The exhaust manifold (fig. 3-14) connects all of the engine cylinders to
the rest of the exhaust system. On L-head engines, the exhaust manifold bolts to the side
of the engine block; and on overhead-valve engines, it bolts to the side of the cylinder
head. It is usually made of cast iron, either singly or in sections. If the exhaust
manifold is made properly, it can create a scavenging action that causes all of the
cylinders to help each other get rid of the gases. Back pressure (the force that the
pistons must exert to push out the exhaust gases) can be reduced by making the manifold
with smooth walls and without sharp bends. Exhaust manifolds on vehicles today are
constantly changing in design to allow the use of various types of emission controls. Each
of these factors is taken into consideration when the exhaust manifold is designed, and
the best possible manifold is manufactured to fit into the confines of the engine
The intake manifold on a gasoline engine carries the air-fuel mixture from the
carburetor and distributes it to the cylinders. On a diesel engine, the manifold carries
only air into the cylinders. The gasoline engine intake manifold (fig. 3-15) is designed with the following
functions in mind:
the air-fuel mixture to the cylinders in equal quantities and proportions. This is
important for smooth engine performance. The lengths of the passages should be near to
equal as possible to distribute the air-fuel mixture equally.
to keep the vaporized air-fuel mixture from condensing before it reaches the combustion
chamber. The ideal air-fuel mixture should be vaporized completely, as it enters the
combustion chamber. This is very important.
manifold passages are designed with smooth walls and a minimum of bends that collect fuel
to reduce the condensing of the mixture. Smooth flowing intake manifold passages also
increase volumetric efficiency.
in the vaporization of the air-fuel mixture. To do this, provide the intake manifold a
controlled system of heating. This system of heating must heat the mixture enough to aid
in vaporizationwithout heating it to the point of reducing volumetric efficiency.
manifold on an L-head engine is bolted to the block, whereas the overhead-valve engine has
the intake manifold bolted to the side of the cylinder head.
manifolds can be designed to provide optimum performance for a given speed range by
varying the length of the passages (fig. 3-16). The inertia of the moving intake
mixture causes it to bounce back and forth in the intake manifold passage from the end of
one intake stroke to the beginning of the next intake stroke. If the passage is the proper
length so the next intake stroke is just beginning as the mixture is rebounding, the
inertia of the mixture causes it to ram itself into the cylinder. This increases the
volumetric efficiency of the engine in the designated speed range. It should be noted that
the ram manifold serves no purpose outside its designated speed range.
earlier, providing controlled heat for the incoming mixture is very important for good
performance. The heating of the mixture may be accomplished by doing one or both of the
a portion of the exhaust through a passage in the intake manifold (fig. 3-17). The heat from the exhaust transfers
and heats the mixture. The amount of exhaust that is diverted into the intake manifold
heat passage is controlled by the manifold heat control valve.
the engine coolant, which is heated by the engine, through the intake manifold on its way
to the radiator (fig. 3-18).
Figure 3-14.Exhaust manifold.
3-15.Typical intake manifold.
3-16.Ram induction manifold.
3-17.Exhaust-heated intake manifold.
3-18.Water-heated intake manifold.