Engines are cool. They make noise and lots of power. That's what we're interested in--power. But in order to make that grunt, there are certain things you really should know. For induction and exhaust systems, the plan is relatively easy--get the air and fuel in and get the exhaust out. To do that, you have to time the opening and closing of the valves at just the right instant.
When you start talking camshafts, the guys in the know begin tossing around all sorts of jargon like lift, duration, lobe separation angles, overlap, and other impressive-sounding terms. If you want to get in on this conversation, you will need to learn these terms so you can get a handle on the best cam for your engine. But don't sweat it, we've got you covered. Stick with us and when you're done with this story, you'll be able to toss those terms around like you were born with 'em. Your car buddies will be impressed--we guarantee it.
Flat vs. Roller
There are two types of camshafts used in small- and big-block Chevys--flat-tappet and rollers. Flat-tappet cams use a lifter, sometimes called a follower, that is essentially flat on the bottom where it rubs on the camshaft lobe. It's actually slightly crowned in the center for wear reasons, but it looks flat. Roller cams use--you guessed it--a roller follower and are usually made of a much harder steel rather than a flat-tappet's cast iron. Each of these cam designs can be divided into either mechanical- or hydraulic-lifter styles. Mechanical, or solid, lifters are just like they sound--a solid connection between the lifter and the pushrod. Hydraulic lifters employ a tiny piston inside a cavity in the lifter. This cavity fills with oil and creates a hydraulic connection between the lifter and the pushrod. One reason for this hydraulic connection is to compensate for expansion in the engine parts from cold to hot. Now let's get into the camshaft itself, shall we?
Lift
Let's start with the shape of the cam lobe. If you start with a circle and add a "bump" to a portion of that circle, you've created an eccentric. This is how a camshaft creates linear (up and down) movement from a rotating device. Lift is defined as the difference in height between the radius of the circle and the height of the eccentric. This is called lobe lift. For example, let's say that we use a dial indicator set on the base circle of the cam lobe. This is the circle area of the lobe. As we turn the lobe, the dial indicator will begin to rise as it contacts the lobe portion of the cam. The dial indicator will eventually hit its highest level, let's say 0.340 inch. This is the lift of that particular lobe. A camshaft for a small- or big-block V-8 Chevy will have 16 lobes, one for each intake and exhaust valve in the engine. Most Chevy cams also come with a fuel pump lobe as well. There are several ways to increase lift. The easiest way is to use different rocker-arm ratios. For example, the stock rocker-arm ratio for a small-block is 1.5:1, but several companies build rocker ratios of 1.6, 1.7, and even 1.8:1 that can increase valve lift with the same lobe-lift height. To determine maximum valve lift, simply multiply the lobe lift by the rocker ratio. In the case of a 0.340-inch lobe lift with a 1.6:1 rocker ratio, this would produce a max theoretical lift 0.544 inch (0.340 x 1.6 = 0.544). We say theoretical because valvetrain deflection and tolerance stack-up can prevent the engine from generating maximum valve lift.
Duration
If you look at a cam lobe carefully, you'll notice that the lift is created gradually using a slope. The amount of time (in degrees) that lift is generated is called the duration of the lobe. Here's where we throw you a mild curve. Camshafts operate at half engine speed. This is easy to see because the gear that turns the camshaft is twice the diameter of the crank gear that drives it. That means that the cam spins at half engine speed. Because of this, camshaft duration is always expressed in crankshaft degrees. This makes it easy when it comes time to degree the cam to ensure it is positioned accurately in the engine.
So let's take a typical cam and look at how duration is expressed. The point at which lobe lift first begins is often difficult to identify since the profile is very gradual at this point. A long time ago, the Society of Automotive Engineers (SAE) decided that 0.006 inch of valve lift was a good place to start, but not all cam manufacturers adhered to that standard. They chose instead to use different heights of >> tappet lift, usually between 0.004 and 0.006 inch. Using 0.004 inch as an example, once lobe lift achieves 0.004 inch, you start recording the number of crankshaft degrees it takes for the lobe to run all the way through max lift and back to 0.004-inch lobe lift on the closing side. Let's say this is 270 degrees. This is the advertised duration of the lobe because this is the number that most cam manufacturers use when referring to their camshaft duration numbers in advertising.
The problem with advertising numbers was that not everyone used the same lobe-lift figure to determine duration. This led to significant confusion when it came time to compare numbers. Legend has it that Harvey Crane suggested that all the cam manufacturers use 0.050 inch of tappet lift as a common lobe-lift point that all cam manufacturers would use so that we could compare the cams. This is the number that most people use when referring to duration specs since it uses a common data point. For example, a Crane PowerMax 278 flat-tappet hydraulic has an advertised duration of 278 degrees on the intake and 290 degrees on the exhaust side. The duration at 0.050-inch tappet lift is 222 degrees on the intake lobe and 234 degrees on the exhaust. Because this camshaft uses different intake and exhaust duration figures, it is referred to as a dual-pattern cam. If the intake and exhaust durations are the same, then it would be a single-pattern cam.
