Manifold Destiny

The Single-Plane
The 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 Considerations
We 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.