even so, as efficiency and output requirements have risen, the double overhead camshaft has become the default for all but the most cost-driven engines.