Engine building is about decisions that can make or break the outcome of a project. Nowhere is that more true than with 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 that describe it-the specifications. If these numbers are as foreign as the DeVinci Code, you'll have problems when it comes time to choosing 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 that an understanding of these concepts will put you more than a fender up on the guy in the losing lane.
LiftLift refers 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 arm ratio. As the camshaft comes around from its 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 stock rockers in factory Mopar big- and small-block engines were rated at 1.5:1 ratio, so the lift, as given by cam manufacturers, is calculated by multiplying the lobe lift by 1.5.
Although the rocker ratio is given as 1.5:1 for stock Mopar wedge rockers, the ratio when measured is not exactly 1.5:1. You'll find the actual ratio is a little less, and as a result, so will be the actual measured valve lift. Aftermarket rockers rated at 1.5:1 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, based on the rarely true assumption that the rocker ratio is exactly 1.5:1. 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 0.484-inch lift Mopar Performance camshaft has a lobe lift of 0.323 inch. The lift with this cam will be 0.484, 0.516, and 0.549 inch with 1.5:1, 1.6:1, and 1.7:1 ratio rockers, respectively.
If you really 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 there are a few other variables that will effect the actual valve lift delivered. Things like the pushrod length will effect the valve lift, because of the angle introduced between the rocker's adjuster and pushrod. Similarly, the working angle of the pushrods, as with offset rockers, or the natural pushrod angle between the lifter and pushrod in a stock small-block will effect the valve lift. Most of these small geometry variations eat up a little lift, and 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.