The intake manifold distributes air to the individual cylinders from a central air inlet. In carbureted or throttle body injected vehicles, the air distributed also carries the fuel charge for combustion. This is called a wet manifold design. Intake manifold design and construction varies widely from model to model. Usually they are cast of iron or aluminum, recently however, some manufactures have been using a plastic based material to construct intake manifolds.
The passages that carry air to the cylinders in the intake manifold are referred to as runners. They are designed to specific length that determines the velocity of the air moving to a specific cylinder. Some high performance engines have elaborate intake manifold configurations that can provide a high velocity charge of air to a cylinder. However, whether an engine is used in an economy car or high performance vehicle, an intake manifold is carefully designed to provide equal distribution to each cylinder.
Exhaust manifolds carry the combustion by-products away from the engine. They are usually constructed of cast-iron or steel tubing. These materials are used for their ability to withstand exposure to the intense heat of the engine exhaust. They also must withstand the constant expansion and contraction they are subject to as the engine heats and cools.
As in intake manifold designs, the design characteristics of exhaust manifolds can affect engine performance. When an engine is on the exhaust stroke, the flow of gases out of the combustion chamber is affected by its ability to flow through the exhaust manifold and the exhaust system. The resistance that the exhaust flow encounters causes a pressure in the exhaust system. This is called exhaust back pressure. Most passenger cars and light trucks use exhaust systems that provide a predetermined amount of back pressure. This can help to provide better performance at low speeds and mid-range by raising internal cylinder pressures. In high performance racing engines, there is a need to flow air through the engine as quickly as possible. Exhaust systems are designed to provide as little back pressure as possible.
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