A New Manifold for Your 4-Barrel

Written by Don Stewart | 29 February 2008

In the February Pony Tricks column, we talked about upgrading to a 4-barrel carburetor. We calculated CFM requirements for a couple of small block Windsors and intuitively recognized that the intake manifold would also need to be changed if we were replacing a 2-barrel setup.

At the risk of being too basic, here’s a brief description of an intake manifold:

An intake manifold is a system of passages which conduct the fuel mixture from the carburetor to the intake valves in the engine. For smooth engine operation, the manifold should evenly deliver an air/fuel charge to each cylinder and at a temperature that minimizes expansion. On a V-8 engine, the intake manifold is mounted between the cylinder heads.

One of the comments that I made in February was to make sure that you use the right manifold for your application. There are several designs available, each for a specific purpose. However, in this pony trick, we’ll only cover the two that are used most commonly: dual plane and single plane. But recognize that manufacturers such as Edelbrock, Weiand, Ford Racing Performance Parts, and others will be glad to discuss specific recommendations with you, based on your mods (if any) and on your performance goals. Remember that the synergy of your modification package depends on all of your parts working together in a complimentary fashion.

The most common 4-barrel manifold for daily drivers is the dual plane design. It has several characteristic that are desirable for street applications. There’s a picture of a representative dual plane further on in this article along with a table summarizing some of its characteristics.

The name dual plane comes from the baffle that splits the manifold down the middle beneath the carburetor. Because of the split, the carburetor is effectively divided so that each side feeds one cylinder bank on the engine (four cylinders). In other words, each bank has one primary throat and one secondary throat dedicated to it. In the table of characteristics, the division is represented by the entry that says 50% of the carburetor capacity is available to each cylinder.

The dual plane works best with an engine tuned to begin making much of its torque at low RPM, which translates to good pull off the line and good driveability. Note the typical RPM operating range in the table.

The downside of the dual plane is its restrictive nature at the upper end of the RPM range. With only 50% of the carburetor’s capacity available to any one cylinder, the engine tends to lean out at high revs.

The single plane, on the other hand, gives 100% of the carburetor’s capacity to each cylinder because there’s no baffle splitting the manifold. This design works best with an engine tuned to perform in the upper end of the RPM range. It won’t work as well as the dual plane on an engine tuned for low end torque. As a result, it can be finicky in traffic at low RPMs. See the picture of the single plane and the table of its characteristics.

There’s much more that could be written about intake manifolds, things such as wet design, dry design, tuned runner length, amount of rise, and much more but that’s all beyond the scope of this column. As mentioned previously, many manufacturers have kits developed for specific performance characteristics. Dedicated applications are best satisfied by talking to those manufactures or to engine shops that specialize in performance. But for basic street performance with a mostly stock engine, the dual plane is probably the best choice.