Some Thoughts on Airflow Testing
Since we devoted some space this month to comments on cylinder heads and intake manifolds, it seemed appropriate to provide a few thoughts on airflow testing, not focused on the more exotic computer-aided means by which such evaluations can be performed. Rather, we figured the Saturday night racer or engine builder could benefit from some "back room" perspectives on airflow bench use. Following are some suggestions to consider.

When flow benches first emerged in the performance parts and engine building community, longer ago than we'd like to admit, the axiom that "more was better" seemed the goal. However, it wasn't long until testers concluded that there's a definable link between an engine's pumping capacity (volumetric efficiency) and the volume of air it can ingest, based on a given piston displacement and engine speed. Concurrent to that realization was the importance of air flow quality, not just quantity.

Underscoring the importance of this notion was the fact air (in both intake manifolds and cylinder heads) is the means by which fuel is admitted to the combustion space. So, the ability of providing combustible air/fuel charges in the most efficient fashion was a function of airflow quality. In addition, air is compressible and (for comparative purposes) fuel is not. And, because they are of substantially different mass (in this case weight), air can change both direction and velocity much more easily. Sometimes this is good, sometimes not. Typical inlet paths can become very circuitous, even with the best efforts of either designing or modifying them otherwise.

Further adding to the complexity of "air as a means of fuel conveyance" is the fact inlet flow is both unsteady and interrupted during a given inlet cycle. Flow benches, obviously, cannot replicate these conditions. Adding a fluid to the system (wet-flow benches) can provide additional information and patterns with respect to how fuel is conveyed or altered from air inlet to combustion space, but the reality of all this is it's a compromise to actual engine running conditions. So the object is to design test methods that minimize the most inconsistent conditions first and then, at the very least, identifying those less inconsistent ones.

So what practical application does this entire dialog have to performing flow bench tests? Let's attempt some answers to that by trying to examine what you can and cannot get from such efforts. At the risk of oversimplification, suppose we try something to make a point. Visualize a given intake path from where air would enter the induction system to where the intake valve stem joins the valve head. Imagine a string positioned at the center of the air inlet, connected to this valve stem-point and pulled taut. In theory, this is the shortest flow path between these two points.

Keeping in mind that air tends to follow the shortest path, its movement will more closely approximate this distance than fuel. Our visualization is intended to emphasize that there are multiple opportunities for fuel and air to become separated between air inlet and combustion space. In a running engine, the object is to minimize the potential differences in actual (dynamic) flow paths between air and fuel. So the obvious question becomes, "How can you attempt to resolve this problem by using a flow bench?" This leads us directly to the flow quality issue.

By at least one definition, flow quality relates to the distribution of pressure (or pressure profile) at any given location in a flow path. Although basic physics utilizes certain Bernoulli principles to describe pressure distribution, let's simply state that this relates to where pressure resides throughout a flow path. In a wet-flow environment, the absence of a low-flow pressure condition is one circumstance that promotes the separation of air and fuel. Another circumstance that encourages mechanical separation of air/fuel charges is a sudden change in airflow direction or when eddies are created.

If you'll think back to our "taut string" example for a moment, you can visualize that in areas most remote from the string, you can expect low pressures to encourage puddling and further variations in air/fuel ratios along the flow path. Of course, none of these conditions are in support of delivering proper and consistent mixtures to the combustion space.

On a flow bench, the use of two types of probes can help identify improper or unwanted pressure conditions. One is a "velocity probe" that utilizes a small, open-ended tube (on the order of 0.030-inch i.d.), connected to a manometer. When pointed in the direction of flow (parallel to the flow), it can indicate flow rate. So-called "mapping" of port cross-sections, along the flow path, can be performed with such a probe. The other is a "J-probe" that consists of another small tube bent in such a way the hook in the "J" is pointed against the direction of flow. In this way, since atmospheric pressure is acting on the open ends of the manometer and probe, only pressures less than atmospheric will deflect the manometer. This type of probe is valuable in performing mapping similar to the velocity probe, differing in the fact its readings indicate areas of boundary flow separation and negative influence on mixture suspension.

You'll note that none of these last few paragraphs have mentioned flow quantity, only quality. Again, keep in mind that aside from its participation in the combustion process, air is the means by which fuel is either conveyed into the combustion space, helps qualify mixtures for combustion, or both-depending on the method of introducing fuel to an engine (carbureted or fuel injected). The point of this discussion is to not become locked into the need for simply increasing an engine's inlet airflow without a measure of concern for flow quality. Whether you're choosing or modifying intake manifolds or cylinder heads, keep an eye on the need for making certain air/fuel ratios delivered to the combustion space have been properly conditioned. And if you're on the dyno and curious about how much raw fuel is actually passing directly through the engine, try sampling unburned hydrocarbons (HC) using a rapid-response emissions instrument. Your observations could be quite revealing-and point you back toward the flow bench for some additional airflow quality measurements. Relying on brake specific fuel consumption data isn't the only way to get a reading on combustion efficiency.