Don't just depend on catalogs...
Don't just depend on catalogs or advertising. Understanding how a cam's design affects performance can help you tune the engine for your car, the track you race, and even the way you like to drive.
For some of us, camshafts are a lot like marriage-we understand the concept but cannot fathom exactly how to make it work. For example, why is duration always measured in crankshaft degrees? And why do you not begin measuring duration until 0.050 inch of lift? Or why do camshaft manufacturers grind a cam advanced? What does the lobe separation angle have to do with performance? And why does advancing the camshaft seem to help low-end torque?
The science behind camshaft design is as advanced as anything in a race car, so most of us-including experienced engine builders-depend on the manufacturer to help spec the right cam for a particular engine package. Still, there is a science that controls every part of the design of your camshaft, and understanding why different parts of the cam are designed a particular way can help you determine what works best for your needs.
Of all the different parts of the cam, most are relatively straightforward (e.g., the journals and distributor gear). The cam lobes, one for each valve, contain all the variables. The cam lobes control not only total lift and when the valves open and close, but also valve speed, acceleration, overlap, and even how much cylinder pressure is developed at speed. There are a few parts of the lobe design critical to achieving this.
A cam chart like this can...
A cam chart like this can tell you just about everything you need to know about a cam. This is a typical design used in the Busch Series. The red line shows where the cam has the exhaust valve positioned in terms of lift for every degree of the crank's movement. The blue line is for the intake. Where the two lines intersect is valve overlap. The circles at the bottom mark lift at 0.050 inch. Courtesy of Comp Cams
Base Circle is the term for the backside of the lobe. When the lifter is on the base circle of the lobe, the valve should be closed. It is also commonly called the heel of the lobe. The size of the base circle is important in relationship to the cam's lift. A smaller base circle allows more lobe lift, but it can also allow the camshaft to flex and throw off the timing events.
Ramps are the parts of the lobe where the lifter is either moved up or allowed to drop. Every lobe has two ramps-an opening ramp and a closing ramp. In performance camshafts, the curve of the ramps changes several times, which is a tool the cam designer uses to fine-tune the speed and acceleration of the lifter.
An asymmetrical lobe refers to opening and closing ramps that are not identical. In order to maximize both valve speed and control, the lifter must be raised in a different manner from which it is lowered. For example, in performance applications the valve is generally opened as quickly as possible, but the speed of the valve slows significantly as it nears maximum lift to keep it from lofting. But on the closing side, the valve must be seated relatively gently to keep it from bouncing. An asymmetrical lobe design allows this.
The nose of the lobe marks the area where the valve is fully opened. The highest point of lift is the lobe's centerline. The intake centerline is measured as crankshaft degrees after top dead center (TDC). The exhaust centerline is expressed as the number of degrees of the crankshaft's position before TDC. Incidentally, a cam's position is always measured relative to the crankshaft's position because that tells you where the piston is and which stroke it is on (intake, compression, power, or exhaust).
Lobe lift is the amount the cam lobe raises the lifter. It isn't the same as valve lift because the rocker arm is a lever that multiplies the amount of lobe lift to get the final valve lift. The lobe lift is equal to the diameter of the lobe at the centerline minus the diameter of the base circle.
Many cams are ground with...
Many cams are ground with 4 degrees of advance built-in, but that isn't always the case with race cams. You can experiment by advancing and retarding the camshaft a couple of degrees with special adjustable timing sets to see if the changes give you any benefit on the racetrack.
Obviously, the primary job of the camshaft is to control the timing of the intake and exhaust valve events. This is done with separate intake and exhaust lobes. The relationship of these lobes to each other is called lobe separation. Lobe separation is measured in degrees between the peak of the exhaust lobe (maximum valve lift) and the peak of the intake lobe. Essentially, it is half the angle in crankshaft degrees of rotation between peak exhaust valve lift and peak intake valve lift. If the duration remains the same, increasing the lobe separation angle decreases overlap, while decreasing it does the opposite.
"Typically, if all other factors are kept constant, widening the lobe separation produces a wider, flatter torque curve that holds better at higher rpm but can sometimes cause a lazy throttle response," explains Billy Godbold, a camshaft designer at Comp Cams. "Tightening the separation generally produces the opposite effect-more mid-range torque and a faster revving engine, but with a tighter power range."
There are other reasons to change lobe separation to influence engine performance. For example, if you are running a long rod package and keep the stroke the same, you will dwell the piston near TDC longer. To maintain similar overlap characteristics, you may need to open up the lobe separation and shorten the duration.