It might seem that the intake manifold is one of the simplest components on an engine. After all, the purpose of the intake is 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 manifold with a series of corridors, using right angle turns to connect the plenum to the heads. 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. For traditional carbureted V-8 engines, the favored layout quickly evolved to the single four-barrel design. 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. 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 vs. Dual-Plane

In OEM carbureted applications, the two-plane design is overwhelmingly favored. 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 manufacturers 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 reason is 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. 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 a natural advantage 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 low-speed velocity.

Taken as a group, 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, making the two-plane system well suited to low- and mid-range power productions and efficiency. Finally, there is the effect of pulse separation in a divided plenum two-plane intake. The induction cycle of any given cylinder of a V-8 engine 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. This way each side “sees” only every other firing pulse, every 180 degrees of crankshaft rotation. That’s why a dual-plane intake is often referred to as a 180-degree manifold. As a result, the induction pulse seen at the carburetor is greatly enhanced, especially at low air speed. This translates to further improved lower-rpm carburetor-booster function and atomization, resulting in better low-end output, enhanced drivability, and economy.

While the advantages of a two-plane design are clear at lower rpm, compromises come into play at higher engine speeds. First, because the runners have to cross over from side to side, the manifold is arranged with upper and lower runners. Since 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 heads port, while the lower runners aren’t usually as fortunate. Second, the smaller divided plenum provides less volume for the engine to draw from as the rpm increases. However, 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 single-plane is a much simpler looking design than a 180-degree manifold, and with its nearly straight-shot runners, it sure looks to have more power potential. Just by its basic layout, it’s far easier to achieve a more equal port flow distribution with a single-plane design. As rpm and airflow demands increase, the direct runners of a single-plane are about as good as it gets for moving air/fuel mixture from the carburetor to the head’s ports. A well designed single-plane intake will have an inherent advantage over a two-plane design in ultimate airflow capacity, without the compromises in airflow distribution typically found between the high and low runners of a dual-plane. The single-plane is an ideal configuration for a great race or high-rpm intake manifold type, with the shorter runners naturally tuning at a higher rpm.

Manifold Mods

While an aftermarket intake manifold appropriately matched to the application and rpm range of an engine will typically result in a substantial power improvement, modifications can take things to the next level. The most basic of these mods is a simple port match to the intake runners of the cylinder head. This ensures a smooth transition at the junction with the cylinder heads, and is typically accomplished by matching both the intake port and the runner exits of the manifolds to a standard gasket size. The value of a port match job depends upon the parts combination in question. Power gains can be significant if you are dealing with restricted smaller sized ports in an engine application hungry for airflow.