Adrienne "Daisy" Zonneville grinds custom cams to help Team Amick Racing Engines push the
Think of the camshaft as your engine's brain. It makes all the decisions as to when the valves should open or close, how long they stay open, when valve overlap occurs, and basically, everything else that happens in the valvetrain. Engine builders own racks of cams that they test with different combinations, but if you have a good idea of how an engine responds to different camshaft characteristics you can zero in on that perfect cam a lot more quickly.
As Much Art AS Science "Cam design is as much an art as it is a science," explains Bo Montgomery, the head engine builder for Team Amick Motorsports. Team Amick supports three NASCAR Busch Series teams with engines, is currently testing the waters in Winston Cup, and has even gone so far in the pursuit of power as to employ its own in-house cam grinder. "The ideal cam would control the valve so that at the right time the valve goes to full open, and when it's time to close it goes immediately to full shut. Unfortunately, a cam/valvetrain combination like that would only live for about three revolutions at engine speed. So what you have to work out is a compromise between your cam's velocities and acceleration rates."
Too much velocity can cause valve loft and/or float. Valve loft is when the valve continues to open beyond the control of the cam. Valve float is when the valve closes on the seat with so much speed that the valvespring cannot control it, and the valve bounces back open. The higher the engine's rpm, the more likely it is to suffer from loft or float; fortunately, lofting is normally not a problem. It's valve float that can cause some serious damage to your hardware.
"The intake valve will normally go into a float condition first because it's normally bigger than the exhaust valve and has more mass," Montgomery says. "When the valve bounces, it's essentially increasing the duration past the optimum point. The result is you lose compression as some of the intake charge flows past the valve and back into the intake port. When that happens the pressure pushes back and we've seen it disrupt the flow of the intake charge all the way back to the carburetor. That movement back out of the combustion chamber disrupts the velocity in the other intake manifold runners as well as the metering signal in the carb, and it just kills your power.
A cam controls all the events of the valvetrain. Once it's ground the only variable left o
"Plus, valve float just beats an engine to death. You are in a situation where the valvespring can no longer control the motion of the valve. So, in addition to beating up the valve and the seat, that valvespring really starts dancing, which causes a lot of heat buildup and, eventually, spring failure."
The normal solutions for valve float are going with stronger valvesprings, limiting rpm (going to a lower-ratio gear), or switching to lighter valvetrain components (less mass equals less force and momentum). But it doesn't take much thought to see how these changes can either limit performance or max out the wallet when it comes to buying increasingly lighter components. Another solution may be going with a slightly different cam grind.
"The ramp on the camshaft going from 0 to 0.050-inch lift is probably the most critical part of a camshaft's design," Montgomery says. "And camshaft guys are really secretive about how they develop that ramp. You've got to protect the valves and the seats, and if you try to sling them open and back shut again with too much force both your performance and durability are going to suffer."