Properly matching parts such...
Properly matching parts such as the intake and exhaust ports on this cylinder head to the intake manifold is but one area that effects volumetric efficiency of a race engine.
Perhaps the most critical element in the selection of race engine components relates to the engine speed range in which you’ll need the highest level of power. Actually, let’s be more specific and refer to torque, not horsepower. A race car’s acceleration potential is keyed more to torque than horsepower. Further, torque is a function of volumetric efficiency or an engine’s ability to ingest the highest possible amount of air, within the desired rpm span.
By way of review, volumetric efficiency (typically expressed as a percentage), is a comparison between the amount of air in an engine’s cylinders at any given rpm and the physical volume of the cylinders. Technically, you could say it’s a measure of actual air capacity to ideal air capacity. But regardless of how it’s defined, volumetric efficiency and torque output go hand in hand. The higher the VE, the greater the potential for torque.
Of the engine components that have a material impact on VE, intake and exhaust system design, mechanical compression ratio, valve timing, carburetor sizing, rod-to-stroke ratios, and inlet air systems play important and influential roles. Improper matching of these parts and conditions can easily reduce an engine’s ability to make torque. In a sense, all of these can be viewed as “tools” applied in a way that optimizes torque in a specific range of engine speed.
Here’s just one example of that. We’ve previously suggested that an intake manifold can significantly affect where torque output is optimized, virtually independent of other parts. A large body of theoretical data and test results supports the idea that at an engine’s peak torque rpm, the mean flow velocity in the inlet path is in the range of 240-260 ft/sec. That flow rate in an intake manifold is a specific function of runner cross-section area, and it’s possible to “size” such areas to produce this flow rate within the range of desired rpm. Stated another way, intake manifold selection can be made based on available runner section areas or, if modifications are in order, you can alter section areas (either enlarging or reducing area dimensions).
In either case (selecting or modifying an intake manifold), the following little equation can be used for computations: Peak torque rpm = (passage section area x 88,200) / cylinder volume. Let’s say you have an available intake with a runner section area of 3.0 square inches, and the engine displaces 350 cubic inches (V-8). This yields an individual cylinder volume of 43.75 ci. Working through the equation, we determine peak torque rpm (for this particular intake manifold) is 6,048.
It’s important to point out that even with this intake installed, the engine might show a dyno’d peak torque rpm less (or more) than 6,048. That’s because there are other factors that make up an engine’s overall volumetric efficiency. Notably, the exhaust (headers) being used will influence the torque curve’s shape (relative to engine speed) and make its own contribution to the net value.
Interestingly, you can treat (size) intake and exhaust systems separately, inasmuch as they both respond in very similar ways to the dimensioning and calculation just described. At the risk of oversimplification, you can also consider that each of these two components will create its own “torque curve,” the net of which is primarily what the overall torque becomes.
So how can we use this observation when selecting an intake manifold or exhaust header system? Well, although there are other and lesser influential components that will affect an engine’s overall torque curve, intake and exhaust systems have a major impact. Actually, you can use the little equation provided to either size or evaluate both intake and exhaust systems. In a collected header system, you can focus on proper sizing of the primary pipes because collectors are basically effective at rpm less than peak torque.
On the chance you may want to delve further into dimensioning intake and/or exhaust systems, there are some excellent PC-based computer software programs that not only enable further study of parts combinations, but allow examination (even optimization) of valve timing, carburetor sizing, and related parts that influence volumetric efficiency. What you want to avoid is the old axiom that “bigger is better” because that’s not always the case. There was a time when it was believed that virtually any increase in airflow, by whatever normally-aspirated means, would be accompanied by a corresponding gain in torque. Just remember that all engines, racing versions in particular, have a specific air capacity. While you may improve steady-state, uni-directional airflow on a bench, it doesn’t automatically follow that power will increase proportionately in a running motor.
I recall a specific instance some years ago when a prototype, single four-barrel intake manifold for a big-block Chevy circle-track engine was configured to match the cylinder head port flow numbers. On the dyno, there was a significant power loss. Turns out the engine wasn’t capable of utilizing the additional bench-measured airflow, based on its ability to displace air during operation.
There’s another factor that should be kept in the back of your mind when selecting and matching components, certainly the more torque-influential one: backpressure. If you’re required to run mufflers, but even if you’re not, backpressure has a negative effect on combustion efficiency. Why? Well, largely because it can prevent efficient evacuation of residual exhaust gases that remain in the cylinder during delivery of the next fresh air/fuel charge. Such dilution is similar to the use of exhaust gas recirculation in on-road vehicle engines. The bottom line is that exhaust gas reduces peak combustion temperature and burn efficiency. It also tends to distract from the potentially good benefits from proper parts selection and integration by tending to prevent correct parts function. For example, it will matter less that you correctly sized or chose an intake or exhaust system if combustion residue is diluting the combustion process. It also tends to desensitize spark timing, create false requirements for air/fuel ratios (or carburetor calibration), and reduce power.
By way of a quick review, try to accurately identify the specific rpm range you plan on using on the track and select parts intended to optimize torque within that span. Many parts manufacturers specify certain ranges of applicable engine speed, so it never hurts to ask for information. As a case in point, we’ve seen some carbureted engine packages that were reconfigured in a way that actually caused slight reductions in top-end power but enhanced lower rpm torque. The result produced quicker corner-exit speeds and reduced lap times. It was a matter of reshaping the torque curve by “rocking” it about the peak torque rpm point, largely by using intake manifolds and header systems more specific to on-track performance requirements.
As suggested, you may want to think about exploring the previously-mentioned possibilities offered by computer-based programs such as what you’ll find at www.proracingsim.com, and there are others. Some of these programs will also allow you to configure parts combinations and then “run” them on a simulated engine dyno.
Given the ability to configure and evaluate a wide range of parts designs or specifications, the process of mixing and matching major components can be accomplished more easily by employing one or more of the available software packages. Not to the extent some engine designers use fully-functional and massive software programs for their work (OEMs, professional race teams, and so on), but more and more high-performance and racing parts manufacturers have begun using computer-based programs for their work as well.