“The valve spring is like a canary in a coal mine – it will usually signal a developing problem before a catastrophic failure.”
When Galileo pointed his handmade telescope at the planets and became the first human to behold Jupiter’s moons and Saturn’s rings, he saw something that had been invisible. Of course these moons and rings had existed for millenia, but they were beyond human perception until the invention of a device that could magnify the faint images. Like Galileo’s telescope, tools such as dynamometers, wet flow benches, and data recorders have given drag racers the ability to “see” events that would otherwise be imperceptible to human senses.
Recently I had an opportunity to test valvetrain components on Comp Cams’ Spintron, a 21st century tool that is as advanced as Galileo’s spyglass was in the 1600s. A Spintron resembles a dynamometer, but instead of a water brake there is a powerful electric motor that spins the test engine’s valvetrain at high speed. Outfitted with lasers and high-speed video cameras, the Spintron gives cam designers and engine builders the ability to observe and analyze valvetrain components at high rpm.
This wasn’t my first experience with a spin fixture. Years ago my late partner Buddy Morrison constructed a spin fixture for our shop that employed a hulking 460 cid Ford V-8 engine to spin the valvetrain assembly in our test engines. A strobe light synchronized to the engine rpm would “freeze” the motion of the valve and spring. The instrumentation and software that were available at the time weren’t particularly user-friendly, but we did learn a tremendous amount about how springs and valves behaved under actual operating conditions. It was a little disconcerting to see the valves continue to accelerate over the nose of the cam and then free fall as the springs slammed the lifters onto the cam lobe’s closing ramp. Maybe that was more information than I really wanted!
The Spintron can record the 1-inch valve lifts that are now commonplace in Pro Stock engines, and the software can distill the information to an understandable format. I secretly hoped that we’d discover some problems in our Pro Stock valvetrains that we could easily cure and thereby improve performance. It turned out that our valvetrain’s dynamics were reasonably good. We weren’t suffering frequent valvetrain failures, so the Spintron confirmed what I already knew: We had a sound setup that wasn’t overtaxing the components. On the other hand, I also learned that perhaps we could push the limits with a more aggressive camshaft design.
Like the dyno, flow bench, data recorder and other tools of engine development, a spin fixture reveals trends, not ironclad answers. The most important information it provides is the knowledge that the cam and valvetrain components are mechanically capable of running to the intended maximum speed. If the engine won’t rev up to full speed on the dyno or the drag strip, then it’s likely that the problem lies somewhere other than the valvetrain.
A spin fixture can lead you down a dead-end path if you pursue smoothness at all costs. In my experience, an extremely smooth profile is unlikely to win drag races. It might be suitable for a NASCAR stock car engine or an endurance racing application, but a cam usually needs to move the valves more aggressively to win on the drag strip. We don’t race for 500 miles, so our cams and valvetrains can be closer to the edge.
Pushrods are a hot topic among engine builders, and an area that we investigated on the Spintron. There is a trend toward pushrods with larger outside diameters and thicker walls. In our Pro Stock engines, for example, we’re using 7/16-inch O.D. pushrods with .165-inch wall thickness. These pushrods would be overkill in a bracket racing engine, but there are definitely gains to be made by reducing pushrod deflection in all types of competition engines.
No pushrod, rocker arm or lifter is infinitely stiff. They all bend and deflect to various degrees. The goal is to limit this deflection to a reasonable level. A pushrod that bends and rebounds under load will change the effective cam timing at the valve, often in unpredictable ways. All of the parts of the valvetrain are inter-related, and the pushrod should not be a spring in this complex system.
If you’re a serious racer, you don’t necessarily need a spin fixture to evaluate your engine. Valve springs are excellent indicators of valvetrain performance. The valve spring is like a canary in a coal mine; it will usually signal a developing problem before a catastrophic failure – but only if you heed the warning signs. If you install a new cam profile that knocks out 50 pounds of valve spring pressure on one run, you probably have a problem.
Both the Spintron and the dyno have convinced me that a valve spring performs best when it runs in a certain relationship with the cam lobe. Installed height and the distance from coil bind are both critical. In the good old days, we’d set up the valve springs at the installed height that produced the seat pressure we wanted, and then add a .030-inch or .060-inch shim when they lost pressure. I now believe that’s the wrong approach for a high-end racing engine. In a perfect world, the spring should operate at its optimum position from coil bind regardless of its pressure. Valve springs are very complex components; evaluating a spring on pressure alone is like choosing a cylinder head solely on its airflow while ignoring velocity and cross-sectional area.
The advent of commercially available spin fixtures will certainly accelerate the development of better cam profiles and valvetrain components. Just as the science of astronomy has progressed from Galileo’s spyglass to the Hubble Space Telescope, new technology is helping racers to see the invisible.