For years, the standard operating procedure for testing new components or ideas has been to eliminate every variable possible, make one small change, and evaluate how that affects performance. It's the scientific method, and it has been preached for years. But what happens when you are testing one component in a complex system of parts?
Take, for example, a race engine's valvetrain. Engine builders have made hundreds of dyno pulls while optimizing their given system. But if an engine builder has found the best cam profile that works with a mildly flexible 51/416-inch pushrod, engine power should be even better when a stiffer 31/48-inch pushrod is swapped in, right?
Not always. Bill Godbold, a cam designer with Competition Cams, has done a great deal of research on the valvetrain and concludes that it must be considered as a complete system: from the size of the cam bores to the shape of the valve seat. And he recently made a very interesting presentation at the Advanced Engineering Technology Conference (AETC), which is a yearly gathering of some of the best and brightest engine builders in the country. The information Godbold presented can be helpful to stock car racers and engine builders across the country, so with his blessing Circle Track is presenting much of it here. Godbold works with all forms of motorsports, and many of his examples and charts reference drag racing engines. But the information still holds true for stock car racing. Everything is for cam-in-block, two-valve engine designs.
The Spring's the Thing Godbold's first example is a case study using two different valvesprings on a test mule motor. The engine is a 406 Chevrolet small-block with what Godbold describes as a very good set of heads, a Comp Xtreme Energy hydraulic roller cam, and other typical components.
The first spring was Comp's nested double spring (PN 954). It is what you might consider the more conventional choice for racing and was installed with 210 pounds on the seat (installed height is 1.900 inches) and 523 pounds over the nose at 1.250-inch valve lift. The second spring was Comp's newer beehive single spring (PN 26918) with 130 pounds on the seat when installed at 1.800 inches and 318 pounds over the nose, which is 1.200-inch valve lift in this case.
The first noticeable disparity between the springs is the nearly 200-pound difference in the open pressures. The high pressure of the double spring is typical of many engine packages in which a strong spring is used to control the valve at higher rpm levels. But this can also have an interesting-and unintended-consequence at the lower rpm ranges.
Godbold provided a dyno graph for the engine with both sets of springs. The double spring (PN 954) is Spring A (blue) while the newer beehive spring (PN 26918) is Spring B (red). The biggest difference is how much better the beehive spring holds power after 6,400 rpm-exactly where the stronger spring is supposed to work best. The beehive spring works better in the higher rpm range because it is significantly lighter. Godbold calculated the difference in the spring's effective mass (since the bottom coil of the spring doesn't move, the total mass of a valvespring isn't relevant). The beehive spring used a smaller retainer, so it was 52 grams lighter than the nested double spring. The weight pulled from the valvetrain allows it to operate more efficiently and maintain good valve control at higher rpm levels. Remember, the valvespring has to overcome not only the inertia of the moving valve, but also its own weight and inertia.
But that's not the only interesting thing to be found in this dyno graph. Godbold also points out that the bigger spring produces better torque, and to a lesser degree horsepower, until approximately 4,200 rpm. The cause of this is a flaw in the valvetrain package. When the cam begins the process of opening the valve at speed, the extreme spring pressure resists it so strongly that it flexes the pushrod and possibly the rocker arm. This, in turn, delays the opening of the valve as the pushrod flexes. The inertia working against the pushrod decreases once the valve starts moving, allowing the pushrod to spring back to its original shape. This extra "push" from the pushrod slings the valve open, causing it to loft at max lift. If you graphed the designed valve lift versus the at-race speed, the line for the actual lift would begin later than the designed-lift line, but the valve would also open more quickly and taller. Overall, it allows more "area under the curve," or more airflow.
So this is good, right? Godbold says not really. Remember, the torque increase for the stiffer double spring only occurs at lower rpm levels. At that point, the valve lofts at max valve lift but settles back down on the backside of the cam lobe before being gently returned to the valve seat. At higher rpm levels, the amount of loft is greater and the cam is spinning faster, so the valvetrain never comes back under control before the valve slams onto the seat. When this happens, valve bounce and a loss of power occurs. That's why the smaller spring maintains valve control better at higher rpm levels. Even though it isn't as strong and should theoretically allow lofting from valve inertia sooner, the fact that it isn't flexing the pushrod actually helps maintain control longer.
The Evolution of Bigger Cams Another example along these lines is the growth in the size of cam cores that has happened in the last several years. This trend began in the drag racing ranks and has caught on among stock car racing engine builders.
