Camshaft Specs Explained: Duration, Lift, and Lobe Separation – Jalopnik

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

For example, upgrading the camshaft can provide more power higher up the rev range for racing or more torque at low revs for street driving. There are even special camshafts for recreational vehicles and towing applications. Let’s face it: Whether you’re shopping online or flipping through an old-fashioned Summit Racing paper catalog, the barrage of camshaft specs can seem like a foreign language. For example, a description might read: “Duration 288/294, lift .520/.540, lobe separation 110.”

To get the features you expect from a new camshaft, it’s helpful to know exactly what those confusing camshaft specs mean. Camshafts are connected to the engine’s crankshaft via a chain, belt or gears and have lobes that operate 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 activated. It’s the pointy end where all the action takes place.

Essentially, the camshaft is a part 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 path to remove spent exhaust gases. The degree to which each valve opens from its fully closed (sitting) position is called lift. For example, a typical high-performance camshaft has a lift of about 0.450 inches to 0.550 inches. In other words, the valves open about half an inch.

Increased valve lift can result in more power by allowing more air and fuel in on the intake stroke of a cylinder and allowing exhaust gases to exit more quickly on the exhaust stroke, but there are limitations. A very high cam lobe that provides excessive lift without other adjustments can cause the pistons to actually contact the valves, which is not an advantage. In addition, too much lift can lead to seizure between cylinder head components, such as the valve springs, which close the valves.

When reading camshaft specifications, the lift number is often referred to as lift. Also keep in mind that you don’t necessarily need to replace a camshaft if you’re just looking for more lift. Higher ratio rocker arms use greater leverage to improve the lift of your old camshaft and are typically cheaper and easier to install than a new camshaft.

If lift is the rate at which the valves open, it only makes sense that duration is the time the cam lobes hold the intake and exhaust valves open. Think of duration as the thick round part of the bottom of the cam lobe. Duration is not measured in typical time units, such as seconds or fractions of a second. Instead, it measures how many degrees the engine’s crankshaft turns while the valve is still open.

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

Typically, long run times improve top-end power – power that occurs high in the rev range – at the expense of low-end torque, which is useful in normal stop-and-go street traffic. A short duration will also make the engine idle smoother. That said, there are some limitations to the amount of lift that can be achieved in a short time because modern hydraulic lifters (automakers’ replacements for solid lifters), which operate the valves, are typically not adjustable.

Discussing camera spec dimensions can be a bit dry, but kudos if you stuck with us. The final piece of the puzzle is the lobe separation angle, the difference in degrees between the peak opening portions of the intake and exhaust lobes. This specification was previously known as overlap because both the intake and exhaust valves are open momentarily at the same time.

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

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

Like many things in life, selecting the ideal aftermarket camshaft(s) is all about compromise. No single camera will do everything universally well in all types of vehicles. It’s important to consider factors such as whether the camera will see a lot of track days or whether it’s primarily intended for street riding. A car or truck that idles erratically, stalls, and has terribly bad fuel economy is not going to be fun to drive around town, no matter how fast it is.

Likewise, vehicles with manual transmissions can handle a noisier cam than automatics due to the automatic transmission’s torque converter – unless you also install a replacement torque converter with a higher RPM stall speed. Factors such as compression ratio, a vehicle’s weight, and even gear ratios and tire size can also play a role in cam selection.

Explaining all these variables would result in an article about 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 help with choosing. This assistance can take the form of calculators, clear feature summaries and personal recommendations based on your vehicle’s description.