We should also go through some information about opening and closing points as well. Each cam company calls out valve opening and closing points differently. For example, a Comp Cams timing card >> will indicate opening and closing points at 0.006 inch of tappet lift. Crane delivers the opening and closing points at both 0.004 inch and 0.050 inch of tappet lift. They both use some abbreviations that you should know. The intake valve opens at a given number of degrees Before Top Dead Center (BTDC) and closes the intake After Bottom Dead Center (ABDC). The exhaust valve generally opens Before Bottom Dead Center (BBDC) and closes After Top Dead Center (ATDC).
There is a simple way to determine duration. Let's say you have a cam in an engine and you're not sure of its duration. Set up a degree wheel on the engine and determine the cam's opening and closing points. Let's say that the intake lobe opens at 4 degrees BTDC and closes at 44 degrees ABDC, both measured at 0.050 inch of tappet lift. Add the opening and closing points together along with 180 degrees and you will have the cam's duration of 228 degrees at 0.050-inch tappet lift (4 + 44 + 180 = 228 degrees). This also works for the exhaust side and you can determine advertised duration the same way. Cool, huh?
Centerlines
We're not talking about wheels here, but rather the term used to determine the placement of the lobes both on the cam and in the engine. Let's take intake centerline first. Each lobe on the camshaft has a centerline, or midpoint in its duration curve. This would mean that both the intake and exhaust lobes have a centerline. Camshaft companies place great importance on the intake centerline for several reasons. Cam companies use the intake centerline of cylinder No. 1 to establish exactly where the camshaft is in relation to the rest of the engine. As you can imagine, the camshaft must be correctly phased in relationship to the engine in order to do its job properly. The cam companies use intake centerline as this reference point. The intake centerline >> is expressed as the number of degrees ATDC. Generally, the intake centerline will be between 104 and 116 degrees ATDC.
The next item on our centerline agenda is something called lobe separation angle. This dimension specifies the distance or spread between the intake and exhaust centerlines. This is important because it establishes the amount of overlap between the intake and exhaust. Overlap is the amount of time (in degrees) that both the intake and exhaust valves are both open in the cylinder. If you look at the cam-timing graph above (9), you can see a triangle that forms an area bounded by the opening side of the intake lobe and the closing side of the exhaust lobe.
If you look at the top of the graph on page 64, you will see two small lines that indicate the position of the intake- and exhaust-lobe centerlines. The distance between these two centerlines is the lobe separation angle, expressed in camshaft degrees. This is the only cam spec that is not expressed in crankshaft degrees. Lobe separation angle can be determined with some simple math. For example, let's say the intake centerline is 106 crankshaft degrees ATDC and the exhaust centerline is 120 degrees BTDC. Add the two figures and divide by 2 (because we are spinning the cam half as fast as the crank) to get the lobe separation angle [(106 + 120) / 2 = 113 degrees of lobe separation angle].
As the spread between the lobes tightens, the lobe-separation number gets smaller and overlap increases since the centerlines of the two lobes are coming closer together. A larger lobe separation angle means less overlap because the lobe centerlines are moving farther apart. This gets tricky because if you increase duration, this automatically increases the overlap with the same lobe separation angle. This is why you will see big cams with wider lobe separation angles since the cam grinder is attempting to limit the amount of actual overlap between the two lobes. If you study the cam-timing graph for a while, this concept will probably start to make sense. Once you are clear about how this works, you've taken a large step toward understanding how camshafts work.
Conclusion
There's much more to cams than this short course can deliver, but these are the basics of how a cam operates and what all those confusing terms are all about. If you've read through this for the first time, don't get down on yourself if you don't fully understand everything we've outlined here. Read this story again and work on each concept before moving on. That will help, and eventually you'll have a firm grasp on the subject. Then you can use that to amaze and astound your friends with your incredible knowledge of that lumpy/bumpy-looking stick that tickles all the valves.
There are basically two types of camshafts, flat-tappet cams (bottom), and roller cams (top). Within each of these categories, you can opt for either solid or hydraulic lifters. |
Roller cams use a lifter with a small, very hard wheel on roller bearings that follows the shape of the lobe. Roller cams are more expensive than flat-tappet cams but offer more lift for the same number of degrees of duration. |
Lobe lift is the height of the eccentric rise over the radius of the base circle. In this case, the rise is 0.340 inch. This lift is generated over 141 degrees of cam-lobe duration. Since the cam spins at half crankshaft speed, we would double this duration to come up with an advertised duration of 282 degrees. |
Rocker arms multiply the valve lift using the rocker ratio. For big-block Chevys, the stock rocker ratio is 1.7:1, which means if the cam offers a lobe lift of 0.330, the theoretical valve lift will be 0.561 inch. These are Crane roller rockers on a big-block. |
Camshafts spin at half engine speed because the cam gear is twice the size of the crank gear. |
Lobe lift is limited to the diameter of the journals on the cam. If the lobe is larger than the journal diameter, you wouldn't be able to install the cam in the engine. |
Degreeing the engine involves setting a degree wheel on the snout of the crank and measuring the relationship between the camshaft and TDC of piston No. 1. This step is essential to determine the exact location of the cam in the engine. Otherwise, you're just guessing. |
The new darlings of performance street engines are the hydraulic-roller cams for early small- and big-block engines. These retrofit cams offer excellent power with relatively short duration numbers. |
Overlap is one of the most interesting aspects of cam operation. This graph illustrates the time, in degrees, when both the intake and exhaust valves are simultaneously open. More overlap decreases low-speed torque but helps top-end power. |