Manifold Destiny

A performance intake manifold ranks as the leading bolt-on upgrade in a performance-minded build. Good examples can work wonders, while the wrong choice can spell doom to the combination. The Edelbrock AirGap two-planes are among the best and most versatile.

At first blush it might seem that the intake manifold is one of the simplest components on an engine. After all, the purpose of the intake is essentially to provide a series of passages to join the carburetor with the intake ports of the heads. That appears to be a fairly simple job description, and on the surface, it is. But, there is big power in just how those passages are connected. In a simple world, a design draftsman could just draw some straight lines between point "A" and point "B," and lay out a manifold with a series of corridors, using right-angle turns to lead were the passages need to go, kind of like hallways in a house. Well, in the old days, it's surprising how many manifolds were designed just that way. Though those early intakes fulfilled their job description, they were found to be far from ideal, and a great deal of potential power was needlessly left on the table. It didn't take long for enterprising hot rodders to see the shortcomings in manifold design as applied to early OEM engines, and an industry was born.

Aftermarket intake manifolds of widely varied design have been offered over the ensuing years-- some good, some decidedly poor, but all cast with the idea of improving on the efficiency of the factory designs in search of more horsepower.

In the '50s and early '60s, aftermarket performance intake manifolds (and even many high-performance OEM systems) were typically designed to accommodate multiple carburetors, but that turned out to be less than optimal. For traditional carbureted V-8 engines, the favored layout quickly evolved to the single four-barrel layout. Driving this evolution was the development of larger capacity four-barrel carburetors capable of handling the airflow requirements of all but the mightiest race engines. There is a variety of single four-barrel intake manifold designs, and though other layouts have surfaced over the years, popular contemporary four-barrel intake manifolds are typically offered in either single- or dual-plane configurations. It's worth a closer look to expand on the differences and applications here.

Single-Plane Versus Dual-Plane
A single-plane intake manifold is the simpler of the two layouts, connecting the plenum under the carburetor with runners leading fairly directly to the cylinder head ports. The two-plane design appears more convoluted with a divided plenum with one side's plenum dropping down lower than the other, and the respective high or low runners routed alternately to the left and right cylinder heads of the engine. Most any motorhead can distinguish a single-plane intake from a two-plane just by looking at it, however, surprisingly, few know what the difference is conceptually. Let's take a look at dual-planes first.Though many early OEM engines used single-plane manifolds, auto manufacturers overwhelmingly favor the two-plane design. Looking at a two-plane, with some runners crossing over from side to side, a two-level divided plenum, and the obviously greater complexity in casting such a piece, we have to ask why the manufactures would go through the trouble. A single-plane open-plenum intake would obviously be easier to design and cheaper to build. Why bother with a two-plane? The design has many advantages.

As a general rule, it has been found that a two-plane intake improves low-rpm response, torque production and idle quality. These are pretty worthwhile characteristics, so it's worth exploring why this is the case. Runner length plays an important part in the rpm range at which an induction system "tunes-in," taking advantage of the natural pressure wave pulses in the intake tract to provide a greater charge density in the cylinder, and therefore more torque and horsepower. As a general rule, longer runners "tune" at a lower rpm range, while shorter runners favor the upper end of the rpm band. Similarly, it has been found that a smaller plenum also favors power production lower in the rpm range, while larger plenums are more inclined to boost the top-end. In these two characteristics, the dual-plane has natural advantages over the single-plane, but there is more. Runner cross-sectional area also plays a part. Smaller runners necessarily result in higher air-stream velocity in the manifold, which improves cylinder filling by improving inertia effect by virtue of the energy contained in the moving gasses. A dual-plane design generally features runners of a smaller average cross-sectional area than a single-plane, lending itself to higher velocity at low rpm.

