Camshafts - The Basics

The camshaft is unquestionably the most complex component in the internal combustion engine. The good news is that despite its complexity, the terminology can be easily understood if we digest it in bite-sized chunks.

The camshaft's role in the engine is to control the valve timing, ensuring that the intake valves open at the proper time to feed air and fuel into the engine. The second part of this operation is to give the exhaust sufficient time to escape out of the combustion space before the whole process starts over again. It's the size, shape, and placement of all those eccentric bumps on the camshaft that makes it all happen.

Now that we know what the cam is and where it resides in the engine, the best thing to do next is learn some basic camshaft terminology. We'll start by looking at the basic camshaft lobe and its various characteristics. A camshaft lobe is an eccentric that converts rotating motion into linear (up and down) movement. To do this, a lobe, or bump, is created from a true circle known as the base circle of the cam—also known as the heel. As the lobe rotates, it pushes downward on the buckets and shims.

The maximum amount of movement is known as lobe lift. The maximum lift point on the cam is called the nose, while the inclined areas leading up to and away from the nose are called the ramps. On our solid lifter camshafts, a small clearance ramp is included on the opening side ramp to gently remove the lash before the cam begins to open the valve. This prevents valve train abuse. At some point on the opening ramp, the lobe begins to create lift. Depending upon the company's definition of "advertised duration," some point on the lift curve of the opening ramp is used to determine where cam lobe duration is measured. Camshaft duration numbers are always expressed in crankshaft degrees. (I.e. 360 degrees would be open all the time and 180 degrees would be open for half the engine cycle).

The majority of standard cams are between 230 and 245 degrees.

Since there are so many different measuring points for advertised duration, a common measuring point became necessary so cams from different companies could be accurately compared. The industry established 0.050 inch of tappet lift as that standard measuring point. At this point, it's worth looking into the effect of duration on engine power. Stock camshafts offer relatively short duration and low lift numbers since the factory is after a crisp, smooth idle and excellent part-throttle operation.

If we increase duration, the intake valve is now open for a longer period of time during the induction cycle. This added duration tends to affect engine power by decreasing idle vacuum and shifting the power curve to a higher rpm.

This reduces low-speed throttle response and power while increasing power at the higher engine speeds. Too much duration, especially in stock-type engines, will kill power everywhere, and you will end up with an engine that is extremely lazy. In the old days of camshaft design, most cams were designed with exactly the same duration and lift on both the intake and exhaust lobes. But decades of engine research have determined that many engines (depending upon cylinder head airflow considerations) prefer more duration on the exhaust lobe than on the intake lobe. These are called dual-pattern cams, while the term single-pattern refers to cams with the same lift and duration on both the intake and exhaust. Now that we have the lift and duration numbers covered, let's move on to a few other more complex areas. Most cam lobe illustrations present the lobe as being symmetrical, which means if you folded the lobe down the middle and laid the opening and closing ramps on top of one another, they would be the same. But today, most cam designs are actually asymmetrical, meaning that the two ramps are not the same. This is especially true with intake lobes that are designed to slow the intake valve as it nears the seat to prevent it from slamming closed. This violent closing rate is usually what causes valve float.

Each cam lobe also has a centreline. Many camshaft companies use the intake lobe centreline of cylinder No. 1 as a way to determine how the camshaft is phased with the engine. This intake centreline is expressed in terms of the number of crankshaft degrees after top dead centre (ATDC). Engine builders have found that advancing the centreline helps low-speed power while retarding the centreline hurts low-speed power and improves top-end power. Of course it's also correct that the exhaust lobe has a centreline that positions it in the four-stroke cycle. Therefore, there is a relationship between the position of the intake and exhaust lobes that is usually described as the lobe separation or lobe displacement angle. This is the angle (in cam degrees) between the intake and exhaust lobe centrelines. This separation angle is used to indicate the relative closeness of these two lobes. Generally, it is the amount of overlap combined with the amount of intake duration that can make a camshaft "lumpy," giving it that distinctive chop, or rough idle. If we lengthen the duration of the intake lobe by opening the intake sooner, the amounts of valve overlap increases. Or, it's possible to increase overlap by merely shifting either the intake or exhaust lobe centrelines closer together.

Conversely, we could also decrease overlap by moving either one or both centrelines apart. As you can begin to see, there are a ton of variables that can be tried when it comes to experimenting with cam timing. This has been a very basic introduction to cam timing and how all the different points of camshaft design interrelate to create this most complex engine component.