Performance Cylinder Heads and Valvetrains - Technology Transfer Part III

Reverse flow Many anti-reversion devices have been tried to keep the direction of the charge (or flow) constant. This theory would be interesting if the anti-reversion device could be timed to prevent reverse flow only when it was undesirable. (This statement assumes reverse flow is desirable.)

We do know that scavenging and sonic tuning must have an effect since racing engines routinely have volumetric efficiency in excess of 100 percent. If scavenging or sonic tuning does occur, then the flow is pulsing in the intake and exhaust tract and going in both directions (not simultaneously) for benefit. If reverse flow can assist scavenging and sonic tuning, then the reverse flow of the port must be of some significance.

In most cases, it turns out that modifications that improve forward flow also improve reverse flow, and it would not be a good idea to sacrifice forward flow for reverse flow. Nevertheless, it is good practice to flow ports in both directions. Any modification that can be made to improve reverse without affecting forward flow would be a good idea, but tuning changes may be required for optimization of any change.

What Are Overlap And Scavenging?
For a variety of reasons, including improved volumetric efficiency (v.e.) at high rpm, camshafts may be designed to delay exhaust valve closing to "overlap" when its corresponding intake valve begins to open. During such time, incoming air/fuel charges may pass directly into a cylinder and out through its exhaust. In one sense, fresh air/fuel charges may be lost from combustion. In another, unburnable exhaust gas from the preceding firing cycle may be removed from the cylinder. If we define scavenging as the process enabled by intake and exhaust event overlap, it's possible to examine how volumetric efficiency greater than 100 percent can be achieved.

Over time, numerous terms have been used to describe this process. Whether called sonic tuning, wave tuning, ram tuning, or some other applicable term, the objective is to achieve a greater volume of air/fuel charge filling of a cylinder (at the time of ignition) than could be achieved under the singular influence of atmospheric pressure. While the term "scavenging" often applies to two-stroke-cycle engines (particularly scavenging ratio and scavenging efficiency), the ability of a four-stroke-cycle engine to utilize some form of pressure excursions/dynamics to enhance v.e. involves valve timing and dimensional properties of the intake and exhaust systems.

Keep in mind that the point at which the intake valve opens is critical to how much residual cylinder pressure (and exhaust gas) is present. As opposed to the suggested benefits of bi-directional flow (in the passage "tuning" process), this initial "reverse flow" or "reversion" pulse can dilute fresh air/fuel charges subsequently flowing into the cylinder. Exhaust flow efficiency also affects the residual volume and pressure of unburnable gas available to the intake system at inlet valve opening. It is beyond this initial point that bi-directional flow may produce benefits to the tuning process.

Ideally, the scavenging process would enable replacement of exhaust gases with fresh air/fuel charges without any loss of such charges to the exhaust system. In reality, due to multiple and highly complex factors, optimum scavenging needs to be "timed" with engine speed closely associated to the rpm most often used. Even the most comprehensive of contemporary computer modeling systems require final analysis on an engine dynamometer (often of inertia type) and on-track evaluation. To further discuss specifics of the issue is beyond the scope of this particular story. But it is fundamental to camshaft, intake, and exhaust system parts selection that components be chosen that perform in a pre-determined range of engine speed (and as a "package"), else the benefits of both overlap and scavenging will not be achieved.