All of these characteristics provide a low-speed advantage in their own right, but the effects are compounded when the carburetor is considered. Taken as a group, a smaller cross section, longer runners, and a smaller plenum create a system that is more responsive at transmitting the induction signal from the cylinder to the carburetor booster. This improvement in signal by the air stream connecting the cylinder to the carburetor improves the carburetor's metering response, and aids in low-speed atomization. All in all, based upon the characteristics discussed so far, the two-plane is a system well-suited to low- and mid-range power production and efficiency, but that's only part of the story. Most of these design characteristics could be incorporated into a simple single-cylinder engine, however, considering the interaction of the multiple cylinders of a V-8 engine, there is another layer of analysis necessary to fully appreciate the two-plane design.

A V-8 engine fires a cylinder every 90 degrees of crank rotation, and the induction cycle of any given cylinder will greatly overlap the induction cycle of the next cylinder in the firing order. The two-plane design isolates each side of the manifold and connects the cylinders in a sequence in which each isolated plenum is connected to every other cylinder in the firing order. Plumbed as such, each side "sees" only every other firing pulse. Rather than having overlapping intake pulses coming into the plenum every 90 degrees as with a single-plane, each side of a dual-plane gets a much cleaner induction pulse every 180 degrees of crank rotation. That's why a dual-plane intake is often referred to as a 180-degree manifold. With the induction pulses coming into the carb every 180 degrees (or actually only one-half of the carb in a divided plenum two-plane), the induction pulse seen at the carb is greatly enhanced, especially at low air speed. This translates to further improved lower-rpm carb booster function and atomization, resulting in better low-end output, enhanced drivability, and economy.

Perhaps the greatest benefit to lower rpm and part-throttle operation with the 180-degree design is that it also largely separates the communication of the induction pulse from the exhaust system. With a single-plane, the wide-open intake valve of a cylinder at peak piston speed on the intake stroke is communicated directly into the plenum, as it should be. At the same time, another cylinder in the overlap phase is also open to the same plenum. At low speed, especially with high-overlap cams, and most acutely at part throttle, this tends to draw exhaust gasses into the cylinder in the overlap phase. This reversion causes rougher low-rpm running and a penalty in torque production until the air speed and overlap tuning effect overcomes the tendency towards reversion at higher rpm. With the 180-degree system, this pathway is greatly reduced, improving idle quality, vacuum, and part-throttle responsiveness.

Given the two-planes' apparent advantage at lower air speed, what about as things get moving faster at higher rpm? The first consideration is the basic runner layout itself. Since the two-plane connects cylinders firing 180 degrees apart--given the firing order inherent in a V-8 with a conventional two-plane crank--the runners have to cross over from side to side. This is physically accomplished by splitting the runner layout in two horizontal planes (thus the two-plane name), with runners from one side routed up high, and the other side of the manifold having runners passing underneath the upper runners. The compromise comes in because there is only so much height that can be practically engineered into the manifold if it's going to fit under the hood.The upper runners generally have a nice straight approach into the head's port, while the lower runners aren't usually as fortunate. Secondly, since the plenum is divided, the volume is necessarily halved, and plenum volume is an important resource for the engine to draw on as the rpm increases. By necessity of the design, there is a disparity of plenum volume, with the low plenum having more room for a more generous plenum with a better transition from the plenum into the runners. The high runners are hampered by a shorter, more abrupt plenum, typically compromising both the plenum volume and the transition into the runners on that side. Benefiting from the available height, the high side of the manifold features an advantage in an improved, more direct runner approach into the cylinder head, while the low side of the manifold is typically compromised by the available height in its runner configuration and approach angle to the head. Due to these constraints, the low runners of a dual-plane typically suffer a flow disadvantage in comparison to the high runners--often substantially so. Nevertheless, many of these potential pitfalls of the two-plane can be greatly overcome with increased manifold height, and well laid-out runner and plenum designs. Ever hear of "high-riser" intakes?

A final consideration with a two-plane concerns the very division in the plenum that defines this type of intake. Each side of a two-plane divided-plenum manifold has only half of the carb capacity to draw on (a primary and a secondary barrel), compared to a single-plane where the whole carb is wide open to whatever cylinder is drawing. The net effect on carb capacity requirements isn't as dramatic as it may appear, since with a single-plane, the carb is being drawn on by an induction pulse hitting the plenum every 90 degrees of crank rotation versus every 180 degrees with the divided plenum of a two-plane.