"Typically, you should run the maximum safe rocker ratio on a given profile. However, some engine builders have found increasing the duration on the more restrictive ports can help smooth out the power curve or increase peak power. Most importantly, the larger the difference in port flow from cylinder to cylinder, the greater the potential to find improvements with these techniques." This last sentence should be underlined.
What are the structural reasons some rocker arms break and some don't? According to Griffin, "Rocker arms are loaded (during normal operation) in bending, which is the most horrible way to load a structure. By making these components out of improved materials with stiff cross-sections, high-rpm loading can be less detrimental. Failures also occur from high-stress points in the threads where the adjuster nut is positioned. Since the rocker arm is bending, the surface is being put into tension. As a result, shot-peening has helped increase the fatigue life of aluminum rockers because of the compressive stresses imparted on the surface of the body of the rockers (by this process)."
As you might expect, material selection is critical to parts life. Aluminum is not just aluminum, no more than parts are parts. Tores is particularly clear on this point. "There are a variety of reasons why one rocker arm design works better than another," he says. "Selecting the proper alloy incorporating high strength at elevated temperatures, low notch sensitivity and high levels of fatigue resistance are important factors.
"At T&D, we use SAE 2024 aluminum. This is the most fatigue-resistant of all the hard-alloy aluminums. It also has low notch sensitivity, which is an indicator of how readily a particular material will start to fracture under high stress. In the final design of a rocker, sufficient material should be used in critical areas to ensure a high safety margin.
"Generally, rocker arms do not fail from tensile strength failures. They usually fail from fractures in highly stressed areas such as the radius under the nose or in the rear of the rocker arm or a cut for a valvespring. Failures can result from these radii being too small, not properly distributing the stress evenly to the areas adjacent to the radius. Rocker-arm failures can often come from alloys that do not have good elevated temperature strength. Remember, a race engine may operate at temperatures up to 250 degrees (F), and these temperatures can cut the strength of some aluminum in half."
From a slightly different perspective, Godbold suggests, "Rocker arm failure is typically an alarm for an engine builder to look for problems in other areas. Aluminum work hardens and becomes brittle. Hence, aluminum rockers should be treated with wear limits and replaced at regular intervals. If you're using aluminum rockers that were hand-me-downs or ones with excessive use, a failure should not be a surprise. However, if a rocker fails early in its life, pay careful attention to wear of the retainers, locks and/or valve seats. It is quite likely that valvetrain instability may have contributed to the failure.
"Comp Cams prefers steel rockers to aluminum rockers in any application where steel is available. Steel rockers should only fail if they are used well outside their design application, sufficient care was not taken with design, or if they are improperly modified. In fact, I cannot recall seeing a Comp Cams full-roller steel rocker fail that had not been modified by the customer. Even in steel rockers, bearings should be replaced periodically.
"Some classes of racing require the use of stamped steel rocker arms. The level of competition in many of these classes often results in the cam forcing the pushrod through the pushrod seat of the rocker. This type of failure is very difficult to resolve without choosing a cam profile that makes less power."