For carbureted or throttle body gasoline injection engines, the intake manifold refers to the intake line after the carburetor or throttle body to the intake port in the cylinder head. Its function is to distribute the air and fuel mixture to the intake port of each cylinder from the carburetor or throttle body.
For port fuel injection engines or diesel engines, the intake manifold simply distributes clean air to the intake ports of each cylinder. The intake manifold must distribute the air, fuel mixture or clean air as evenly as possible to the individual cylinders, for this reason the lengths of the gas runners in the intake manifold should be as equal as possible. In order to reduce the gas flow resistance and improve the intake capacity, the inner wall of the intake manifold should be smooth.