Engine building is about decisions--decisions that can make or break the outcome of a project. Nowhere is that more true than in the camshaft. There are a variety of choices in camshafts, but the only real way to differentiate one from the next is by the numbers. Essentially, everything you will know about a given camshaft and how it will perform in an engine combo comes down to the numbers describing it--the specifications. If these numbers are as foreign as the Da Vinci Code, you've got problems when it comes time to choose the right stick. Even if total mastery of a cam's numerology is a little deeper than you want to go, a fundamental understanding of the key specs will greatly improve your ability to avoid getting the shaft with the wrong grind.
We're not sure if the information presented here will qualify you as the neighborhood cam whiz, but we can say for certain an understanding of these concepts will put you more than a fender up on the guy in the losing lane.
LIFTThis one is the easiest terms to understand, with "lift" referring to how far the valve is opened off the seat in fractions of an inch. What creates the valve lift, as quoted on a cam card? The gross lift is the cam's actual "lobe lift," multiplied by the rocker ratio. As the camshaft comes around from the base circle to the lobe ramps, the lifter is displaced and rises, until it reaches the point of peak lift at the nose of the lobe. The amount of this displacement is the actual lobe lift of the cam. The lobe lift isn't the same as the amount of lift at the valve, since this lift is multiplied by the rocker ratio. The rocker ratio is created because the pushrod side of the rocker is closer to the rocker's pivot point than the valve side. The rockers in most factory V-8 engines ranged from 1.5:1 to 1.75:1, while aftermarket rockers can be had in a variety of ratios for many engine types. Usually the lift given in manufacturer's catalogs is based upon the cam's lobe lift multiplied by the engine's original rocker ratio.
Actually, although an engine's rocker ratio is given as a specification for that given style engine, the ratio when measured is often not exactly what the specifications will indicate. You'll find that the actual ratio can be a little more or less, and as a result, so will be the actual measured valve lift. Some things that will affect the measured lift, or rocker ratio, are the pushrod angles, pushrod length, and the particular geometry on the heads being used. Aftermarket rockers rated at 1.5:1 also do not usually deliver exactly that ratio, and sometimes deliver a slightly higher ratio. With all the variation in actual delivered ratio, valve lift as given is just an approximation (though usually it is pretty close), based on the rarely true assumption that the rocker delivers the exact ratio quoted. On the other hand, the "lobe lift" does not include the variable of rocker ratio, and is a very precise number. The "lobe lift" can be used to calculate the valve lift with any rocker ratio. If aftermarket rockers with a non-stock ratio are being used, the "lobe lift" multiplied by the rocker ratio will provide the valve lift number. For example, the popular COMP Cams XE 268H hydraulic flat-tappet cam for the small-block Chevy has a lobe lift of 0.318-inch on the intake side. The delivered lift with this cam will be 0.477-inch, 0.509-inch, and 0.541-inch with 1.5:1, 1.6:1, and 1.7:1-ratio rockers, respectively.
If you want to know exactly what the valve lift is going to be in a given engine combination, you actually need to mock-up the camshaft and valvetrain, and measure it with a dial indicator. Just doing the math with the lobe lift and rocker ratio will typically provide an approximation that is close enough, but as mentioned previously, there are a few other variables that will affect the actual valve lift delivered. These variations are of little consequence in a typical street or mild performance engine. However, no serious race engine should be put together without physically measuring the actual valve lift at the valve.