For carburetor or throttle gasoline jet engines, the intake manifold refers to the inlet pipe after the carburetor or throttle body to the inlet of the cylinder head into the airway.
Its function is to distribute air and fuel mixture from carburetor or throttle body to the inlet of each cylinder. For airway fuel injection engines or diesel engines, the intake manifold simply distributes clean air to the inlet of each cylinder. The intake manifold must distribute air, fuel mixture or clean air as evenly as possible to each cylinder, so that the length of the gas runner in the intake manifold tube 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.