We were nothing less than thrilled about the performance of the 406ci small-block we built for January's "Old School Meets New" showdown. Faced with a formidable 402ci Gen IV opponent, our mondo-cammed motor matched its high-tech opponent pony for pony, churning out 582.9 hp and 532.7 lb-ft of torque. But even as we celebrated our creation's prodigious power output, we began to wonder if we'd left something on the table. Our peak horsepower number happened at 6,300 rpm, at which point the power curve took an abrupt nosedive. "It's going into valve float," said our dyno guru, Westech's Steve Brul. With that, we decided to see if we could get better control of our 406's valves and moderate the post-peak power drop.
Our first step was to call in some professional help, namely Comp Cams engineer Billy Godbold. We shared our dyno-day observations, and as it turns out, valve float, which indicates loss of lifter contact with the camshaft lobe, isn't really an accurate description of what was happening in our 406's valvetrain. The issues are much more complex than that. We don't have room here for a dissertation on valvetrain dynamics, and fortunately, Godbold was able to give us the info we needed in layman's terms. Better yet, he was able to prescribe a solution, and an effective one at that.
Godbold described the phenomenon we experienced as "a very light valve bounce." Unlike valve float, valve bounce indicates that the valve is not staying seated. "The spring mass itself wasn't enough to hold the valve down," Godbold continued. "The spring starts to surge, so the valve control isn't good. When a spring is having problems controlling its own mass, it doesn't have as much ability to close the valve, and doesn't hold it closed. It can even push the valve open."
Substitute the word "resonance" for "surge," and you've got a quick explanation of our problem. All valvesprings resonate at a certain frequency, and they reach this frequency at a given rpm. Godbold calls this the spring's limit speed, but it isn't necessarily tied to high-rpm antics. "When a spring gets close to the limit speed, it resonates," he told us. "Many people think it happens at high rpm, but it actually happens whenever the spring approaches its limit speed." This resonance, according to Godbold, is a function of both a spring's rate and its mass. "As spring rate goes higher, frequency goes up," he told us. "And as mass goes down, frequency goes up."
Although spring limit speed isn't always tied to high rpm, in this case, our spring's limit speed was close to the engine speed at which our 406 produced peak power-thus the spoon-shaped dip at the end of our dyno runs. What we needed to combat the resonance issue was a lighter spring that also has a higher natural frequency. Godbold figured that a bit of Gen III/IV tech was in order, specifically a set of beehive-style valvesprings, first introduced for the Gen III LS6 powerplant.
"The nature of a beehive spring is that it has a progressive rate," Godbold explained. "Each coil has its own resonance frequency. It's not just magic-there's a reason it works. It's hard to make the whole thing resonate." Remember, however, that resonance is also a function of a spring's weight, as well as its spring rate. Higher spring rate usually leads to a physically heavier spring, but using a beehive spring allowed us to maintain the same spring rate while cutting valvespring and retainer weight in half. "All we did was move the limit speed up, and the limit speed was close to peak power," Godbold summed up. When compared to the "gorilla method," which would have mandated a bigger, heavier spring, this is a finesse approach. We are actually able to obtain better control with less seat load, open load, and spring rate by substantially reducing the mass, thereby increasing the natural frequency. Our post-peak dip is gone, and the thing revs to seven grand without a complaint. Sure, power still drops off after the peak, but not as abruptly-up high, our rev-happy 406 is now making 40-some horsepower more than it did. We'll take it.
Control high-rpm valve instability on a 406ci stroker small-block
The Bottom Line
A set of beehive valvesprings eliminated our post-peak power drain.