If the passage had a circular section area, there would be only one “shortest path” in a turn. Using the “D” section shape provided multiple shortest paths along the flat portion of the “D” (see illustration). During one particular manifold design, we discovered that a trapezoidal section not only helped control flow pressure on the large side of this shape but provided additional pressure control by varying the length of the shorter part of the shape (see illustration). And even if you cannot produce a true trapezoidal section, trending toward this configuration helps minimize the pressure differential from short- to long-side of the turning passage. If you’ll spend a little time will the sketches, this should appear more apparent.

It all comes down to creating (or modifying) manifold runners that minimize energy differences across the sections of a passage, particularly during changes in flow direction. Obviously, air will follow a change in direction much easier than the heavier fuel. You’d like for both to change direction as if no change had occurred, and trending toward a trapezoidal shape can help resolve the problem. As you would expect, once air and fuel separate, reuniting them in a fashion that supports combustion can be problematic.

Air/Fuel Charge Distribution vs. Dry Flow Measurements

Several factors are involved with this. First, we need to seek ways of making cylinder-to-cylinder dry flow as equal as possible. This at least provides us a shot to balance out comparative cylinder-to-cylinder power. And while it doesn’t account for the dynamics of flow direction changes and the differences in momentum between air and fuel, balancing dry flow to all cylinders (manifold runner paths) is a starting place.

The use of “downstream” and “upstream” Pitot tubes connected to a water manometer will help identify regions in a runner where boundary layer separation is occurring. It is in these areas where a degree of flow restriction and air/fuel separation can take place. (Note sketches showing two type of Pitot tubes.) While the “J-tube” shape tube will show where airflow shear or separation is occurring at or near flow surfaces, the “I-tube”) can be used to determine comparative flow rate (velocity) at any given point in the path. You can even perform flow section “mapping” by using this tube at the runner entry as well as further into the passage.

Intake Manifold Performance vs. Cylinder Head Port Flow Performance

It is sometimes helpful to think of an intake manifold’s runners as extensions of the cylinder head ports. This is certainly a consideration when attempting to design an intake “tuned” to certain engine speeds; e.g., the total intake path. But for purposes of manifold selection and use of existing designs, you can focus on intake manifold runner section areas and apply the previously shared equation about mean flow velocity, piston displacement and peak torque rpm.

Depending upon piston displacement and anticipated range of rpm, circle track racers have discovered it’s sometimes beneficial (particularly for corner exit torque) to use intake manifolds of slightly smaller runners than cylinder head ports. Generally speaking, as engine displacement increases, the internal size of intake manifold chosen should grow, accordingly. The same rule of thumb applies to increases in operating rpm, especially higher levels. However, since many racing intake manifolds have longer runners than the cylinder head ports, it’s possible to have a material influence on torque range by, once again, applying the little “sizing” equation mentioned earlier.

There is also another option to modify manifolds in a way that broadens the torque curve, using runners of two different section areas (every other cylinder in the firing order). Some of you may recall an Edelbrock intake manifold once named the Victor 4+4. In this case, the four inboard runners (comparatively short) were intentionally sized with slightly larger section areas than the out-board four (longer than the inboard four runners).


Depending upon piston displacement and anticipated range of rpm, circle track racers have discovered it’s sometimes beneficial (particularly for corner exit torque) to use intake manifolds of slightly smaller runners than cylinder head ports


Once again, based on the equation we provided, you can see that the desired m.f.v. required a slightly higher rpm to be reached for the inboard runners than the outboard runners. The result was a somewhat flattened and broadened overall torque curve, at least as contributed by the intake manifold. And, by the way, when matched to a set of headers of two-size primary pipes (linked to the different in intake manifold runners), the combination was particularly effective for corner exit torque and then again at the flag stand.

Flow Surface Texturing

We decided to include a few thoughts on this subject, even though there may be some skeptics about the value of working certain flow surfaces in an intake manifold. Actually, the concept can be carried out beyond the intake manifold to such places as piston tops and combustion chamber walls. When properly applied, the notion is to provide some boundary layer excitation, especially in locations where air and fuel tend to separate. By so doing, some of the separated fuel becomes re-suspended in the inlet air stream. We know for a fact that this technique is particularly beneficial when also applied to piston crowns and certain areas of the combustion chamber.

Inside the intake manifold, roughening the entire length of the runners (at least to the extent surfaces can be reached), plenum floors and around the entries to the runners can help, depending upon the fundamental design of the manifold. You’ll be able to read the effects by slightly reduced b.s.f.c. data. The best barometer is on-track performance because some of the benefits show up only in a transient mode of operation, as provided on the track.

Dynamic Cylinder-to-Cylinder Mixture Distribution Fixes

Particularly in circle track applications, you will likely discover that distribution fixes sorted out on an engine dyno will not always satisfy what is required on the track. We recall years ago working on an intake manifold project with Smokey. In fact, it was a cross-ram, single 4V design he’d done for Chevrolet and their Trans-Am program. After a fair amount of work on the engine stand, it turned out the “fixes” did not apply to the track at all. Especially in this particular design, there was a considerable amount of fuel running around in the manifold as a function of not only air/fuel separation but from the sheer centrifugal forces present on the track.

Even in contemporary single-plane, single 4V racing intake manifolds, such forces can upset distribution patterns otherwise worked out on the dyno. That’s not to say the static engine method is of no value. Just don’t be surprised if what works on the dyno fails to translate directly to the track.

Some Concluding Thoughts

It’s important to understand how influential an engine’s intake manifold can be to the total volumetric efficiency landscape. If you consider that v.e. and net torque are closely linked (and they are), then manifold selection becomes a focal point to having an engine that delivers power where you’d like and need it to be. In particular, the decision needs to include the range of engine speed where power will be required most often, and then you can make intake and exhaust system selections that match this range. The little equation provided earlier will help you through this process as well.

One suggestion is that you also seek the advice of either your favorite engine builder or the parts manufacturer. The chances are good that both have knowledge they’ll share and intended to help with the manifold selection process.