This engine dynamometer sheet provides a "real world" look at how manipulating intake mani
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.