We dig into some aspects of intake manifolds that you may not have contemplated, fully understood, or thought worthy of further thought. You may discover some useful information.

When you consider that a chunk of metal standing between a carburetor or fuel injection system throttle body can affect so many engine functions, maybe we should look a bit deeper into how it’s able to become so involved. But before we lay out some of these reasons, causes, and what we might do about them to optimize power and efficiency, let’s define the basic types. We’ll be referencing the following fundamental manifold designs as we develop the balance of this story.

Single-Plane Design

As a rule, although not always a requisite, all intake passages (runners) that are joined to a common volume (plenum), may be defined as a single-plane arrangement. Even such a design for which the plenum is partially or fully divided can fall into this category. In most applications, depending upon runner cross- section area, single-plane manifolds are associated with a higher range of engine speed than other types. In fact, so-called high-rpm “Tunnel Ram” manifolds are of single-plane design. So also was the Smokey Ram that Edelbrock manufactured some time ago (that’s an entirely different story and a recollection for some of you “older timers”).

Two-Plane Design

This design contemplates two different levels (planes) to which runner sets connect with a common volume (plenum). Both V-type and inline cylinder arrangements can accommodate a two-plane (or single-plane) concept. Whether such a plenum is partially or fully divided, these manifolds are usually noted for volumetric efficiency boosts in an rpm span somewhat lower than a single-plane design.

Independent Runner design

If none of a manifold’s runners connect to a common volume, even if some sort of a “pressure” or “pulse” balancing tube joins one or more runners, the design is defined as an independent runner (IR) design. Some mechanical and/or electronic fuel injection systems use this concept. Typically, they can be rpm limited and prone to fuel “stand-off,” a condition related to reversion and to be discussed later in this story.

Since we know that flow in a intake manifold, regardless of design, is unsteady and bi-directional (during some phases of the intake cycle), we next need to examine what such instabilities are, what causes them, when they occur, and what may be done about mitigating their negative influence on net power and combustion efficiency. In one way or another, they are all related and need to be understood.

Manifold Pressure History, the Intake Cycle and Reversion

Let’s examine events and conditions that occur throughout a complete intake cycle. Try to visualize these events in very slow motion. Depending upon the efficiency (completeness) of a given exhaust cycle, pressure in the cylinder is higher than atmospheric pressure at the time the intake valve begins to open. At this point, the most prevalent material in the cylinder is exhaust gas (byproducts of combustion). So until cylinder pressure becomes equal to manifold pressure (approaching atmospheric in a normally-aspirated engine), exhaust residue flows back into the intake manifold. There are various terms that describe this event. Perhaps the most common one is “reversion.” Once these two pressures (cylinder and manifold) pass beyond being equal, and while the piston continues its descent during the intake event, fresh air/fuel mixture charges and un-burnable exhaust residue from the previous event are passed into the cylinder. When the intake cycle is complete, we have a cylinder with fresh air/fuel charges diluted with uncombustible exhaust residue. So you can see that the extent of the reversion event can materially affect both combustion efficiency and net power.

What is Intake Manifold Runner “Cross Talk”?

Reversion also introduces another of its influences on an engine’s overall combustion efficiency as what we’ll call “cross talk” among the manifold’s runners. Remember we said that for a brief period there will be some flow back into the manifold? Suppose during this brief time, another of the engine’s cylinders is already into its intake cycle. At this point, some of the reversion material passes out of the runner experiencing reversion and is inducted by that other cylinder.

Until cylinder pressure becomes equal to manifold pressure (approaching atmospheric in a normally-aspirated engine), exhaust ...

Stepping back from all this for a moment, this “cross talk” follows the engine’s firing order over and over again as engine speed increases. But as it does increase, there is less time for these reverse pulses to participate in “cross talk” or even influence the pressure differential across the carburetor. Don’t forget, carburetors are pressure differential devices, so they’ll deliver fuel whether flow is normal or reverse (leading to the fuel vapor hovering above the carburetor), leading to the “stand-off” condition previously mentioned. Stated another way, as rpm increases, contamination of adjoining cylinders gives way to increased contamination of each cylinder in its intake cycle. Reversion becomes contamination, as a function of higher rpm

Over the years, a number of devices and techniques have been employed that are applicable to an engine’s tendency toward reversion. We won’t delve into them here, but the type of intake manifold can affect its containment. For example, a single-plane manifold tends to dampen reversion pulses more effectively than a two-plane design and certainly better than an IR version. Of these, the IR design (having no plenum volume to absorb reversion pressure pulses) is often characterized by a “fuel cloud” or “vapor cloud” hovering above the runners. Such conditions play havoc on attempts to develop an efficient air/fuel charge calibration (or fuel curve). And in some instances, reversion conditions can be so severe as to leave exhaust gas residue in the intake manifold, all the way to the underside of the carburetor. The stuff just doesn’t burn well, a second time around.

Further to the issue of fuel curves, if we define “metering signals” as the positive net pressure (atmospheric and other carburetor-influencing pressures), the reversion phenomenon and fuel delivered to the engine varies with manifold type. In particular because of its larger overall internally-connected volume, single-plane manifolds cause weaker (softer) metering signals than either a two-plane or IR, in that order. In other words, if you calibrated a carburetor using a single-plane manifold, you’d likely be pulling jet size out of the carburetor if you switched to a two-plane design, simply because of the stronger metering signals typical of a two-plane design, no other changes involved. In fact, when plenum dividers come into play (single- or two-plane manifold design), metering signals typically increase as well.

The Effects From Changes to Runner Cross-Section Area

Even though we stated that intake manifold flow is both unsteady and bi-directional (we just discussed reverse flows or reversion), there is ultimately a net rate and volume of flow. Let’s define this net rate as “mean flow velocity.” A fair amount of research and data collection has been conducted over time supporting the concept that the m.f.v. at peak volumetric efficiency (essentially peak torque), is around 240 ft/sec. Hold that thought for a moment.

Now visualize that we have a flow passage across which we place a pressure differential. In reality, this is what we have during the main part of the induction cycle; e.g., atmospheric pressure pushing air and fuel into the engine. Since atmospheric pressure (for purposes of this discussion) is relatively constant, the rate of net flow will be governed by the cross-sectional area of the passage. If we increase this area, still maintaining a constant pressure drop across the passage (same rpm), the net flow rate decreases. If we make the area smaller, flow rate increases. Hang on. This is pretty important stuff.

Now, remember we said that the m.f.v. at peak torque will be pretty close to 240 ft/sec, so we’ll use that value. Visualize an engine on an engine dyno, running at peak torque, constant rpm. Two factors will affect this m.f.v.; runner cross-section area and cylinder displacement. But if we fix cylinder displacement to a given value, only runner section area remains to move this m.f.v. to wherever we want in the rpm range. The concept becomes a tool, either for the design or modification of any of the type intake manifolds we’ve included in this story.