Overhead cams are a more efficient method of actuating the valves than with a pushrod, but someone in Detroit never got the memo. And thank goodness they didn't. With production pushrod mills turning 7,000 rpm, NASCAR Sprint Cup engines pushing 9,500 rpm, and NHRA Pro Stock motors creeping closer to 11,000 rpm every day, breakthroughs in valvetrain technology have pushed OHV architecture far beyond what was ever imaginable. Furthermore, while street motors will never wind out as high as dedicated race motors, with today's high-flow cylinder heads, turning 7,500 rpm isn't out of the question. While durable valvesprings, and lightweight valves and retainers get much of the credit, these stratospheric rpms wouldn't be possible without commensurate progress in the rocker arm front.
Stuck between the opposing forces of a valvespring and a pushrod, and responsible for multiplying the motion from the cam lobe onto the valve stem, a rocker arm is one of the most highly stressed components in the entire valvetrain. Current trends such as steeper cam lobe profiles, stiffer valvesprings, and higher rpm only compound the stresses. To find out how engineers build rockers that survive these strenuous conditions, we took an in-depth look at the latest in rocker arm innovations with Chris Mays of Comp Cams and Brad Rounds of T&D Machine Products. Some of the things we discussed included the different aluminum and steel alloys used to manufacture rockers, the effects of weight on rocker performance, how to optimize valvetrain stability, and the benefits of a shaft-mount rocker. Needless to say, rocker arms are no longer simple pieces of stamped steel seesawing on a ball pivot.
Steel Or Aluminum?
Brad Rounds: "There is certainly a tradeoff in strength and durability between aluminum and steel rockers. The vast majority of rockers we manufacture are aluminum, but the NASCAR teams we work with demand the durability of steel. Generally, aluminum rockers are lighter, easier to manufacture, and provide a dampening effect on the valvetrain. However, all aluminum rockers have a finite cycle life. On the other hand, steel rockers are more durable and provide a slight increase in cycle life over aluminum. The tradeoff is that they're heavier, harder on other valvetrain components, and more difficult to manufacture. Although much of what T&D builds for top racing series like NASCAR Sprint Cup is proprietary, and therefore somewhat secret, it can be stated that there has been a strong exodus from aluminum to billet steel in rocker body construction. The reason is quite simple: A rocker arm must last through the high-rpm, fully orchestrated chaos that is now the NASCAR norm.
"Through years of development and testing, we found the absolute limit of aluminum rocker arms. Some of our customers wanted a rocker that had longer cycle life with higher ratios, and the answer was steel. For us, the progression to steel was actually a natural one. The initial customer for T&D steel rockers was Mercury Marine, a company that uses T&D as an OEM product on many of its offshore powerplants, even in non-race applications. Once the piece was engineered, it did not take long for others to ask for steel. The initial steel offerings were not just an aluminum design milled out of steel. They were designed as steel rockers from the ground up. The current NASCAR Cup rockers have evolved light-years from those initial attempts, not only in design but in material as well. Our proprietary manufacturing process ensures both strength and ductility for maximum reliability. Outside of NASCAR, steel rockers have become very common in blown and nitrous applications, tractor pullers, dirt late-models, and sprint cars."