to the valves either directly (using a tappet) or indirectly via a rocker arm. Double overhead camshaft (DOHC) A double overhead cam, dual overhead cam, or twin-cam engine has two camshafts per bank of the cylinder head, one for the intake valves and another for the exhaust valves. Therefore there are two camshafts for a straight engine and a total of four camshafts for a V engine or a flat engine. A V engine or flat engine requires four camshafts to function as a DOHC engine, since having two camshafts in total would result in only a single camshaft per cylinder bank for these engine layouts. Some V engines with four camshafts have been marketed as "quad-cam" engines, however technically "quad-cam" would require four camshafts per cyinder bank (i.e. eight camshafts in total), therefore these engines are merely double overhead camshaft engines. Most DOHC engines have four valves per cylinder. The camshaft usually operates the valves directly via a bucket tappet. A DOHC design permits a wider angle between intake and exhaust valves than in SOHC engines, which improves the air-fuel mixture's flow through the engine. A further benefit is that the spark plug can be placed at the optimum location, which in turn improves combustion efficiency. Components Timing belt / timing chain The rotation of a camshaft is driven by a crankshaft. Many 21st century engines use a toothed timing belt made from rubber and kevlar to drive the camshaft. Timing belts are inexpensive, produce minimal noise and have no need for lubrication. A disadvantage of timing belts is the need for regular replacement of the belt; recommended belt life typically varies between approximately . If the timing belt is not replaced in time and fails and the engine is an interference engine, major engine damage is possible. The first known automotive application of timing belts to drive overhead camshafts was the 1953 Devin-Panhard racing specials built for the SCCA H-modified racing series in the United States. These engines were based on Panhard OHV flat-twin engines, which were converted to SOHC engines using components from Norton motorcycle engines. The first production car to use a timing belt was the 1962 Glas 1004 compact coupe. Another camshaft drive method commonly used on modern engines is a timing chain, constructed from one or two rows of metal roller chains. By the early 1960s most production automobile overhead camshaft designs used chains