Mazda Training manual — part 242

4 – VALVE TRAIN

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OVERHEAD CAM VALVE TRAIN

The overhead cam (OHC) design provides more direct control over valves
than the OHV design. This advantage may explain why overhead cams are
the most commonly used design. The overhead cam design includes
engines with a single overhead cam (OHC or SOHC), as well as engines
with dual overhead cams (DOHC).



Overhead Cam (OHC or SOHC)

In an overhead cam engine, the camshaft is installed in the cylinder head
above the valves. The camshaft, shown in Figure 36, is a solid or hollow
cast iron shaft with several cams on it. Each cam has an off-center bulge
on one side called a cam lobe. On most Mazda OHC engines, the
camshaft runs directly on the cylinder head journal surface without insert
bearings.

FIGURE 36. The
camshaft rides
directly on the
cylinder head
journal surface.

Cam lobes

Camshaft
journal

Cams

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FIGURE 37. As
the camshaft
turns, the lobe
contacts the
rocker arms,
opening and
closing the
valves.

Rocker arms

Camshaft

Valves

Cam lobes

To make sure that the valves open and close at the right time in the
four-stroke cycle, the camshaft is driven by the crankshaft. The camshaft
turns at one half the speed of the crankshaft because it takes two
revolutions of the crankshaft to complete one cycle. So the camshaft
completes a full cycle in one revolution.

As the camshaft turns, a lobe on one of the cams contacts a rocker arm
which presses on the valve stem end and opens the valve, as shown in
Figure 37. As the lobe turns past the rocker arm, it releases the valve,
allowing the spring to close it. The single overhead camshaft includes
two or more cam lobes for each cylinder — one lobe for each valve — so
that the intake and exhaust valves can open and close at different times.



















On V-6 and V-8 engines with an SOHC, two separate camshafts are used,
one for each bank of cylinders. Even though two camshafts are used, this
is still an SOHC design because one camshaft operates all the valves for
the cylinder bank.

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Dual Overhead Cam (DOHC)

The dual overhead cam (DOHC) design is also very common in
light-weight, multi-valve engines that operate at high speeds. Dual
camshafts reduce the weight of the valve train parts, which means the
engine can generate more power without an increase in displacement and
weight.

V-6 and V-8 engines with dual camshafts actually have four separate
camshafts, two for each bank of cylinders.

In addition, the OHC design also allows four valves per cylinder — two
intake valves and two exhaust valves, as shown in Figure 38. This
arrangement improves the intake and exhaust efficiency of the engine.



CAMSHAFT DRIVES

This section covers four types of camshaft drives:

• OHV

Drive

• OHC

Drive

•

Belt and Chain Drive

•

Gear-driven Camshaft with Friction Gear

FIGURE 38. Dual
camshafts
reduce the
weight of the
valve train and
improve intake
and exhaust
efficiency.

Camshafts

Exhaust
valves

Intake valves

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FIGURE 39. OHV
drives typically
use a timing
chain to drive the

camshaft.

Camshaft

Camshaft

gear

Timing chain

Crankshaft

gear

OHV Drive

In an OHV engine, where the camshaft is mounted below the valves in the
block, a crankshaft gear is used to drive a timing chain, which drives the
camshaft gear. Figure 39 shows a typical drive for an OHV engine.