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-PlaneA 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.
The Single-PlaneThe single-plane is a much simpler looking design than a 180-degree manifold, and it sure looks racier by eyeball. The plenum is usually large and wide open, with short, direct passages leading directly to the head's ports. Just by the basic layout, it's far easier to achieve a more equal port flow distribution with a single-plane design. As rpm increases, the air velocity and valve overlap tuning effects negate the low-speed advantages of a two-plane intake, and satisfying the engine's airflow requirement becomes the primary consideration.
The direct runners of a single-plane are about as good as it gets for moving the air/fuel mixture from the carb to the head's ports, and therein lies much of the single-plane's high-rpm advantage. Without having to route the runners across and underneath each other and splitting between a high and low plenum, the single-plane can be designed to do its main job well-the job being to flow air. A well-designed single-plane intake will have an inherent advantage over a two-plane in ultimate airflow capacity without the compromises in airflow distribution typically found between the high and low runners of a dual-plane. Add the effective port runner entrance of a well-designed single-plane's open plenum, and we have the formula for a great race or high-rpm intake manifold. Further enhancing the high rpm ability of the single-plane is the shorter runner length. Shorter runners tune in at a higher rpm.
Other ConsiderationsWe have given many of the theoretical considerations involved in differentiating between the two major configurations of intake manifolds, but a survey of any manufacture's catalog will typically show a wide range of selections within each group. Here, we'll look at some of the more practical aspects of selecting the right manifold for the job. Starting with the basics, we have to consider the aspects related to fit, and that can take several factors into account. Foremost, the manifold needs to fit the vehicle it is intended for. Hood clearance is often a major factor in the decision making process. While our theoretical discussion pointed out that greater height offers the potential for improved runner design and, potentially, performance with both single- and dual-plane manifolds, available height in the vehicle could present limitations. By contrast, in a race application, various hood scoops eliminate this as a constraint. Manufactures such as Edelbrock list the manifold height for each of its designs, so fit can be determined ahead of time, rather than relying on trial and error. Additional height can often be accommodated, even under stock hoods, and lower-profile aftermarket air cleaner assemblies can gain further clearance.
In addition to fitting in the available space, the intake manifold is still the connection between the cylinder heads and the carburetor. On a basic level, that requires that the desired carburetor will physically bolt onto the manifold being considered. Essentially, there are three basic configurations of carburetor mounting pads, with three distinct bolt pattern configurations normally available. The standard aftermarket performance intake manifolds are designed with a standard-pattern Holley carburetor flange in mind know as the "square-bore" 4150 pattern. This 5.16x5.62-inch pattern has become the standard of the industry, and though the bolt pattern is technically not square, it is referred to as a square-bore pattern. This bolt pattern is shared with most aftermarket Carter AFB carburetors, as well as Edelbrock's Performer Series variations of the AFB and AVS carb designs. Some early square-bore carbs, most notably the early OEM Carter AFB, and WCFB carried a smaller 4.25x5.62-inch bolt pattern, and many aftermarket square-bore intakes are double-drilled with this smaller bolt pattern in addition to the standard 4150 Holley pattern.
Another popular configuration of manifold flange is designed to accommodate the "spread-bore" pattern of the popular OEM and aftermarket spread-bore carbs. Among these, the Rochester Quadrajet and Carter Thermo-Quad were used extensively as OEM equipment, while Holley and Edelbrock also produce spread-bore carbs to this day. These carbs are distinguished by large secondary barrels and comparatively small primaries, and can offer an excellent balance of performance, response, and economy in street performance applications. Typically, manifolds offered with the spread-bore pattern are targeted for just such applications normally designed with street performance in mind. While factory spread-bore intake manifolds were singularly designed with this type of carburetor in mind, most aftermarket intake manifolds can serve double-duty. These manifolds are designed to accommodate the spread-bore carb's bolt pattern and large secondaries, but are double-drilled and flanged to accept a conventional square-bore carburetor, sometimes with a thin 1/8-inch plate required beneath the carb to ensure vacuum sealing.
The final type of single four-barrel carb pattern is found only on maximum performance race intakes, designed to accommodate the 5.38x5.38-inch 4500 Dominator carb. These carbs were designed for race applications requiring airflow unavailable within the limitations of the conventional 4150 body, and were never used in any OEM applications. Typically, engines that can take advantage of the 4500-series carb's airflow capacity are very high-rpm and/or big cubic-inch race powerplants. As might be expected, the Dominator flange is almost exclusively found on all-out race-style single-plane intake manifolds.
With that, we have the carburetor end of the installation handled, but there are considerations where the other end of the manifold bolts on at the cylinder head. Here, the two major factors to be aware of are the port configuration, and in some cases, the bolt pattern. While traditional V-8 engines retained these essential design elements over long production runs, there have been revisions on some engine lines that create unique manifold requirements. Two examples here are the Vortec Chevrolet heads, and the Magnum small-block Mopar, both of which did away with a long-standing manifold flange configuration in favor of rearranged vertical intake manifold fasteners. Port type can also be a relevant factor in selecting the correct manifold. A good example is the Chevrolet big-block, which was offered over the years in a variety of port configurations, including the large rectangular port heads, the more common oval ports, and the diminutive "peanut port." The advice is really simple here: make certain that the manifold matches the engine configuration.
While we have focused thus far on performance and fit criteria in selecting an intake manifold, there is one other at atribute that needs some attention, particularly in the realm of street-driven machines, and that is legality. In some localities, functional emissions equipment is required by law, thereby legislating the choices in manifolds to those that will support such equipment. Laws vary by region, but in some instances, the codes may require that legal manifolds are certified with an E.O. (exemption order) number to establish its legality as an emission-compliant replacement. In some applications, it's something to think about.
For such a seemingly simple component, it is surprising how much is involved in creating an effective design. We touched on a few of the major elements here, but the engineers and racers involved in development go much further. In their realm, considerations of port cross-section, port taper angle, length, plenum size volume, and configuration begin to define the final product. Each manifold is designed with a specific range of applications loosely applicable to power level, rpm range, and displacement, and you might note that all of these factors are closely related. Manufactures typically offer a variety of intakes to target these varied levels of output and rpm.
To aid the enthusiast in making an appropriate selection, Edelbrock provides a qualitative description of each manifold's intended application, as well as the designed operational rpm range. Don't make the mistake of presuming that adding an intake manifold cataloged as effective to 7,500 rpm will ensure the highest output from your street-bound powerplant. The specified range provides that the intake manifold will support an engine built to operate most effectively within the designated rpm band. A realistic appraisal of the performance goals and potential of the engine combination, in conjunction with reliable information from a reputable manufacturer such as Edelbrock, will take much of the mystery out of selecting the correct intake manifold.