Now, and because space is limited in this column, I'll both rewind and fast forward some events I'd like to share. During the mid-70s while I was still involved with Edelbrock R&D, one of the concepts embodied in some of the manifolds dealt with specific runner sizing relative to intended torque boosts at equally specific rpm. Further, we were incorporating more than one runner crossection in the same manifold, intended to provide v.e. boosts applicable to how engines were being operated. An example was the small-block Chevy Victor 4X4 where the inboard four runners (Nos. 4, 6, 3, and 5 cylinders) had larger section areas than the other four longer runners. Each set was torque resonant at different rpm; e.g., one set for off-the-corner torque, the other aimed at mid-track power.
On the pages of this magazine, we've shared periodic thoughts about the relationship between an engine's torque and volumetric efficiencies. Briefly, and to refresh your memory, here's a functional snapshot of that relationship.
Under the conditions of normal aspiration, an engine ingests air not exclusive to piston displacement, rpm, potential restrictions in flow paths (intake and exhaust), and pumping efficiency. Within a span of operational rpm, there will be a point in time at which inlet air flow efficiency will be "peaked," before which there was insufficient rpm to optimize the induction process and after which there is insufficient time to maintain that optimization.
Somewhere in that time frame, among the many calls we fielded from both racers and on-road enthusiasts, I received one from a Peter Vorum. It turned out Pete was a graduate student at Ohio State University and completing his mechanical engineering Masters degree on, of all things, tuning techniques for engine induction systems. Based on some similarities in Pete's work and ours, and the fact nothing proprietary appeared involved, we began sharing ideas and experiences about our work. In fact, we even provided him some data that eventually found its way into his thesis.
If we compare the amount of induced air in a given cylinder (at any point in time) with the volume that would occupy the same cylinder if left open to atmospheric pressure and at BDC piston position, this relationship could be described as the engine's volumetric efficiency, at that engine speed. Typically, v.e. is expressed as a percentage. We've also suggested that there's a relationship between v.e. and an engine's ability to produce torque. In fact, were you to graphically plot the two on a rectilinear grid, they'd look remarkably similar, differing essentially from the representation of pumping losses during running of the engine.
Further, as you would expect, certain intake flow rates play into this issue. For purposes of discussion (and at the risk of oversimplification), we could say that means flow velocity in an intake system varies as a function of rpm, for all practical purposes, in a linear fashion. In application, we find that both intake and exhaust systems behave in this fashion, enabling (by proper mathematical modeling methods) the design of these engine systems to produce volumetric (or torque) peaks at desired rpm. In fact, such methods are a tool by which torque characteristics can be influenced for specific application purposes, such as off-the-corner torque and down-track passing power.