The purpose of the intake manifold, or inlet manifold, is to evenly distribute the air-fuel mixture to the cylinders as well as to the carburetor on a non-fuel-injected engine. An intake manifold is the opposite of an exhaust manifold, which collects gases from the cylinders.
After the carburetor completely mixes the air and fuel, the combustion mixture goes through a manifold of tubes on its way to each cylinder. In the case of a fuel-injected engine, the fuel is injected through the injectors into the intake manifold.
The intake manifold then evenly distributes the fuel-air mixture to the cylinders. Even distribution is a key function of the intake manifold to maximize the efficiency and the proper performance of the engine. Other engine components may be mounted on top of the intake manifold, including the carburetor, throttle body and fuel injectors.
The intake manifold creates a vacuum from downward piston movement and throttle valve restriction. The vacuum becomes a source of power to drive basic car functions such as power windows, power brakes, cruise control and others. The same vacuum also functions to remove piston blow-by gases from the engine. The gases are then burned along with the combustion mixture.