Camshaft Specs Explained: Duration, Lift, And Lobe Separation

When the function of a camshaft(s) is explained to non-mechanically-inclined types, it's sometimes referred to as the "brain" of an engine. Most engines will live their entire lives with the stock cam(s) installed by the manufacturer. However, it's not totally unheard of for a cam to wear out on an extremely high mileage vehicle. Alternatively, some enthusiasts may opt to voluntarily replace a cam with one that has different specifications, which is a surefire way to change the performance characteristics of an engine. 

For example, upgrading the camshaft can produce more power higher up in the RPM range for racing or more low-end torque for street driving. Heck, there are even specialty camshafts for recreational vehicles and for towing applications. Let's face it, whether shopping online or browsing an old-fashioned Summit Racing paper catalog, the barrage of camshaft specifications can be look like a foreign language. For example, a description might read "Duration 288/294, Lift .520/.540, Lobe Separation 110." 

To get the characteristics that you want from a new cam, it's helpful to know exactly what those confusing camshaft specifications mean. Camshafts are connected to the engine's crankshaft via a chain, belt, or gears and have lobes that actuate the valves. To visualize the lobes, think of the shape of a pear. The round circular part of the pear/lobe does nothing while other cylinders are being activated. It's the pointy end where all of the action happens. 

At its most basic, the camshaft is a component that opens the cylinder valves. Each cylinder has at least two valves — an intake valve and an exhaust valve. These valves allow the air and fuel mixture to reach each cylinder and open a pathway to remove spent exhaust gases. The amount that each valve opens from it's fully closed (seated) position is called lift. For example, a typical high-performance camshaft will have around 0.450 inches to 0.550 inches of lift. In other words, the valves will open approximately one half inch.

More valve lift can result in increased power by letting in more air and fuel on a cylinder's intake stroke and allowing exhaust gasses a faster exit on the exhaust stroke, but there are limitations. A very tall cam lobe that provides excessive lift without other modifications can cause the pistons to actually make contact with the valves, which is no bueno. Additionally, too much lift can cause binding between cylinder head components such as the valve springs, which close the valves.

When reading camshaft specs, the lift number is often labeled as lift. Also, note that you don't necessarily need to replace a camshaft if all you're looking for is more lift. Higher-ratio rocker arms use increased leverage to improve lift from your old cam and are typically less expensive and easier to install than a new camshaft.

If lift is how much the valves open, it only stands to reason that duration is how long the cam lobes hold the intake and exhaust valves open. Think of duration as the fat round part of the bottom of the cam lobe. Duration isn't measured in typical units of time, like seconds or fractions of a second. Instead, it's measured by how many degrees the engine's crankshaft rotates while the valve is still open.

Do keep in mind that inside a four-stroke engine, the camshaft(s) rotate at half the speed of the crankshaft. Therefore, a spec like 260 degrees of duration actually means that a valve is held open for 130 degrees of camshaft rotation. But to keep things confusing, cam duration is expressed as crankshaft rotation degrees. 

Typically, long durations improve top-end power – power that occurs high in the RPM range — at the expense of low-end torque, which is useful in normal stop-and-go street driving. A short duration will also help an engine idle more smoothly. That said, there are some limitations as to how much lift can be achieved over short durations because modern hydraulic lifters (automakers' replacement for solid lifters), which actuate the valves, typically aren't adjustable. 

Discussing the measures of cam specs can be a little dry, but kudos if you've stuck with us. The final piece of the puzzle is lobe separation angle, which is the difference in degrees between the peak opening portions of the intake and exhaust lobes. This specification used to be known as overlap because both intake and exhaust valves are briefly open at the same time. 

The smaller the lobe separation angle is, the more overlap there will be between intake and exhaust valves. For example, 105 degrees of lobe separation angle will place the peak valve opening points on the cam lobes closer together than a camshaft with 112 degrees lobe separation angle.

As you might imagine, pressure within an engine's cylinder drops when both valves are open simultaneously. This can lead to a rougher idle, which can be particularly problematic for cars or trucks with automatic transmissions, but a short lobe separation angle is also known to produce additional mid-range power that's beneficial in street driving scenarios. 

Like so many things in life, selecting an ideal aftermarket camshaft(s) is all about compromise. No cam is going do everything universally well in all types of vehicles. It's important to consider factors like whether the cam will see a lot of track days or whether it's primarily for street driving. A car or truck that idles rough, stalls out, and gets terrible fuel economy won't be fun to drive around town, no matter how fast it is.

Similarly, vehicles with manual transmissions can handle a more rowdy cam than automatics because of the automatic transmission's torque converter — unless you also install a replacement torque converter with a higher RPM stall speed, that is. Factors like compression ratio, a vehicle's weight, and even gear ratios and tire size can also play a role in cam selection.

Explaining all of these variables would result in an article of approximately the same length as Tolstoy's "War and Peace," but at least you now know the basic concepts. Fortunately, many camshaft manufacturers and online performance parts stores offer assistance in choosing. This help can take the form of calculators, plain-speak summaries of characteristics, and personal recommendations based on your vehicle's description.