Intake Manifold Science

How the mixture gets from the carburetor to the combustion chambers may be more significant than you imagined.

We all know the fact that air passage through a carburetor mixes air and fuel in some proportion for combustion. But what hasn’t been talked about is the fact that there are mixture conditions from carburetor to engine cylinder that can reduce both power and efficiency. Typically, the region where this can take place is called the intake manifold. This month we’ll discuss some of the features that separate good intake manifolds from those that might not be worth the time it takes to read about ‘em.

Essentially, there are two types: (1) single-air-cavity (single-plane) and (2) two-air-cavity (two-plane). Each has specific features to be discussed later in this story, but both are required to provide properly conditioned air/ fuel mixtures to an engine for which good efficiency is anticipated. First of all, you need to understand that air and fuel do not have the same weight (equal volumes of air and fuel do not weigh the same). As a combined mixture flowing through passages in a given intake manifold,

each will respond differently to changes in direction (as shown in Figure D). What usually happens is that fuel will not follow the same flow path as air, resulting in nonatomized fuel that passes into and out of the combustion chamber (unburned and reducing the amount of power produced by the engine).
So it becomes the responsibility of an engine’s intake manifold to (1) provide equal amounts of air (and air/fuel mixtures) to each cylinder and (2) prevent the separation of air and fuel between carburetor(s) and engine cylinders. And given the freedom of design space, there is the chance that intake manifold runner length and size can contribute to a given engine’s ability to produce torque. From the standpoint of design, this air cavity between carburetors) and engine cylinder head(s) can be reduced to a single or dual air space. If the manifold connects all of the engine’s cylinders into a common volume, the design is said to be a single-level (single-plane) manifold.

Separation of half the engine’s cylinders from the remaining number classifies an intake manifold as a two-cavity (two-level or two-plane) design. Such manifolds tend to increase the “fuel delivery signal” (amount of fuel flow) at low engine speeds and improve torque output below 4000 rpm.
But regardless of the types of intake manifolds you may be considering, there are certain features affecting the ability of each design to produce optimum engine performance. For example, in multiple-cylinder engines, there is a requirement for each cylinder to receive the same (or nearly the same) amount of air as any other. Unequal amounts of air to an engine’s cylinders means unequal amounts of power produced. And if the intake manifold being used is responsible for these inequalities, nothing short of a change in manifolds is going to solve the problem.
As a matter of fact, unequal amounts of cylinder-to-cylinder air flow (air distribution) is a common problem with “factory” intake manifolds.

A. Pressure “excursions” (variations in intake manifold mixture flow pressures) are shown here in a trace of pressure vs. time (or crankshaft angle) in only one runner of a single-plane manifold. Since the measuring pressure transducer was located in the No. 8 runner of the test manifold, reversion pulse pressure (or spikes) is the strongest in this manifold runner. Note that even after the intake cycle for the No. 8 cylinder is completed, there are pressure peaks (spikes) measured in a “non-flowing” runner. Also note that the strength of the reversion pulse is greater than the strongest “vacuum” or cylinder filling pulse . Reversion (intake manifold back-flow) is a significant problem in overall combustion efficiency. B. At the same engine speed, the smaller the carburetor throat, the higher the flow energy and the deeper the penetration in the plenum chamber of air/fuel mixtures, increasing the chance for mixture separation. As a rule of thumb, the smaller the carburetor the higher it should be from the floor of the intake manifold’s plenum chamber. This applies to both single-plane and two-plane manifold designs.

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