Performance may always lay heavy odds on power, but you also need the brains to go with it. The four-stroke internal combustion engine has been around for over a century, but we're still learning incredible things about using camshafts to make more power. That elliptical lobe may look simple, but it represents the brains for your engine's brawn. Let's take a look at what makes this simple eccentric device so powerful.
Lift
We'll start with the easiest concept involved with the camshaft--lift. A cam lobe starts off from a simple circle, called the base circle of the cam. From that circle, the designer creates additional lift using a ramp so that as the circle rotates, it converts rotation into a linear or vertical motion by using a follower or tappet. This lift eventually rises to its highest point beyond the basic radius of the circle. The difference in height between the top of the lobe and the radius of the circle is the lift component of a cam lobe. For most street-type cams, this will be roughly between 0.275 and 0.450 inch. This is called the lobe lift of the camshaft. But we don't generate this lift all at once--it's created by gradually moving the tappet from the base circle to maximum lobe lift. This is where duration comes in.
Lobe design can vary immensely, even within a family of camshafts. The key to selecting the right cam is to match it not only to the type of use the engine will see but also to the other components in the engine. The goal is to create a strong overall power curve.
Duration
Ideally, you could slam a valve open, hold it open, and then slam it closed, and many drag race camshafts attempt to perform this feat, but this harsh action is incredibly abusive on valvetrain parts, especially valves and springs. To make these parts live over hundreds of thousands of miles, the cam lobe lift curve must be gentler. The easiest way to measure the amount of time the lobe is creating lift is with degrees of duration. A long time ago, our cam-building forefathers decided to use crankshaft degrees to measure cam lobe duration. So a typical performance camshaft may have a duration of 280 crankshaft degrees. Keep in mind that a camshaft actually spins at half engine speed.
But this created confusion because all the different cam companies measured the beginning of the lift curve at different points a few thousandths of an inch above the cam's base circle. This is called advertised duration and it can get complicated because, for example, Comp Cams begins measuring its advertised duration for hydraulic lifter cams when the lifter rises 0.006 inch off the base circle. Crane uses 0.004 inch, which would make the same lobe "appear" a few degrees longer in duration because the duration would be measured over a 0.004-inch-longer distance (0.002 inch on the opening and closing sides added together). This difference between companies eventually led to the selection of 0.050 inch as the standard checking point where all the different companies' lobes can be compared. So now we have both advertised duration and duration at the 0.050-inch checking figure. Now that you have the basics, we can dive into a few more items that make camshafts complex, but also fun.
The best way to install a cam is to degree it into the engine so you know exactly "where" the cam is located. Advancing the cam will move all the lobes to open and close sooner in the cycle. Generally, advancing a camshaft will increase low- and mid-range torque at the expense of some top-end power.
For each lobe there is an opening and closing point. Let's say you are measuring an intake lobe on a camshaft in the engine using a dial indicator and a degree wheel. Once the lifter rises off the base circle 0.006 inch, let's say that the degree wheel reads 20 degrees before top dead center (BTDC) and closes 90 degrees after bottom dead center (ABDC), then you can add those two numbers together along with 180 degrees and come up with the advertised duration: 20 + 180 + 90 = 290 degrees. This same formula can also be used to determine duration at 0.050 inch tappet lift.
Duration is a major contributor to the torque curve and where it occurs in the engine's rpm band. Generally speaking, as you increase the amount of intake lobe duration, this makes for an earlier-opening and later-closing intake valve. This additional duration also extends the rpm point where peak torque occurs. This tends to increase peak hp (depending upon the other components used on the engine) while sacrificing low- and mid-range torque. Conversely, a very short duration camshaft opens the intake valve later and closes it sooner, reducing the potential for high-rpm horsepower but increasing torque at a lower engine speed.
Intake Centerline
Now that you have lift and duration mastered, we can move on to more of the measurement values in camshaft design. Imagine looking at a lobe with a vertical line running right down the middle as viewed from the end. This line would represent the centerline of the lobe. If this were an intake valve, this would be the intake centerline of the lobe. Cam designers and engine builders use this centerline to establish where the lobe is located relative to the piston. For example, a Comp Cams 268 Xtreme Energy cam has an intake centerline of 106 degrees ATDC. This means that the midpoint of the lobe (which may or may not be maximum lift, since some cams are asymmetrical in design), will occur when the Number One piston is positioned at 106 degrees after top dead center.
The camshaft spins at exactly half crankshaft speed, which is easy to tell with the timing chain and gear, with the cam gear twice the size of the crank gear. Performance gear and chain sets often allow you to easily advance or retard the cam with optional slots on the crank gear.
When installing a camshaft in an engine, performance engine builders and the blueprinting process demand that you measure or "degree" the camshaft to ensure that it is installed where the engine builder desires. It's not enough to merely line up the dots on the cam and the crank gear. This way, if the engine builder would like to change the phasing of the camshaft, he can use that installed point as a reference. It's tough to know where to go if you don't know where you are.
This phasing of the camshaft is important because when the valves open and close has a serious effect on engine performance. This is where we get into advancing or retarding the position of the camshaft relative to the Number One piston. All references to positioning a cam are always around the intake lobe for the Number One cylinder. So if we wanted to advance the previously mentioned 268XE cam with its 106-degree intake centerline by 2 degrees, this would open the valve earlier in the cycle. It would place the intake centerline at 104 degrees after top dead center (ATDC).
Conversely, if we wanted to retard the camshaft by 2 degrees, this would move the original 106-degree centerline to 108 degrees ATDC. This is an important point that many people get backwards, so take a minute or so to study why these numbers are correct by looking at the cam timing graph.
Advancing the camshaft means that you are starting the opening and closing process sooner in the cycle. It generally improves low-speed torque and mid-range power while sacrificing top-end hp. Conversely, retarding the cam detracts from low- and mid-range power in order to help top-end power. Generally, moving a camshaft a couple of degrees will not make dramatic changes to the engine's power curve.
The camshaft is only one part of the overall valvetrain. Rocker arms are also important players in the overall engine performance scheme. From the left is a stock stamped steel rocker followed by a roller-tipped rocker, and then a series of different design true roller rocker arms.
Lobe Separation Angle
Here's where we get into some meaty stuff, so stay with us. If you look at the lobe graph, you can see the relationship of the intake and exhaust lobes. One of the variables that make cam designing such a challenge is the relationship of the intake to the exhaust lobe. The number of degrees between the intake and exhaust lobe centerlines establishes what is called the lobe separation angle. As an example, many Crane street camshafts are built using a 112-degree lobe separation angle. This means there are 112 camshaft degrees between the exhaust and intake centerlines. This can be determined from a cam card by adding the exhaust and intake centerline numbers together and then dividing by 2. So if you add a 111-degree exhaust and a 113-degree intake lobe centers and divide by 2, you'd get a 112-degree lobe separation angle. Keep in mind that often the intake centerline and the lobe separation angle will be the same number, but they represent completely different functions.
Valve overlap is a function of both duration and lobe separation angle. If the lobe separation angle remains the same but you increase the duration, the amount of overlap will also increase. Overlap is the time, measured in crankshaft degrees, when the exhaust valve and intake valves are both open. Overlap helps improve engine performance by starting the intake cycle before the exhaust cycle has ended. As overlap increases, this tends to make the idle quality more erratic, or lumpy, while improving midrange and top-end power. This is a very complex subject that we'll just touch on here, but even slight changes in overlap and intake opening and closing points can make a big difference in engine performance.