What does it mean when a car has dual overhead cams?

A camshaft plays an integral role in the functioning of an engine. And dual overhead cams ramp up a machine's performance even more. Find out how dual overhead camshafts work in this episode.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from house Stuff Works dot com where
smart Happens. Hi Am Marshall Brain with today's question, what
does it mean when a car has dual overhead cams? Generally,
engines with dual overhead cams or d o hc are

(00:24):
higher performance engines. They produce more power and they can
run at higher speeds. The camshafts have the job of
opening the valves that let air into an exhaust out
of the engine. The camshaft uses rotating lobes called cams
that push against the valves to open them. Springs on
the valve return them to the closed position. This is

(00:47):
a critical job and can have a great deal of
impact on the engine's performance at different speeds. The main
benefit of dual overhead cams is that they allow an
engine to have four valves per cylinder. Each camshaft operates
two of the valves on a cylinder. One camshaft handles
the intake valves, the other one handles the exhaust valves.

(01:09):
Having four valves per cylinder gives an engine two big advantages. First,
by having four valves in a cylinder instead of two,
a larger portion of the area can be used to
let air in an exhaust out. The engine can make
more power if more air enters the cylinder, and it
weighs less power if it's easier to pump the exhaust

(01:30):
out of the cylinder. At higher engine speeds, the engine
pumps a lot of air through these cylinders. Having four
valves per cylinder allows the engine to pump enough air
to run and make useful power at these higher speeds.
A second interesting thing that some car makers do is
to have a separate intake runner for each of the

(01:52):
two intake valves in each cylinder. One of the intake
runners is wide and short for maximum airflow, and the
other is a tuned intake runner. When the intake valve
is open on the engine, air is being sucked into
the engine, so the air and the intake runner is
moving rapidly towards the cylinder. When the intake valve closes suddenly,

(02:16):
this air slams to a stop and stacks up on itself,
forming an area of high pressure. This high pressure wave
makes its way up the intake runner away from the cylinder,
and when it reaches the end of the intake runner
where the runner connects to the intake manifold, the pressure
wave bounces back down the intake runner. If the intake

(02:38):
runner is just the right length, that pressure wave will
arrive back at the intake valve just as it opens
for the next cycle. This extra pressure helps to cram
more air and fuel mix into the cylinder, effectively acting
like a turbocharger for moral This and thousands of other
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