How can you equate Spintron testing to actual engine operation, given the fact valvetrain components aren't operating in the changing environment of cylinder pressure? Historically, there are problems associated with relating "simulated" conditions to "operating" conditions, almost independent of the science involved.
It's the nature of "modeling" complex events by whatever method employed, be it via computerized simulation or the use of laboratory devices. In the case of "spinning" valvetrain components and comparing stability and durability performance with that of a running engine, there are issues to be considered.
Countless hours of Spintron work have caused Griffin to draw certain conclusions. "Oddly enough, valvetrain failure rates occur close to the same number of cycles, Spintron to dyno," he says. "Besides the lack of cylinder pressure, engines do not experience the input of combustion heat and its affect on the viscoelastic properties of the valves and seats. As a material's temperature changes, its stiffness changes. Along with that, different materials will have different changes in these properties. It is entirely possible that we never see this effect and it is also possible that the effect is minimal. We just don't know!"
In this same context, Godbold has some additional opinions. "We compare dyno results and other engine data to validate much of our Spintron data," he says. "While some differences appear when making direct comparisons, we've seen almost identical pushrod strain data on the Spintron and a running engine. There's always the possibility that cylinder pressure at exhaust valve opening is substantial. However, the pushrod loads at exhaust opening on the Spintron sometimes exceed 2000 pounds. Hence, forces to overcome the inertia of valvetrain components is most likely much higher than any force due to cylinder pressure.
"On the intake side, pressure differences should be zero at intake closing and small at intake opening. In most applications, we see the same characteristics on the Spintron and a running engine. Other factors, such as valvetrain harmonics and pressure differences across the valve (in a running engine), tend to create data shifts from one measurement environment to the other. Overall, there are significant benefits comparing Spintron information to that obtained from a 'live' engine."
Cycle testing of rocker arms is another viable method of evaluation as companion to Spintron analysis. In this regard, according to Tores, "We test rocker arms on a hydraulic load machine. At loads of about double typical valvespring pressure, we increase the load and cycle frequency until failure is induced. The 'cycle to failure' numbers mimic that of race conditions. The most critical part of testing is the operating temperature, and this can be duplicated on the Spintron. We feel only a small percentage of the load on a rocker arm is from cylinder pressure."
I'm a Saturday-night circle track racer, building my first "serious" engine. What guidelines can I follow in selecting and installing valvetrain components? Caution and exercising judgment is a pretty common thread on this topic. Even the three experts made comments that fall into this line of thinking. Godbold offers, "Every application is so different that very little can be held as true across the board. It's wise to establish a relationship with someone you can trust at a major camshaft company. While you may experiment with other vendors later on, that primary relationship can be extremely helpful toward getting a competitive engine built that will stay together. The second piece of advice is to not let your first few engines become an R&D project for the cam company. Make sure some variation of what you are running has been used successfully in someone else's engine.