"In the late '90s, most NHRA Pro Stock engine builders thought that the Ford block's larger 2.124-inch journals gave an advantage over the Chevy's 1.948 stock journals," he says. "The first step was increasing to 2.124 babbit, then 2.165 roller, and later to 2.362 roller journals. These larger journals increased the core barrel size. [To go with this change,] engine builders found they needed to tweak their lobe selection to take advantage of the larger cores. Typically, they would decrease the 0.050 duration by about 2 degrees while selecting a profile with a similar 0.200 duration."
What engine builders had not realized was how much deflection they were getting in the smaller cams. By moving to the larger cams they were creating a stiffer valvetrain. As with the smaller valvespring that created less pushrod flex, the stiffer cam core created a valve movement that was closer to the lobe design.
The flexing camshaft produced a later valve opening but compensated by raising the valve more quickly. Unwittingly, camshaft designers and engine builders had optimized their cams around this design weakness by using lobes with more duration and a gentler opening ramp.
Godbold says that when engine builders began using the larger cam cores, they noticed that power was jeopardized even though they were able to get more rpm out of their engines. While some builders ditched the idea at that point, the smarter ones realized they needed to re-optimize the system because of this change and began tweaking the cam lobes. By using a lobe with less overall duration, a more radical opening ramp, and a higher overall lift they not only reproduced the previous power levels, but also exceeded them while keeping the ability to spin the engine faster. This re-optimization was necessary every time a change was made to a larger camshaft core and was an evolution that Godbold says took approximately five years.
Pushrod Flex The final example Godbold provides concerns pushrod flex. "Over the past few years, 71/416-inch pushrods have become commonplace in applications such as NHRA Super Stock," he says. "However, several engine builders noted a decrease in low-rpm torque and less peak power when making the switch. They also noted that the power held on much better past peak and that overall durability improved. To take advantage of the newer components, many engine builders swapped from the Hi-Torque -8 cam lobe profiles to the newer HXL designs. The newer designs' higher lift appeared to fix most of those issues, but often the duration was slightly reduced."
Stiffening the pushrod has the same effect as using a larger camshaft core or switching to lighter springs. The valve follows the cam lobe more accurately once the deflection is removed from the system. This is also true in the case of stock car engine builders switching from 51/416- to 31/48-inch pushrods, or using pushrods with a thicker wall.
If the cam lobes had already been optimized for the more flexible valvetrain, the effect of the bigger pushrods would have opened the valve sooner and raised it more slowly with less lift. (Remember, with a smaller, more flexible pushrod, the effect is much like a pole vault. Movement is slow as the pole bends, but once that energy is released it flings the vaulter up at a very high rate of speed.) Power is reduced because the ideal cam lobe has a very short duration to maximize valve seat time. It opens the valve as quickly as possible and then holds it at max lift until the very last minute. To compensate for this, the cam lobes had to be re-optimized with less duration, a faster opening ramp, and more overall lift.
So What Does It Mean? One of the most interesting points of these tests is learning how much valvetrain flex can affect duration. "If you asked a bunch of stock car racers if they ran a variable-duration cam, nobody would raise their hand," Godbold says. "But they all do when you look at the results of some of the SpinTron and dyno tests we've done. And the amount of change can be staggering. We've seen a 7-degree change in valve seat duration just by making a spring change. We've also seen duration change 4 degrees inside of 3,000 rpm on the same engine."
This amount of change in duration is a direct result of valvetrain flexibility. For years we've tried to increase rpm levels in an engine by lightening everything possible, especially the valvetrain. But now it appears the secret to reaching new rpm levels in race engines may be to ignore weights and take a long, hard look at component stiffness.
It can be tempting to say, "Well, if I've already got a system optimized around the flexible components in my valvetrain, why change it?" Godbold says the answer is that while you may be able to optimize the system around a specific rpm level, it is impossible to get the most out of a flexible system over a wide rpm range. Plus, flexibility almost always limits maximum rpm levels.
Finally, while many of these discoveries were made with extensive use of Comp Cams' SpinTron, Godbold says that engine builders with no access to a SpinTron aren't necessarily going to be left behind. You just have to remember that single part changes aren't always going to reveal the entire picture. If you are dyno testing an engine with a new component that should be better-a stiffer rocker arm, for instance-and the results aren't what you expect, it may be necessary to consider the entire system. If you were running a more flexible rocker arm before, a stiffer arm will change the effective duration of the cam. Maybe that should be changed, too. The key is to consider the valvetrain as a whole, not as a collection of individual parts.