Ask Away with Jeff Smith: Running a Single or Dual Plane Intake Manifold

I keep seeing all kinds of information on the best intake manifold for a carburetor and all the opinions just make this even more difficult. I have a 5.0L small-block Ford fuel injected engine that I’m putting in a Falcon Ranchero with a C4 automatic and the stock rear end. This isn’t a race car, just an around-town cruiser. Thanks,
J.M.

Carbureted intake manifolds are generally categorized into two main areas: single plane and dual plane designs. The single plane is the easiest to describe so we’ll begin with that one.

In a single plane manifold design, the area directly underneath the carburetor is called the plenum which on a single plane intake is a somewhat large common area that leads to all eight of the separate intake ports. This large common area tends to work well for high rpm engines that tend to make their best power at higher engine speeds. There are several advantages to this design for high rpm use as it offers a direct and nearly straight shot from the carburetor to the intake ports in the cylinder head. This is why it works well at higher engine speeds. The disadvantage is that single plane intakes generally use very short runner lengths which sacrifice low and mid-range torque.

The dual plane intake manifold is really the design that is better suited for your application so we’ll spend some time on a few reasons why.

A dual plane actually splits the plenum into two separate but much smaller areas. Each of these plenums feed four of the eight cylinders. The main reason for this is to increase the length of each of the eight runners. By lengthening these runners, this tends to increase low- and mid-range torque which is beneficial for street engines that spend a majority of their life at low and mid-range engine speeds.

We won’t get into the technical reasons why a longer runner improves the torque as the explanation is fairly complex involving reflected pressure waves and lots of physics. So you will just have to trust us on this.

Of course increasing the length of the runners generally also adds turns which tends to hurt overall flow. If you’ve looked into the potential market for intake manifolds, you may have noticed a huge number of options even for just a dual plane intake for a small-block Ford.

We’ve listed several dual plane intakes from three different companies along with a spec for carburetor height. This is the dimension from the bottom of the manifold to the carburetor mounting flange. We performed a massive dyno test several years ago for Car Craft/Hot Rod magazine evaluating over 20 different dual plane intakes for a small-block Chevy. What we discovered was that power generally followed carb pad height.

In other words, the taller the intake, the more overall power the engine created.

However, carb pad height also affects how close the air cleaner will come to the bottom side of the hood. There are lots of variables here but you might consider that as part of the selection process. For example, the tallest of the manifolds we listed measures nearly five inches while the shortest is less than four inches.

If outright power is the goal, then the tallest manifold is a good choice. That would be the Weiand Stealth, Summit Racing Stage 2, or the Edelbrock Performer RPM. If power isn’t that important, it would appear the Summit Racing Stage 1 or Edelbrock Performer would work very well. (Check out the table at the bottom for links to more specs on each one.)

You didn’t ask about a carburetor, but with a mild 5.0L engine, a 600 cfm carburetor with an electric choke would be a good choice. There are lots of opportunities here. The Edelbrock AVS2 is a great choice and offers outstanding throttle response as it uses an annular discharge booster that enhances light throttle operation.

Another carburetor that looks good also with annular boosters is Summit Racing’s 600 cfm electric choke carburetor.

Both of these carbs offer the linkage hookups for the C-4 automatic so you might consider either of these as possible opportunities. Summit Racing even offers a package that combines a dual plane intake manifold with that carburetor, along with all the gaskets you’ll need—it’s almost like one stop shopping.

Ask Away! Dual Plane Intake Manifolds for Ford 5.0L