Looking toward the side of...
Looking toward the side of this RO7 intake, you can see how the manifold's runners (the "short" or "inside" pairs) are elevated to come more into alignment with the cylinder head's intake ports. In such cases, it's desirable to have manifold runners that essentially become "extensions" of the intake ports...where possible.
Where rules allow fuel injection and open intake manifold design, the "trumpets" of the individual runners clearly resemble organ pipes whose lengths can be adjusted to provide desired power gains at the intended rpm.
Unfortunately, the geometry of common single-plane, single-carburetor inlet manifolds, and most circle track regulations, do not allow for radical changes in runner lengths. Depending on the rules for your particular series, modifications to the intake manifold can have a profound effect on the shape of the volumetric efficiency vs. engine speed curve.
With a common-plenum, single-carburetor manifold, the overall volume of the manifold correlates inversely with runner length. A larger volume manifold, when modified efficiently, will have shorter runners. A smaller volume manifold tends to have longer runners. Cup teams traditionally built one type of "open racing" engine and tuned the engine to the track with the intake manifold only!
If you don't believe the importance of a circle track engine's intake manifold, today's top teams may run a completely different valvetrain and cylinder head at short tracks than at tracks 1.5 miles and longer. Until very recently, the only difference between a Cup engine raced at Martinsville and one raced at Michigan was the intake manifold.
This fire breathing 410 sprint...
This fire breathing 410 sprint car engine runs on alcohol. Notice the organ pipe style injectors.
If your racing takes you to tracks you have not been to before, try a few different manifolds to see if one suits the track or track conditions better. Check the volume of your intake manifolds by filling them with water and try the smaller volume manifolds when you need to move the power down and larger ones when you need more high-speed power.
Here's one more point on intake manifolds. Airflow in an intake manifold is neither one-directional nor continuous. Remember this last statement; not continuous. Each time an inlet cycle begins, there is residual "energy activity" in the manifold. In fact, this is a subject unto itself. The point is that an engine's volumetric efficiency (as it affects torque output) is a function of how quickly and efficiently each succeeding cylinder can be filled, in the firing order. As the pressure differential builds between cylinder pressure and atmospheric (as influenced by engine displacement and rpm), the smaller the intake manifold's volume, the quicker it can contribute to v.e. Keep in mind that smaller volumes associated with this condition can become flow restrictions (and contributors to the mechanical separation of air and fuel) at higher engine speeds. You simply need to determine the specific rpm range in which the engine will be run and select intake manifolds accordingly.
Today, cylinder heads, intake manifolds and valve event timing are developed as a system. The intake track starts at the carburetor (or fuel injector manifold's base) and ends at the inlet valve. Cylinder heads and intake manifolds have almost become one word. The tuning of the engine is done by bringing port design, manifold design and valve event timing together. The extraordinary levels of performance of today's Cup engines can be directly related to the development of these separate systems as one whole--which, in fact, is an approach that can be applied directly to the weekly racer.
Next month, we will discuss push rod valvetrains, component by component, in addition to some applicable fundamentals about problems controlling valve motion and what happens when it isn't. We'll see you next month and--don't spin out.