All the testing was performed...
All the testing was performed with a 360 small-block on Westech Performance Group's Superflow 901 engine dyno. The results were surprising.
Capping any high-performance V8 engine will usually be an aftermarket chunk of cast aluminum with a bunch of passages linking the carb (or carbs') barrels to the head's intake ports. Though technically any intake manifold can be seen as a simple conduit for the air-fuel mixture, there are a wide variety of designs on the market, with each designed and created to handle the job a little differently. The most significant variation on a given intake's design concept is in how the runners are arranged, with the most common types being either single-plane or dual (two)-plane designs. Though most any motorhead can distinguish a single-plane intake from a two-plane just by looking at it, surprisingly few know what the difference is conceptually. Let's take a look at dual-planes first.
Though the earliest Mopar small-block engines used single-plane manifolds, for the most part, auto manufacturers overwhelmingly favor the two-plane design in OE applications. 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, so why bother with a two-plane?
The lineup. In this corner...
The lineup. In this corner representing the dual-planes, we have (1) the Edelbrock Performer, (2) the Edelbrock Performer RPM, and (3) the Mopar Performance 340 Six Pack; the single-planes are the tough Edelbrock Torker II (4), and the race Holley Strip Dominator (5).
It has been found that a two-plane intake favors low-speed response. The longer runners and smaller plenum typical of a two-plane are easy to see, and these are the factors generally attributed to the low-speed (read street) advantage of a two-plane, but that's only part of the story. A V8 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 such a way that 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 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 enhanced lower rpm carb booster function and atomization, resulting in better low-end output, enhanced driveability, and economy.
Perhaps the greatest benefit to lower rpm and part-throttle operation with the 180-degree design is that it also separates out 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. However, 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, this tends to draw exhaust gases 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 toward reversion at higher rpm. With the 180-degree system, this pathway is eliminated.