Intake Manifold Science

Most stock engines are fitted with a single carburetor that delivers fuel based on “average” intake manifold flow signals, so it stands to reason that some cylinders will receive more or less fuel than others (depending upon how correct the intake manifold design may be). And since most factory intake manifolds are of the two-plane (two-cavity) design, problems of unequal air distribution have historically been associated with this type of configuration.
In order to avoid some of the problems brought about by sudden pressure variations inside a given manifold, it has been found beneficial to join each of a manifold’s runners into a common volume (or plenum). This design (single-plane or single-cavity) typically has better cylinder-to-cylinder air distribution characteristics than a two-plane, but there is usually some loss of low-rpm flow velocity (and subsequent reduction in carburetor fuel flow signals). Through the combination of the better features of each type, it is possible to provide flow rates that are quite high (as if the engine were being operated at very high rpm) but with cylinder-to-cylinder air distribution approaching the characteristics of a single-plane design.
For want of a better description, you could call such a design a single-plane/two-plane manifold.
Assuming that each of a given engine’s cylinders is getting the same (or about the same) amount of air, the next objective is to provide the required amount of fuel. This is a particular problem where pressure (or energy) conditions in an engine’s intake manifold vary, resulting in the separation of air and fuel as both are conditioned for combustion within the intake manifold. If for some reason fuel droplets are caused to increase in size, the normal combustion processes will not consume such liquid fuel. Intake manifolds that allow the collection of atomized fuel into “liquid” particles reduce the overall horsepower efficiency of a given engine. Such lost power also contributes to an increase in exhaust emissions, since unspent fuel shows up as unburned hydrocarbons out the tailpipe.

Regardless of how well fuel may be atomized as it passes out of the carburetor, some amount of flow velocity within the manifold can be used to help keep fuel suspended all the way to the intake ports of the cylinder head(s). Typical flow velocities (for passenger car engines) range from around 120-140 feet per second (fps) to nearly 320 fps, depending upon size of intake manifold, total piston displacement of the engine and rpm. And while it aids fuel suspension to keep flow velocities in the higher ranges, the problem of air/fuel separation is increased in proportion to mixture and speed and directional changes.
You might think of it this way: Air and fuel moving through the intake manifold have some amount of kinetic energy (flowing energy or pressure). Mathematically, kinetic energy is directly related to the mass (weight) and speed (velocity) of the air/fuel mixture as it flows through the manifold.

C. Air flow velocity into an engine can be calculated in units of feet per second (ft./sec), resulting in the indicated graph. At standard conditions of temperature and pressure (60° F. and 14.7 psi), differences in pressure (based on inches of water) provide the indicated rates of air flow. If you have (or are planning construction of) an air flow bench, flow velocities in the range of 15-25 inches of water would relate very well to typical race engine operation. D. A basic problem in the movement of air and fuel into an engine is the prevention of air/fuel separation along the induction system path. Since fuel is much heavier than air (comparatively speaking), there is considerable difference in flowing (kinetic) energy between the two. Mixture flow around corners can cause separation of air and fuel, which leads to reduced power and overall engine efficiency.

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