By way of review, last month we shared responses from three highly qualified valvetrain authorities: Thomas Griffin and Billy Godbold from Comp Cams and Larry Tores from T&D Machine Products. Their comments were woven together with some personal perspectives on the subject. The following questions and comments are geared toward expanding the Saturday-night racer's understanding of valve motion and how it relates to making power with parts that live.
With some Winston Cup engines now using multiple lobe profiles on the same shaft, how can rocker-arm ratios be selected accordingly? Let's talk about this for a moment. The issue takes us back several months to material that appeared in the Circle Track Sept. 2001 issue dealing with single-cylinder tuning and optimization. The discussion included ways to address how cylinders "talk" to each other in single-plane intake manifolds and "collector-joined" exhaust systems. Various and predictably varying pressure excursions can influence intra-cylinder combustion efficiency and power output, especially in the intake system.
Stated another way, some cylinders receive more air and/or different air/fuel charges than others. Among the reasons for that are inequalities in cylinder-to-cylinder airflow created in a running engine, despite efforts to "calibrate" or equalize flow during air bench tests. As a result, some cylinders make more power than others. While the ability to identify and confront this problem typically requires precision measuring equipment and a fair amount of analysis experience, Engine Cycle Analysis or ECA (also a prior CT subject) is gaining in use and popularity among the more aggressive race teams and engine builders.
Even though some responses to this question may appear a bit off target to its specific content, there's clearly some "between-the-lines" information you can gather from the following comments.
"Most, if not all, of the Winston Cup teams have very extensive dyno testing programs as well as on-track testing," Tores says. "Part of the tuning process for a given racetrack (length, banking, etc.) will be the rockers, as well as valvesprings and valves. Usually, the results of Spintron and dyno testing will dictate which rocker-arm ratios are used; more aggressive for a qualifying engine or configuration and more conservative for the longer race." Obviously, engines for qualifying are not the same for racing.
Griffin responded differently. "Cylinder pressure analysis (ECA) helps to understand the interaction of the 'mixing' of cam profiles on overall performance," Griffin says. "Seat-of-the-pants determines how well any optimization routine works." OK, this goes directly to the prior suggestion that individual cylinder performance follows individual cylinder analysis. Nothing new here. "Reading" spark plugs is age-old technology. However, in-cylinder pressure analysis as a function of crankshaft angle (ECA again) brings the measurement technique inside an operating cylinder to help identify "weak" or "strong" cylinders. Then you can go outside the cylinder to take steps that could include multiple profiles and rocker-arm ratios.
Now, if you'll hold those thoughts for a moment, what Godbold said falls right in line with this reasoning. "By looking at in-cylinder data (perhaps by experimenting with individual cylinder rocker ratios), engine builders sometimes find certain ports are weaker than others," Godbold says. "The weaker the port, the more duration and area (under the lift curve) the cam needs to provide to make power at a given rpm. Every profile Comp designs is intended to work within a range of rocker ratios, hence you cannot just keep adding more rocker ratio to weak ports.