The time has come to buy a new intake manifold for your hot rod. There are plenty of choices on the market: single-plane, dual-plane, dual four-barrel, three two-barrel, tunnel-ram, and so on. You've narrowed it down to a single carb intake. Single four-barrel manifolds come in two basic configurations: dual-plane and single-plane. Both designs have been around for years, and each has its own benefits. But which is better for your motor's combination? The answer is it depends. But let's back up a bit and see how the two manifold types differ.
Different StrokesFirst, let's look at the most common race manifold (which, obviously, is the single-plane). Generally speaking, a single-plane intake includes a large, centrally located plenum which has reasonably straight runners leading from the plenum to the port entries in the cylinder head. In a single-plane configuration, there is a large common plenum under the carburetor. According to the experts, this "common" plenum allows each runner and cylinder intake port combination to draw from all four of the carburetor venturis at wide-open throttle.
As the partially vaporized air-fuel mixture leaves the base of the carburetor venturis, it forms as four individual mixture streams. When each of the cylinders places a demand on the plenum chamber, these mixture streams (or in some cases, portions of the streams) physically bend in the direction of demanding runner-port entry. The mixture "streams" combine to form a single "mixture river" which flows into the runner, eventually feeding the cylinder which is making the demand.
The beauty of a single-plane manifold configuration is that it allows each runner to withdraw a larger volume of air-fuel mixture during the available induction time span. Unfortunately, life isn't always simple-and neither are intake manifolds. As each cylinder withdraws a charge from the plenum, the "mixture streams" are forced to change direction constantly. Creating more havoc inside the manifold are pressure pulses which travel backward from the cylinder into the manifold runner and eventually into the plenum. And some engine combinations have more of this reverse pressure pulsation than others do. These constant directional changes in the plenum, along with pressure pulses, can create a healthy amount of turbulence inside the plenum.
Dual-plane manifolds, on the other hand, were used in great numbers on carbureted production engines. In these cases, low-rpm performance of the car was the primary concern. To enhance the low-rpm ability of the respective street car engines, a dual-plane manifold was almost always used. In essence this manifold is a two-in-one arrangement. Each half (or "plane") of the intake routes the air-fuel mixture from a separate plenum area to an individual group of cylinders (obviously, four cylinders in a V-8).
With each side of the intake separated from the other, individual intake runners are grouped so that 180 degrees of crank rotation separates the intake cycles of the cylinders fed by the same half of the manifold. Given the layout of the manifold, the runners can be (and, in practice, always are) long. Coupled with 180-degree separation of the cylinders, these manifolds, at least in theory, are best suited to engines which operate up to approximately 5,500 rpm. Of course, this also means that, in theory, this manifold configuration should be best used in a low-rpm, high-torque application.
Much For TheoryFrom our perspective, at least, we thought it would be interesting to compare two very good manifolds to see what the differences in dual- and single-plane manifolds really were. So we picked up the phone and called John Heida at Speedway Testing and posed the intake manifold question. Now, John flushes quite a few engines (drag race, street/strip, and street) through his dyno facility, and we thought he might have a perfect candidate for the test. He did. The small-block he had available was very typical of a solid, basic bracket engine. Specs for the engine are as follows: