Types of Cams
Flat tappet cams (foreground)...
Flat tappet cams (foreground) tend to be less expensive than roller cams, but the rollers offer some distinct advantages.
The basics apply to all camshaft designs, but it's worthwhile to also go over the different lifter applications. The most basic type of lifter is the mechanical flat tappet. While these tappets look flat, they are actually ground with a slight crown in the center that allows them to rotate in the lifter bore. These lifters are designed to operate with a clearance, or lash, usually set between the rocker arm and the valve stem tip. The next is the hydraulic version of the flat tappet. The hydraulic tappet uses a small piston inside the tappet that allows for expansion and clearance changes with a small amount of preload. This preload eliminates the need for semi-regular maintenance to check lash on mechanical flat tappets.
While roller cams may appear to be a relatively recent performance engine phenomenon, the reality is that roller cams go back almost to the earliest performance engines. The roller eliminates a major source of friction in the engine but does require a much stronger steel core as opposed to cast iron for the flat tappet cams. These steel cores, along with the increased cost and complexity of a roller tappet, are the major reasons that roller cams cost more. But as you'll see if you read our "Cam Secrets" story on page 40, there are significant power gains to be made with roller cams.
Lobotomy 101
If you take some time to study this graph, it will probably make the concept of cam timing a little easier to understand. Starting on the far left side, the vertical scale is the amount of lobe lift in inches while the horizontal scale is crankshaft degrees through 720 degrees or two full revolutions of the crankshaft. This is because the cam spins at half engine speed.
The first lobe (in red) is the exhaust lobe. You can see how advertised duration is greater than duration at 0.050. At the top of each lobe is a point indicating the lobe centerline and how the lobe separation angle in degrees can be computed by adding the centerline points and dividing by 2, since this angle is expressed in camshaft degrees.
The most interesting point in this graph is the small triangle created by the overlap of the exhaust closing and intake lobe opening points. This area will change depending upon the amount of duration in either lobe as well as the position of either lobe centerline. Any change to any of these dimensions will have a major impact on valve overlap. For example, if you merely increase duration in the intake lobe but leave everything else the same, overlap will increase. If you tighten the lobe separation angle from, let's say, 112 degrees to 108 degrees with no other changes, that will increase overlap, making that little triangle area larger. Keep in mind that in order to change lobe separation angle, you will have to have a new camshaft ground.
There are literally dozens if not hundreds of variables that you can plug into this basic graph to help you see the effect of even the slightest camshaft changes. This can help you visualize the changes that a cam makes to the engine.