An intake manifold is comprised of the main trunk that diverges into separate passages leading to the individual intake valves for each cylinder, which routes the incoming air from the throttle body or carburetor to the cylinder head.
The basic intake manifold will be designed for minimum resistance to airflow, lightweight, and ease to manufacture at a relatively low cost. Minimum resistance is achieved through relatively straight runners, the individual ducts leading to the cylinders.
If turns must be present in the ducts of the manifold, they should be generally designed with big radii, unless a right angle is desired to promote fuel vaporization by shattering fuel droplets against the interior walls.
The manifold design should distribute the incoming air equally to each of the cylinders for optimal performance and incorporate smooth inner surfaces to aid in laminar flow. Smooth walls reduce the viscous friction, but some designs may use a slightly rough wall to assist in the evaporation of the fuel from the accompanying turbulence.
Roughness may also be desired to reduce the speed of flow near the inner radius of bends in the ducts.
Basic Operation and Design
The main design constraint for a carbureted or throttle body fuel injected system is for equal distribution of air and fuel to each of the cylinders, which is not necessary in multi-port injection because of the alternative fuel delivery method.
Distribution is further complicated through the use of multiple carburetors. To combat the problem of some cylinders receiving a lean or rich mixture, a mixing box may be designed to assist in a more equal distribution.
Another potential problem arises from the suction created by the opening of the intake valves, which may rob neighboring tubes of their fuel mixture. To combat this problem, manifolds may be divided into two subsections separating the runners for opposing cylinder induction strokes.
Another important design consideration is to avoid depositing of liquid gas on the walls of the manifold. For example, at idle, the manifold walls are dry and the air is virtually saturated with vapor. If the throttle is suddenly depressed, the density of the air suddenly rises which effectively squeezes the fuel vapor from the air leading to condensation on the manifold walls.
In a cold starting engine, the fuel may not fully evaporate because of the lower temperatures causing for pools of fuel to form in the manifold. The liquid fuel cannot be allowed to drip into the cylinders in order to prevent misfiring and dilution of the lubrication.
To prevent these conditions, a well beneath the riser can be designed to capture the liquid fuel as well as assist in the re-evaporation of the fuel. Some runners may also be slightly sloped down from the cylinder head to prevent entering liquid fuel into the cylinder.
Buffered ends also assist in straight rake-type manifolds to lead the condensed fuel into the well. Lastly, to ensure fuel 70 is not deposited on the walls of the manifold, in general, the runners should be sized so the velocity of the incoming air is no less than 70 m/s.