Corvette Intake Shootout - Deep Breathing Exercises

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.

Single-Plane/Dual-Plane
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.