One of the charms of our favored V-8 engine layout is a cylinder head topping the cylinder bores at each bank, processing the fuel/air mixture, and releasing spent gases in an efficient manner. As a necessity, bridging the gap between these cylinder banks is the intake manifold, fittingly the crowning component of the engine and garnering a large share of the visual attention. Besides looking good, an intake manifold is a major contributor to an engine's power production and performance characteristics. An intake manifold is charged with the seemingly simple duty of routing air and fuel via passages from the fuel and throttle source to these twin rows of cylinder heads. Although its function is simple enough to recognize, the subtleties of an intake's configuration and design play a key role in performance.
Early in the development of the internal combustion engine, intake manifold design considered little more than the requirements of connecting the various cylinders to the carburetor. It didn't take long for hot-rodders and racing teams alike to realize the relevance of the intake manifold to engine output. High-performance aftermarket intake manifolds have become one of the most popular components in seeking to enhance engine output, with a broad range of choices, even from a single manufacturer. The difficulty can be narrowing down the selection to a specific style and type.
One of the key distinctions in intake-manifold type is related to the layout of the various runners; and, of these, the commonly available configurations are single- and dual-plane manifolds. The difference is fairly easy to see. With a single-plane, all the runners originate from a common chamber under the carb, referred to as the plenum, and connect to the ports at the cylinder head directly, in as uniform a layout as practical. In a dual-plane configuration, the layout is more convoluted, with various passages laid out in two levels, originating from a divided plenum and crossing in a seemingly random way to the cylinder-head banks. The distinction here has a greater relevance than is apparent at first blush. Closer scrutiny will reveal that the runner layout mirrors the firing sequence of the engine, providing that induction is drawn from alternating sides of the intake manifold with each induction sequence of the engine.
Without getting deep into engine design theory, a V-8 engine fires a cylinder each 90 degrees of crankshaft rotation. A dual-plane intake manifold essentially isolates the induction to resemble two four-cylinder systems, doubling the separation of induction pulses to 180 degrees. As it relates to valve timing, this separation allows for a cleaner communication of the induction pulse of a cylinder drawing an induction charge from the carburetor than is the case with a single-plane intake manifold. The positive effects are improved throttle response, torque production, manifold vacuum, and engine smoothness, primarily at lower rpm. While a single-plane manifold disregards the low-speed benefits of 180-degrees pulse separation, at higher rpm the dynamic effects of high-speed airflow become more significant, and the direct passages of this intake manifold configuration often provide for improved high-rpm output.
Running The Numbers
Talking theory makes for good conversation at the club, but analyzing hard numbers is the best way we know to get to the facts. To this end, we assembled a collection of Edelbrock's intake manifold offerings for the small-block Chevrolet, and took the trip out to Westech's dyno facility to run them for the numbers. Our test engine is a typical hot-street small-block, with a displacement of 355 cubic inches and a mild Comp street roller camshaft. Topping the mill is a set of Air Flow Research 190 cylinder heads, providing enough induction draw to produce serious power, while taxing the ability of an intake manifold to keep pace. Here is how they stacked up.
The standard Edelbrock Performer is a conventional dual-plane divided-plenum performance manifold, and a popular performance upgrade in milder applications. Edelbrock markets and has certified this intake as a legal stock-replacement item for many applications. Despite the stock-replacement tag, the Performer was designed, as its name implies, as a performance upgrade over the OE intake manifolds.
We used the Performer as our baseline intake, and were somewhat surprised to find that even on our stout 350, this intake offered credible output. The relatively low manifold height makes it an attractive alternative to stock in applications where hood clearance is an issue.
The Performer RPM was a milestone design in dual-plane intakes, created with the intention of providing the low-end performance benefit of a dual-plane, with the top-end performance attributes of a single-plane at higher engine speeds. To this end, the manifold height was increased, allowing the traditionally poor lower plane runners of the intake to provide a more direct path into the cylinder-head port. The runners are laid over in a form akin to a gradual curve rather than the traditional abrupt angular pathway found in OE or earlier aftermarket manifolds.
In output, the RPM closely followed the power curve developed by the standard Performer, but by 5,500 rpm the difference became apparent. Higher in the rev range, the RPM showed a clear advantage. The Performer RPM is about 0.7 inch taller than the standard Performer intake, and, like all the high-rise intakes tested, represents a hood clearance problem in most Corvette applications with stock flat hoods.
Performer RPM AirGap
The AirGap was the next evolution in the Performer series of dual-plane intakes, characterized by divorcing the runners from the rest of the intake manifold. This design feature isolates the runners from heat gain via the tappet valley of the engine, and allows the surrounding air to keep the runners cooler. Cooler runners allow for a denser mixture charge, which in turn promises improved output.
We never run out of good things to say about the AirGap intake. This manifold provides the torque advantage inherent in a dual-plane configuration, while consistently providing top-end power rivaling a single-plane right to the top of its rated rpm range. In an engine application running up to 6,500 rpm, there is little if anything that will touch the AirGap in output. The AirGap provided by far the strongest average output numbers in our tests.
The Victor E is another single-plane manifold, similar to the Victor Jr., but having a larger, deeper plenum, providing more volume and more generous runner dimensions. These attributes lend themselves to operation at higher rpm in applications such as drag racing, while our engine combination was strictly hot-street. The Victor E retains the manifold height dimension of the Victor Jr.
We were curious to see how the larger-volume intake would affect the output of our 355-cube test engine. The consensus at the dyno facility was that the Victor Jr. was more appropriately sized, and the best choice in a single-plane intake. We were surprised to find a nearly identical power curve to the previously tested manifold.
The Super Victor is a true race intake manifold, designed for high-rpm output in racing applications. This manifold is substantially taller than the other single-planes tested, providing for a more advantageous approach to the cylinder-head ports. The Super Victor is large, not only in terms of height, but also in runner cross-sectional area and plenum volume.
Again, this manifold wouldn't represent an obvious choice for an engine configuration as tested, but we deemed it worthy of the experiment. The Super Victor provided an unexpected improvement in average torque as compared to the other single-plane intakes, and tied the Victor Jr. for top output honors, edging out the Victor E by a mere 1 hp.
The Victor Jr. was the first of our single-plane entries. Long regarded as the single-plane intake of choice for a variety of race and hot-street applications, the Victor Jr. is quite a versatile manifold. With a moderate runner and plenum volume, the Jr. is responsive, while providing power comparable to more race-orientated intakes in street/strip or moderate race use.
True to form, the Victor Jr. made more outright horsepower than the dual-plane intakes; but as compared to the AirGap, not by a large margin on our test engine. Closer scrutiny, however, shows that the averages were down in comparison to the dual-plane intakes. In a racier engine combination at much higher rpm, the single-plane would likely be more in its element; but even in the range of our test, the enhanced power production up top was clearly shown.