Understanding Vibrations

Understanding how a valvetrain will react at very low rpm levels is relatively easy. At low rpm levels, the inertia in all of the various components does not come into play as they move. Predicting how a valvetrain will behave at 1 rpm-or even 1,000 rpm-is relative child's play compared to predicting what will happen to that very same valvetrain at 7,000 rpm.

At higher rpm levels, the speed at which the valvetrain moves is much greater. The inertial forces acting on every component comes into play in a big way. "At slow speeds when the valve is opening and closing, compressing the valvespring is the main activity that creates a force in the valvetrain," Griffin explains. "As the engine speed increases, the pushrod and rocker arm are required to move the valve and compress the valvespring at a faster rate. When this happens, the magnitude of the forces seen in the valvetrain increases. The additional inertia force causes more deflection in the components than the force of the valvespring alone. To aggravate the situation even more, because these forces are cyclical, vibrations can occur in the components." In other words, at high rpm you have to consider the valvetrain a system of vibrating components.

By definition, a vibration is a motion that happens repeatedly within an object. It is measured by how frequently this repeated motion occurs within a period of time (hertz). Every object has its own natural frequency, or the speed at which a vibration will travel through it. When the speed of the input causing the vibration (engine rpm) matches the natural frequency, the vibrations build upon one another, gathering strength each time. This can be very damaging to the component and even the components attached to it. Likewise, it is also possible that the cause of the vibration can be timed to conflict with the component's natural frequency so that the two cancel each other out. By working with a general understanding that the valvetrain is a system of vibrating components, we can work to mix and match the components (and their natural frequencies) to minimize the effect of vibrations on the system. The result is more dependable performance, better valve control in the upper rpm ranges, and increased component life.

A good example of how vibrations can alter the intended performance of a component is the change in loading on a pushrod as the rpm changes. As the cam begins raising the lifter and pushrod, the pushrod sees a spike in load as it tries to overcome the inertia stored in the mass of the rocker arm, spring, valve, retainer, and locks. The loading drops once those components begin to move, and there is a vibration that travels up and down the length of the pushrod. At the 7,000- and 8,000-rpm levels, you can see how radically the loading changes on the pushrod versus the lower rpm levels. Griffin says the vibratory waves also grow and contract with respect to crankshaft rotation as an effect of the pushrod interacting with other components. This is one reason many engine builders are going with larger pushrods in high-rpm applications. Even though you might think the lighter weight in a smaller pushrod would be more beneficial at high rpm, it is often actually better to go with a stronger (and thus, heavier) pushrod that exhibits less deflection as a result of these loading forces.

When you consider the effect of vibrations on the valvesprings, it gets even more interesting. Valvesprings have flexibility, so they suffer from the largest deflections because of vibrations. Not only that, but the frequency of the vibrations (and the loading on individual coils) changes because the coils actually collide and create a force spike that changes the shape of the vibration wave as the valve is opening. This can cause a phenomenon called "spring surge," which is a major culprit behind the loss of valve control in many racing engines. When allowed, progressive rate springs are very effective at controlling this. They actually utilize the closing up of the active coils in the spring to damp out vibrations due to spring surge.

Of course, we don't want to go too much deeper into this or we will quickly reach the point where racers will become frustrated physics students. We'll leave the actual mathematical models to the engineers who drool over that kind of stuff. The point here, whether you are an engine builder or a racer who simply wants more power ( who doesn't?) is that many problems caused by poorly matched valvetrain components are mistakenly blamed on other parts of the engine. If the engine is lying down at half-track, don't automatically assume that the heads are too small or the ports need more work. It could be that poor valve control caused by vibrations is keeping the engine from working as efficiently as it can.

Always Test, Never Assume

While helping Circle Track with this article, Griffin also gave us the following example of how assuming anything when it comes to the valvetrain can lead to trouble. Sometimes, it just makes you feel better to know that even the Cup guys make mistakes.

"The pressure to reach higher engine speeds and more valve lift is always present. In a particular case several years ago in the Nextel Cup world, the approach for some engine builders was to take some existing cam profiles and throw more and more rocker ratio to them to achieve this goal. The word got around through the garage that the Chevy guys were able to do this, so quite obviously, the Ford guys tried it.

"But the same combination of parts that were running in the Chevy engines didn't work out quite the same in the Ford engines. The cam profile, the pushrod, rocker arm ratio, and valvespring were all the same, and the weight of the valve, retainer, and locks were all the same. The assumption that everything here is the same was proven incorrect, though.

"The barrel size of the two camshafts were different, and the geometry of the valvetrain components were different. The valvetrain is a system of components that must be properly selected to operate in a violent environment, and a poor-but common-assumption is that the valvetrain is insensitive to subtle changes. The Ford engine pushed the limits of valve bounce farther than the components allowed. What's bad is that the Ford valvetrain customer failed a few valvesprings in a couple of races before getting around to testing the setup on a SpinTron."

SOURCE
COMP Cams