A Note from Jim: The stated objective of this story is to introduce you to some notions that may be of imme-diate help to circle track engines operating under rules that allow non-OEM parts. It is based on years of design and experience dealing with flow passages on both sides of the cylinder. Likely, you won't agree with all the points. That's OK, but try them out on the flow bench. Build some sample flow boxes. Measure the indicators. U.S. Patents have expired on most of them, so we'll reach behind the patent claims and talk about how they were derived. You can take it from there.

Elementary physics confirms that when two different pressures exist, there will be some form of "activity" directed toward establishing an equilibrium condition. In short, nature would like for the two pressures to be equal. For example, when the inlet cycle of a normally aspirated engine begins, there is a difference in pressure between cylinder and atmospheric pressures. Air outside the engine is not "sucked" but forced by atmospheric pressure into the cylinder.

What you should remember is that this flow would like to take the shortest path into the engine, which is key to this discussion. In fact, on the exhaust side when cylinder pressure replaces atmospheric pressure and expands into lower pressure conditions outside the engine, the natural flow path is still the shortest route, even though this may not be possible at high rpm (simply based on fluid dynamics). If you take nothing else from this story, remember both these points. Now, let's get specific.

Intake Systems Suppose we walk through the basic pressure relationships in an intake path, beginning when the intake valve first opens. Initially, residual cylinder pressure tends to be higher than that in the inlet track (manifold and cylinder head ports). Containing residual and unburnable combustion by-products, cylinder pressure causes flow back into the inlet path. Over time, this has been called "reversion" or "back flow" into the induction system.

For purposes of this discussion, we'll omit any comments about how valve overlap may play into reversion. It doesn't relate to what follows. Also, at least in this story, we'll not include the effects of "wave motion" and other unsteady flow conditions that exist in both intake and exhaust systems. Our topic here deals with port passage considerations in the presence of steady-state flow. It's a premise that forms the basis for a number of flow models.

Further into the initial stages of the inlet cycle, cylinder pressure will equal atmospheric, after which cylinder filling begins. Obviously, the shorter the reversion period the greater the potential for increased volumetric efficiency and power increases from additional fresh air-fuel charges. So it's critically important that flow toward the cylinder is optimized. It is here that port shape becomes important. And it's not just about port size or volume, as you'll soon see.

Now, remember we spoke about how airflow tends to seek the shortest geometric path, all else being equal. If we're dealing with a flow path that is without bends, pressure distribution across any given port section remains relatively the same (constant). Just as soon as the path provides a directional shift, however, conditions can change significantly (Figures 1, 2, and 3). Note how the pressure distribution profiles change, favoring (again) the shortest flow path.

In an intake system for carburetor-equipped engines, we're dealing with a "working fluid" comprised of both air and fuel, each of which has materially different mass (weight). In a steady-state flow environment, the ability of air and fuel to remain "in suspension" is not much of a problem, certainly by comparison with what happens during directional changes. When subjected to changes in flow direction, the kinetic energy differences between the two elements (air and fuel), based on differences in mass, causes them to mechanically separate, and the consequences of this are classic.