During the course of the past few months, some of you have sent letters to Circle Track either posing questions or raising issues worthy of further discussion. This month, we elected to share a couple of them, along with various comments directed to clarification or stimulate additional thought.
In reference to the May '09 Enginology column, a reader wanted further discussion about "tuning" engines on an individual cylinder-to-cylinder basis; e.g., focusing on treating cylinders or groups of cylinders as "engines" unto themselves. You may recall we suggested, aside from the "cross-talk" that goes on among cylinders linked by way of a collected exhaust system or common-plane intake manifold, it's possible to adjust intake/exhaust paths and valve timing to "tailor" torque curves to specific track conditions. (You can even provide individual cylinder ignition spark timing, but that's a subject for possible later review.) Well, maybe that's too general but it describes the concept. The reader appeared to want more information about how this might apply to race classes requiring use of stock intake manifolds, referencing a recall he had about using 1.6 rockers on an engine's No. 1, 2, 7, and 9 cylinders in combination with 1.5 rockers on cylinders No. 4, 6, 3, and 5. I'm going to assume he was dealing with a V-8 GM engine and a firing order of 1-8-4-3-7-5-7-2.
For purposes of discussion, visualize a single-plane intake manifold on a V-8-type engine of this firing sequence. You could apply what follows to a two-plane design, but results would be less significant. Now, all else being equal, shorter intake (or exhaust) passages tend to "tune" to higher rpm than longer ones. Based on our example manifold, the inboard four runners (cylinders Nos. 4, 6, 3, and 5) would tune to higher rpm than the outboard four runners (Nos. 1, 7, 2, and 8). Of the options this poses, one is to treat these two "sets" of cylinders in a different tuning fashion, with respect to lengths of intake/exhaust passages and valve events (lift, duration, lobe separation, and phasing). As you'd expect, rocker arm ratio plays into this.
If I recall correctly, applying rocker ratios to the specific cylinders the reader indicated tends to affect where power is produced but not in a fashion that addresses intake-passage length. His pattern is the reverse of what you'd do to optimize runner length. Instead, the approach he mentioned tends to compromise the difference in intake runner lengths by increasing the net time each longer runner contributes to the inlet cycle and less time for the shorter ones, likely with the intent of narrowing the effective rpm range of most efficient operation. It would be akin to making the longer passages work better at higher rpm and the shorter ones at lower engine speed. This compromise, and that's what it amounts to, would be intended for engines operated in a comparatively narrow span of rpm; e.g. encourage the longer runners to perform better at high rpm and the shorter ones at lower rpm. Actually, both sets would thus be discouraged to work best where their geometry suggests.
On the other hand, if we'd like to take advantage of intake manifold passages already determined by a part's design, reversing the approach to optimize the length benefits of the inboard and outboard passages has proven more beneficial, especially in a broader range of rpm, like off-the-corner torque and increased speed at the flag stand. Unfortunately, when designing the majority of racing engine components, compromise is unavoidable, whether it relates to structural, material, manufacturing, or functional requirements. Once designed, however, there are "adjustments" to functionality that can be made, and tailoring valve events to intake and exhaust flow characteristics is one area for exploration.