Darin Morgan: At the Pro Stock and Comp Eliminator level, the increases in horsepower we’re seeing today are directly related to the loft curve built into the cam. Do it right, and it’s like having a variable-duration camshaft. At 8,000 rpm is where the loft curve comes into play. Generally, a smooth loft curve will give an additional 0.008 inch of lobe lift by 8,000 rpm. That figure increases with rpm, so by 10,000 rpm you end up with an extra 10-15 degrees of duration. However, accomplishing this is much easier said than done. You want as much valve speed as you can get, but you have to balance it with the proper rate of valve acceleration to maintain valvetrain control. The rate of the springs, and the weight of the coils, retainers, and locks must all be optimized. With larger base circle cams we can now get up to 0.600-inch lobe lift. This allows us to use lower ratio rockers so the initial valve acceleration isn’t as quick, which helps stabilize the valvetrain.
Getting the valvesprings to live isn’t difficult if you have a limited amount of valve lift. However, cylinder head technology has improved dramatically in the last 10-15 years, so now we’re picking up the valves much more than ever before. That puts much more stress on the valvesprings. For a while, the focus was on the metallurgy, wire thickness, and alloy of the springs, but several years ago manufacturers realized that impurities in the spring wire were causing them to break. As a result, using clean wire in the springs is a top priority these days. Furthermore, what we’ve learned in the last four to five years is that you don’t need enormous spring diameters anymore. Not too long ago, we used to have double- and triple-duty springs with 1.600-inch diameters. What happened was that the springs were getting so big and heavy that you needed higher spring rates just to control the weight of the spring. With the better, cleaner metal we have these days, engine builders are using smaller-diameter springs. Smaller springs also let you use smaller retainers, which further reduces valvetrain mass. A great example of this is a beehive valvespring. By reducing the diameter of the top of the spring, it cuts down on mass as well. Our ’99 Camaro drag car has an LS motor that turns 9,600 rpm. The exhaust valvesprings are just 1.550 inches, but they have 1,000 pounds of open pressure.
COMP Cams: In recent years, a lot of progress has been made with regard to valvesprings. One of the newer trends is that we are now designing springs for specific applications. In the past, we would try to find a spring that we thought would suit a specific engine combo. In a lot of cases now, we will create a clean-sheet design to get the spring ideally matched to the rest of the system. The biggest advantage to the newer springs is the reduction of mass. I’ll also say we have only started to scratch the surface with regard to spring design and materials. The metallurgy, spring design, and overall size of the spring all contribute to the performance of a spring.