This engine dynamometer sheet...
This engine dynamometer sheet provides a "real world" look at how manipulating intake manifold runner section areas and lengths can affect overall torque output. By design, runners in Comp's LSX EFI manifold are changeable for the express purpose of "tuning" overall torque curves.
Among theories advanced to explain the phenomenon, there appear to be two prevailing concepts that deal with the "tuning" of intake and exhaust systems. One involves the study and application of how variable pressure excursions can be used to affect volumetric efficiency (torque) throughout an engine's rpm range. Commonly known as "wave motion," it involves the dynamics of how specific wave events within intake and exhaust systems affect net volumetric efficiency.
The other focuses on the belief that there are certain so-called "critical mean flow velocities" that either occur or can be deliberately created to also affect torque boosts. Interestingly, both involve piston (or cylinder) displacement, engine speed, and time, among other variables. So to say the two concepts are not related could be a misstatement. However, rather than present further arguments for either model, the subject we'll address is how both can be applied in practical applications for torque gains at predetermined rpm.
On more than one occasion, this column has suggested it's possible to not only tune an engine's cylinders on an individual basis but how a torque curve can be manipulated by treating each cylinder as an "engine" unto itself. In the course of these discussions, we've spoken about varying intake and exhaust passage section area and length, knowing both have an impact on where torque is enhanced (or not). And because valve event timing also affects volumetric efficiency, further benefits have been suggested by including multiple intake and exhaust valve timing patterns, cylinder specific and on the same camshaft.
Conceptually, none of this is new. Years ago, as some of you may recall, Edelbrock introduced a version of its early-design "Victor" manifold for small-block Chevrolet V-8 applications called the "Victor 4+4." One design of a single 4V inlet manifold for a V-8-type engine requires essentially two different lengths of runners; e.g., cylinders 4, 6, 3, and 5 were all shorter than cylinders 2, 8, 1, and 7. Regardless of the tuning theory to which you might subscribe, it's known that "short" passages (intake or exhaust) generally tune to a higher rpm than "long" passages, all else being equal.
Further, as previously suggested in this column, increasing runner section area tends to raise the rpm point at which torque boosts occur. This is why the inboard (shorter) four runners in the Victor 4+4 were of larger section area than the outboard four (longer). The concept tended to "flatten" or "broaden" the torque curve over a wider range of engine speed, correspondingly increasing the area under the torque curve and thereby enhancing off-the-corner torque and past the flag stand, for circle track applications. An experimental camshaft was also configured to further enhance the manifold's benefits; one set of four intake and exhaust lobes for the inboard four runners and another set of lobes for the outboard runners.
However, setting aside any notion all this ignores conditions within an intake manifold that distract from the concept just described, we need to consider that we're also dealing with a single-plane design to which all runners are connected to a common chamber (a plenum). As a result, pressure excursions occurring in any one inlet passage can, and do, affect similar pressure excursions in any other inlet passage because of their common connection to the plenum. As a matter of fact, the use of a connecting pipe between the exhaust header collectors for a V-type engine essentially creates a "single plane" exhaust manifold.
There is ample data pointing to the fact such a system is much like a single-plane intake manifold, flowing backwards. We'll not go there this month. Maybe some other time, if you'd like.
Moving on to more contemporary intake manifolds, we'll focus on one similar to the version being used in our Project G.R.E.E.N.; an EFI manifold for Chevy's LS engine series. However, when we discovered that the composite LSX EFI manifold currently produced and offered by FAST (a division of Comp Cams) includes a feature enabling the installation of differently-sized inlet passages, it was the clear choice as a practical illustration for this month's discussion.
The theoretical basis for this manifold's development included wave motion analysis as applied to runner section area (involving passage diameter), length, and taper. Computational Fluid Dynamics (CFD) was also employed.
By the use of Computational...
By the use of Computational Fluid Dynamics (CFD) software, Comp was able to step beyond the limits of traditional airflow bench analysis to focus not only on quantitative investigations but to include qualitative analyses of dynamic conditions within the manifold. In the illustration shown, it was studying boundary layer conditions at runner entries.
According to Comp's VP of New Product Development, Brian Reese, "Our objective with the design was to shift the torque curve 'up' over stock, across the entire torque band. This is no small task, as typically it is easy to move a torque peak within the rpm band, but such tuning typically comes at the expense of a lower torque average at another point in the rpm band.
"For instance, lengthening the runners will shift peak torque to a lower rpm, but at the expense of erasing higher rpm torque. We found this an unacceptable solution for 95 percent of the market. To make our manifold commercially viable and applicable to 95 percent of the market, we set a design requirement to increase torque 'all over.' To accomplish this goal, the tuning is absolutely critical.
"Our CFD capabilities today are a tremendous tool, as they correlate to a flow bench within 1 percent, which negates the need for extensive prototyping and physical flow bench work. So if the CFD doesn't look good, it isn't usually worth pursuing any further."
It's particularly noteworthy that Comp had sufficient belief and confidence in the concept of optionally-sized intake passages that fit into the same basic manifold to have incorporated this feature as an available feature. From experience, I will share that enhancing the performance of individual cylinders (or group of cylinders in a given firing order) through specific intake and exhaust dimensions and then linking these with the appropriate valve events can have a material influence on where torque is boosted in the rpm range.
During development of the Victor 4+4, a street performance engine was modified using the following approach. Given a firing order of 1-8-4-3-7-6-5-2, cylinders 1, 4, 7, and 5 were sized for a torque boost at an rpm lower than for cylinders 8, 3, 6, and 2. Correspondingly, the primary header pipes for these two sets of cylinders were dimensioned in a similar way, relative to the overall rpm range. This was capped off by installing a camshaft with intake and exhaust lobes (and displacement angles) associated with a "low" and "high" rpm torque output.
Since dimensional variations were deliberately applied to alternating cylinders, the engine not only ran smoothly but produced a very broad torque range. We also learned that cylinder-to-cylinder air/fuel charge distribution corrections could be made by changing runner and/or plenum configurations.
With regard to Comp's manifold, according to Reese, "We experimented with different runner configurations in an effort to equalize cylinder-to-cylinder air/fuel ratios. Air doesn't like to 'bend' and even less so once it gets some velocity and momentum. So every cylinder fills differently, particularly when the throttle is central, not individual. This problem is not exclusive to EFI either. Carbureted engines are actually worse off, as they lack the individual cylinder fuel control option. There is power to be had by balancing individual cylinders and it is best to do it by way of airflow, not by way of fuel trimming, whenever possible."
Fortunately, contemporary computational means and methods far beyond what was available 25 years ago have moved induction system design and development considerably past prior techniques. It's comforting to know that such technology evolution has accelerated the process of scrutinizing a variety of internal combustion engine components and functions in ways that clearly benefit the racing community. You can be assured such tools often find homes in the more progressive specialty automotive parts manufacturers and race engine shops very quickly, sometimes before just about any place else.