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.
Interestingly, since single-plane 4V intake manifolds are in common use, we should re-mention that they are inherently provided with two basic (and of different length) runner designs. While much has been said and explored regarding how to optimize these two sets of runners (camshafts ground for boosting torque at two different engine speeds, exhaust systems designed to complement both runner lengths, different rocker ratios, and so on), the fact remains that exploiting such methods is a viable way to "integrate" these types of manifolds with companion parts.
A similar approach can be taken with respect to matching exhaust systems to a particular engine displacement and intended rpm range. Although we are dealing with a "dry flow" system (as compared to an intake system), differences in working fluid temperatures (air/fuel charges vs. exhaust gas) and piston position when peak flow rates are generated (roughly b.d.c. exhaust cycle and mid-stroke intake cycle), we can apply the same approach used for intake manifolds. If you missed it during earlier discussions, the calculation equation is as follows:
Peak torque rpm = (pipe or passage cross-section area x 88,200) / cylinder volume.
You can certainly perform some algebraic operations on this equation to solve for a required pipe or passage cross section area or cylinder volume (one cylinder).
Again, as with intake manifold runner length, header pipe length changes (longer or shorter) tend to rotate a given torque curve about the peak torque rpm point. Shorter pipes increase torque above the peak while decreasing it below this point, longer pipes increase lower rpm torque and decrease it above the peak, all else being equal.
Before we leave this little math model, there's one more instance where it can be applied. Let's say you have some engine dyno data for a particular engine that includes a full torque vs. rpm data stream and that you've measured the cross-sectional areas of the intake manifold and exhaust header passages (primary pipe only). By comparing the dyno data's peak torque rpm points with what you've computed and averaged for the intake and header system used, you can determine if the engine is "over-ported" or "under-ported." For example, if the actual peak torque rpm point is higher than what you computed, chances are it's an over-ported combination of parts. And, of course, the opposite is true as well.
Moving on to camshafts and armed with a specific range of targeted engine speed, you'll find that most camshaft manufacturers have a wealth of experience and technical information to help in the selection process. Just be realistic in your expectations about the rpm range for which you plan the most on-track use.
Cylinder heads? Not so much because intake and exhaust port path lengths are comparatively short (as measured against intake runner and exhaust pipe lengths), it's the need to help make transitional and directional flow changes into and out of the combustion space that cylinder head ports are believed to be important. There are indications that providing efficient and effective transitional flow into and out of the cylinders is more important than port tuning.
Some thoughts about practical applications
As you're probably aware, there are several very good sources available today for PC modeling of engine component packages. Their use will enable you to circumvent many of the problems often associated with sorting out specific engine parts combinations. Considering the costs associated with "live" testing (either on a dyno or at a track), these packages are certainly a cost-effective alternative. The benefits are even more pronounced when you consider the expense associated with full-on CFD and other such computer models.
At least from experience, regardless of the analysis method you choose to use, it's critically important to make your best estimation about what engine speed range you want to target, well in advance of any steps to determine the best combination of parts. The results often trail down to racing cost reductions and more black and white flags hanging in your shop.
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