The most meaningful statement you can make about power production is that it all starts with cylinder heads that can flow large quantities of air. But having the greatest flowing heads counts for zero if the valves are not opened sufficiently or at the right time in relation to the crankshaft's rotation, and that very important function falls to the camshaft.

The problem is, if you are something of a novice at this engine business, just about everything to do with cams and valvetrains looks complex, and the truth is, it's that and more. If cam and valvetrain design at the top level is in your future, you had better thinkin terms of a Ph.D. in mechanical engineering. OK, so most of you are not looking to do that, but would like to understand cams and valvetrains sufficiently to make truly informed power-generating decisions. Being able to do so can easily mean choosing a cam for a typical street/strip small-block that will make 20-30 lb-ft and 20-30 hp more than your buddy who bought a generic grind out of a catalog solely on the basis of duration. (If it's bigger, it must be better, right?) If that 20-30 extra lb-ft and hp are important to you, then what you are about to read will give you the knowledge you need to get it.

The lowest denominators for a power-producing valvetrain are the lift and duration delivered to the valves. Like most statements that appear to sum things up elegantly in just a few words, this is a gross simplification, but we have to start somewhere. First, duration. This is the number of degrees the valve spends off the seat, or the degrees the lifter is above a specified lift. In catalogs, two numbers are commonly quoted. These come under the heading of advertised duration and duration at 0.050 inch (50 thousandths). The first of these is usually measured, for a hydraulic cam, at 0.006 inches and, for a solid cam, 0.020 inches (6 or 20 thousandths) of cam follower lift, while the second is at 0.050 inches (50 thousandths). A third duration figure, which is often confused with the advertised duration, is the duration at the lash point or, as it is also called, "off-the-seat" duration. Assuming a totally rigid valvetrain, the engine sees the last of these three.

Because the valve lash operates through a step-up rocker ratio, the lash between a solid lifter and the cam is usually smaller than the 0.020 inches used for the advertised duration, so the duration at the lash point is often longer by some 6-12 degrees. The exception here is that some of the older solid designs were designed to run 0.28-.030 inches lash. With this valve lash and the commonly used 1.5:1 ratio rocker, the lash point at the lifter was 0.020 inch--the same as the lifter rise used for quoting duration. This meant advertised duration and duration at the valve lash point were the same. At the other end of the scale, a modern tight-lash solid can be as much as 14 degrees longer than the advertised duration. For a hydraulic cam, lifter collapse is assumed to be about 0.006 inch, so the advertised duration is about the same as the duration at the valve. This makes duration comparisons between hydraulic cams much more realistic than between solids.

Now that we have defined the cam lobe's duration and lift, we can move on to looking at the cam's collective attributes; that is, the intake's relationship with the exhaust. The point here is that the engine's output will be optimized when the valves are open and closed at certain points in relation to the crank's rotation. To be able to specify what you want to your cam grinder, you need to understand the terminology involved. The cam attributes diagram shows what you need to know.

What dictates the cam's success in the quest for maximum area under the output curve along with highest peak torque and horsepower is not (as is so often assumed) the duration involved. The most important factor is actually the overlap and the Lobe Centerline Angle, often referred to as the LCA. I realize this may fly in the face of everything you have been told or have read before, but it's not that hard to see it must be so. Let's do one of those mental experiments to establish overlap as a highly influential criteria. First, and of prime importance to a street-driven performance machine, is manifold vacuum. Let's be clear that we are talking V-8s and single-plane manifolds here. As far as idle quality and vacuum are concerned, it degrades rapidly with increasing overlap. The reason it does so stems from the fact that a V-8 has an induction phase every 90 degrees. This means that when one piston is moving rapidly on its induction stroke (about 90 degrees down the bore) there is another piston stationary at the top of its stroke with the valves in the overlap position, i.e. both open.

With a typical 280-degree hydraulic cam, the valves (in the TDC overlap position) will be a little over an eighth inch off the seat. This means the piston moving rapidly down the bore in the middle of its induction stroke can draw through the intake manifold and right on through the overlapping valves to that cylinder's exhaust. The area it has to accomplish this in a typical V-8 is about equal to a 15/16-inch-diameter hole. On the other hand, the through-flow area via the carb's butterflies is only equal to a hole about 5/16 to 3/8 inch. It's not hard to see that, even in the case of a cam of only about 280 degrees with 64 degrees of overlap, the overlap acts like an unwanted hole nearly an inch in diameter in the intake manifold.

OK, so overlap, or at least too much of it, is not good for a street-driven engine. But let's not overlook that the actual amount of overlap is not the sole issue toward killing vacuum. If we replace the open plenum intake with a two-plane (180 degree) intake, in one fell swoop, we space out the induction pulses seen in each half of the manifold to 180 degrees instead of 90. This means there is no time when the one cylinder's intake can draw through the exhaust of another. As a result, the intake vacuum goes up by something in the order of 50 percent.

Overlap: How Much Is Too Much?
Assuming we are choosing a cam for a streetable engine, how much overlap can we use before it becomes a problem? The answer here is that it depends on the valve sizes in relation to the cylinder displacement. If the heads have small valves in relation to cylinder cubes, then the amount of overlap we can use is significantly more than the same cylinder with much larger valves. For instance, a 500-inch big-block Chevy can tolerate not only more overlap, but a much bigger cam because the cylinder heads are so under-valved for the displacement. A 350 small-block with a set of decent heads has a lot more valve-per-cube, so it does not need so much overlap to get the job done.

Now that we have covered the effect overlap has on street manners, it is time to look at its effect on power output. Let's make one thing clear here: Big (but not excessive) overlap is a prime key to big power numbers, but only if your exhaust system sucks. Literally. If you have ever heard that an engine needs a little backpressure, you might want to ask yourself why an engine would want an exhaust system that literally pushes exhaust back into the combustion chamber rather than sucking it out. The simple answer is, it doesn't. If a big-overlap, big-cammed engine has an exhaust system with any measurable backpressure, the price paid is a big drop in output.

Although the foregoing might be interesting info, it doesn't actually help you make a decision as to how much overlap your engine needs. Just in case you might ask, I use a one-off computer program that was 18 years in the making to do what we are doing here, but that is no help to you. So that you have something of a guide, I have made up the nearby chart. To make the most of this chart, you will need to take into account where, in terms of valve size per cube, your engine falls. For, say, a 302-inch engine with decent-sized valves, the overlap selected needs to be toward the short end (left side) of the segment that fits your application. If it's a typical 350-inch small-block, then choose something around the middle of the relevant segment. If the engine in question is a big-block or a really big-inch small-block (both of which are typically under-valved), then select the overlap toward the larger, righthand side of the relevant segment.