Comp Cams' Billy Godbold Talks
Simply tweaking an engine's cam specs can transform it from a bring-on-those-3.08:1-gears torque beast to agive-me-rpm-or-give-me-death screamer. In other words, a cam has the power to impart bipolar disorder. Although cylinder heads may more profoundly influence how much power a motor produces, no other component determines how that power is produced and defines the overall character of a motor as significantly as the camshaft. With the thousands of camshaft options available for your Mouse or Rat, it pays to study up.
There's far more to understanding bumpsticks than duration, lift, and lobe separation angle (LSA). Furthermore, optimum camshaft performance is impossible without the valvetrain hardware to back it all up. Recent breakthroughs in valvetrain technology have allowed engineers to push the envelope of camshaft design. Some trends offer genuine performance benefits, while others don't quite live up to the hype. Fortunately, we've compiled a cheat sheet for you, courtesy of Billy Godbold of Comp Cams, to distinguish fact from fiction.
"We have more than 8,000 lobe designs at Comp," Billy reveals. "Many of these have similar duration and lift figures, but their acceleration curves are different. On the R&D side, we spend a more extensive amount of time trying to tweak the acceleration-curve shape of lobes to maximize engine speed for a given area, duration, and lift than we do trying to pick the right overall duration, lift, and lobe separation angle. An analogy using cylinder heads would be runner cc and valve size. There are hundreds of different ways to make a 180cc small-block head with 2.02-inch intake valves. However, the real trick is in the port shape. In cam design, it's important to understand where you can really push the tappet and valve hard without upsetting the spring, and know where to take it easy. This is why we have such a large R&D staff, two Spintrons, and our own data analysis software. We also have invested a substantial amount in valvetrain modeling software. Anyone can look at our Web site and copy lift, duration, and lobe separation angle. The question is, can you get as much air in the engine and keep it all together dependably at the required engine speed?"
Duration Vs. Lift
Increasing either duration or lift can improve power, so how do you know which is more critical when selecting a camshaft? "Under ideal circumstances, camshaft design would allow for isolating the effects of duration and lift from each other in terms of how they affect power," explains Billy. "However, they are typically closely related in flat tappet profiles due to the velocity limit imposed by the tappet, and there is only so much lobe lift you can get for a given amount of duration. Choose the duration based mostly on the engine's target operating rpm and the lift based mostly on cylinder head flow." In addition to that, other engine factors to consider when selecting lobes include valvespring selection and valvetrain durability.
"Many years ago, springs were not designed around the solid stress state, when the coils are pressed together," Billy says. "That has since changed, and the damping effect of springs is used to reduce any surge waves resulting from a rapid valve opening as the spring approaches a solid state. If you do not check the solid height of the spring and the installed height, then you can't control how close you get to a solid state. Likewise, if you don't check installed height, you can only guess how much load you have on the seat or anywhere else to control the valve. Obviously, if you try to lift the valve past solid height, then something has to fail. However, now we know that if you are too far away from solid, you will not be able to use the coil interaction over the nose to reduce surge. Depending on the application, it's ideal to have the spring come to within 0.060-0.120 inch of solid over the nose."