Exhaust Manifold
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 compartment.

Intake Manifold
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:

• Deliver 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.
• Help 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.
• The 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.
• Aid 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 vaporization—without heating it to the point of reducing volumetric efficiency.

The intake 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.

Intake 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.

As stated 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 following:

• Directing 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.
• Directing 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.

Figure 3-15.—Typical intake manifold.

Figure 3-16.—Ram induction manifold.

Figure 3-17.—Exhaust-heated intake manifold.

Figure 3-18.—Water-heated intake manifold.