During a recent conversation with an experienced engine builder I realized we'd not recently discussed the importance of, reasons why and areas of concern about proper engine component integration. Based on some of his questions, I came to the conclusion that I'd probably not done a sufficient job when last this was a topic of discussion, some time ago.
For the most part, a successful circle track engine relies on torque, whether it's for improved corner exit speeds or helping acceleration past the flagstand. As in many forms of racing, horsepower may make a car fast, but acceleration wins races. If you're a frequent reader of Circle Track, you have likely followed our G.R.E.E.N. racing initiative that included the use of E85 fuel and an EFI system. By itself, the EFI's intake manifold design featured a set of longer runners than found in the single 4V manifold it replaced. As such, that design led to quicker corner exiting speeds (thanks to improved torque), resulting in lap time reductions, compared to the 4V setup. The future is forming, right before our eyes. EFI can enable benefits beyond enhanced combustion efficiency. So let's begin by examining concepts and parts that relate directly to torque. At the possible risk of oversimplification, we'll get right to the points at hand.
Understanding volumetric efficiency and its relationship to torque is important. By one definition, we can say that v.e. is the ratio of an engine's actual air capacity to its ideal air capacity, expressed as a percentage. Stated another way, it's a measure of the volume of air occupying its cylinders at any given engine speed vs. the physical volume (piston displacement) of the engine. Even though an engine operating at 100 percent volumetric efficiency might seem to be optimum, it's possible to achieve higher values through proper intake/exhaust system design and valve operating characteristics. But that's an entirely different topic for a future discussion.
Regardless, aside from the effects of pumping losses, volumetric efficiency and torque curves appear quite similar to each other. And as an interesting side-note, if plotted together, v.e. and torque curves are mirrored by brake specific fuel consumption (b.s.f.c.) curves. We've included a little sketch this month of how this relationship turns out to be. Note that the lower b.s.f.c. numbers (as they relate to combustion efficiency) appear at or near peak volumetric efficiency. That's not by accident. But we're digressing.
It is because of this close relationship between v.e. and torque that one more objective needs to be considered; the engine speed range during which the engine will be operated, most of the time. Given this goal, we can focus on parts designed for or best suited to this critical range of rpm. If a narrow range of rpm is the target, intake and exhaust systems can be "tuned" to operate more closely to each other and within the limited range of engine speed. If a wider range of rpm is the object, these two systems can be tuned more separate from each other. Some ways to do this will be included as we proceed here.
There are several major engine components that can affect volumetric efficiency. Of them, one is obviously the intake manifold. As has been postulated in past "Enginology" columns, runner cross section area dictates values for mean flow velocity as a function of piston displacement and engine speed. The so-called "critical" mean flow velocity (that flow rate associated with peak v.e. and peak torque) is on the order of 240-260 feet/second, depending upon which theory you base the reasoning. For example, consider an intake manifold with a cross-section of 3.0 sq.in., installed on an engine with a peak torque (240-260 ft/sec mean flow velocity) at 3,800 rpm. If we were to install this same manifold on a larger engine, peak torque would occur at a lower rpm, so some "matching" of manifold to a target rpm and piston displacement is necessary to optimize a given manifold/engine combination. Also, as frequently recounted here, while section area influences peak torque rpm, passage length change tends to "rock" or rotate a given v.e. or torque curve about this point.