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What is DOHC?

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					Overhead cam (OHC) valvetrain configurations place the engine camshaft within the
cylinder heads, above the combustion chambers, and drive the valves or lifters in a more
direct manner compared to overhead valves (OHV) and pushrods. Compared to OHV
pushrod (or I-Head) systems with the same number of valves the reciprocating
components of the OHC system are fewer and have a lower total mass. Though the
system that drives the cams may become more complex, most engine manufacturers
easily accept that added complexity in trade for better engine performance and greater
design flexibility. Another performance advantage is gained as a result of the better
optimized port configurations made possible with overhead camshaft designs. With no
intrusive pushrods the overhead camshaft cylinder head design can use straighter ports of
more advantageous crossection and length.

The OHC system can be driven using the same methods as an OHV system, which
include using a rubber/kevlar toothed timing belt, chain, or in less common cases, gears.

In conjunction with multiple (3, 4 or 5) valves per cylinder, many OHC engines today
employ variable valve timing to improve efficiency and power. OHC also inherently
allows for greater engine speeds over comparable cam-in-block designs, as a result of
having lower valvetrain mass.

There are two overhead camshaft layouts: single overhead camshaft ("SOHC"), and
double (or dual) overhead camshaft ("DOHC").

Single overhead camshaft

A single overhead camshaft cylinder head from a 1987 Honda CRX Si.

Single overhead camshaft (SOHC) is a design in which one camshaft is placed within the
cylinder head. In an inline engine this means there is one camshaft in the head, while in a
V engine or a horizontally-opposed engine (boxer; flat engine) there are two camshafts:
one per cylinder bank.

The SOHC design has less reciprocating mass than a comparable pushrod design. This
allows for higher engine speeds, which in turn will increase power output for a given
torque. The cam operates the valves directly or through a rocker arm, as opposed to
overhead valve pushrod engines which have tappets, long pushrods, and rocker arms to
transfer the movement of the lobes on the camshaft in the engine block to the valves in
the cylinder head.

 A WW I-era Hispano-Suiza V8 aviation engine, which used single overhead camshafts
for each cylinder bank.

In the early era of the liquid-cooled aircraft engine field, single overhead cam format
engines were in existence during the First World War, for both the Allies and the Central
Powers. The Hispano-Suiza 8 V8 engine, designed by Marc Birkigt in the Allied camp,
and the series of Mercedes inline-6 aviation engines, culminating in the Mercedes D.III
for the German Empire, both used rotary shaft-driven single overhead camshaft valve
drive systems, and were among the most prominent aviation powerplants of the First
World War era. The late-war Liberty L-12 V12 configuration American aviation engine
also used the general Mercedes D-series single overhead camshaft design, based
primarily on the later D.IIIa's drive system from rocker box to valvestem.

SOHC designs offer reduced complexity compared to pushrod designs when used for
multi-valve heads in which each cylinder has more than two valves. An example of an
SOHC design using shim and bucket valve adjustment was the engine installed in the
Hillman Imp (4 cylinder, 8 valve); a small, early 1960s 2-door saloon car with a rear
mounted alloy engine based on the Coventry Climax FWMA race engines. Exhaust and
inlet manifolds were both on the same side of the engine block (thus not a crossflow
cylinder head design). This did, however, offer excellent access to the spark plugs.

[edit]
Double overhead camshaft

Overhead view of Suzuki GS550 head showing dual camshafts and drive sprockets.

A double-overhead-camshaft (also known as dual-overhead-camshaft) valve-train layout
is characterized by two camshafts located within the cylinder head, one operating the
intake valves and one operating the exhaust valves. Some engines have more than one
bank of cylinder heads (i.e. V6, V8 where 2 cylinder banks meet to form a 'V') and these
have two camshafts in total, but they remain SOHC, unless each side has two camshafts.
The term "twin cam" is imprecise, but will normally refer to a DOHC engine. Some
manufacturers still managed to use a SOHC in 4-valve layouts. Honda, for instance, with
the later half of the D16 family, utilizes the 4-valve-per-cylinder, SOHC layout to reduce
overall costs. Also, not all DOHC engines are multivalve engines—DOHC was common
in two-valve-per-cylinder heads for decades before multivalve heads appeared. Today,
however, DOHC is synonymous with multi-valve heads, since almost all DOHC engines
have between three and five valves per cylinder