Measuring Duration
Cam manufacturers publish both advertised duration and duration at 0.050 inch of lift, which can be a bit confusing, but the difference between the two figures is simple. Advertised duration can be given at any lift point. Cam companies pick different points at which to measure, making advertised duration figures relatively useless. "A simple measurement was needed to keep everybody honest about how big their cams really were, and that's how duration at 0.050 came about," explains Billy. "Low-lift numbers between 0.001 and 0.020 inch tell an engine builder a great deal about vacuum and responsiveness, while high-lift numbers greater than 0.200 inch tell an engine builder more about power potential. The 0.050-inch number is relatively easy to measure with a dial indicator and a degree wheel. It does the best job of predicting the operating range of a given lobe in a specified application." Note that this is tappet lift and not valve lift, so those numbers can tell a very different story, depending on rocker-arm ratio.
Shaft-Mount Rockers
With shaft-mounted rockers becoming more popular and affordable, are they a worthwhile investment for a typical street/ strip motor? Stud-mounted rockers inherently lack the stiffness of a tied-together shaft system. Some racing classes and budgets require stud-mounted rockers, and some of these applications can operate in the 8,000-rpm range even without a stud girdle. "Typically, we recommend either shaft-mount rockers or a girdle above 7,000 rpm, but our Pro Magnum rockers are very stiff even compared with a good shaft rocker setup," explains Billy. "With 71/416-inch studs and a girdle, you'd be surprised how close you can come to the same performance as with a shaft-mount system. It often becomes a question of where to spend your performance dollar. Shaft rockers become more and more attractive when all the other bases of an engine buildup have already been covered."
Retainer Weight
In an effort to promote valvetrain stability, many weekend warriors pay a hefty premium for titanium retainers, but innovations currently in the works promise to give racers the benefits of titanium at a fraction of the cost. "We could go racing in every series without titanium retainers if necessary, and lightweight steel retainers are already very far down the production chain and should be available soon," says Billy. "Some steel retainers are already in use in NASCAR, but these are very thin and made out of special alloy that costs considerably more than titanium. Our lightweight steel retainers should fall somewhere in between standard steel and titanium retainers in terms of cost while weighing within a gram or two of a titanium retainer. And the durability will be much better than with a titanium retainer."