Camshaft Theory

If the crankshaft is frequently called the "heart" of an engine, there is no doubt that the camshaft is the "brain." Its job is to open and close the valves at just the right time during engine rotation, so that maximum power and efficient cleanout of exhaust can be obtained. The camshaft drives the distributor to electrically synchronize spark ignition.

Camshafts do their work through eccentric "lobes" that actuate the components of the valve train. The camshaft itself is forged from one piece of steel, on which the lobes are ground. On single-camshaft engines (by far the most common pre-1980 engine) there are twice as many lobes as there are cylinders, plus a lobe for fuel pump actuation and a drive gear for the distributor. A typical V8 camshaft has five bearing journals to keep it in precise position while it rotates.

Driving the camshaft is the crankshaft, usually through a set of gears or a chain or belt. The camshaft always rotates at half of crank rpm, taking two full rotations of the crankshaft to complete one rotation of the cam, to complete a four-stroke cycle.

The camshaft operates the lifters (also called tappets or cam followers) that in turn operate the rest of the valve train. On "overhead valve" engines the lifters move pushrods that move rocker arms that move valve stems. (On "overhead cam" engines there are no pushrods or rocker arms, just tappets. We won't talk any more about such engines in this article.)

Lifters can be of several types. The most common are hydraulic, mechanical and roller lifters. Hydraulic lifters fill with oil that acts as a shock absorber to eliminate clearance in the valve train. They are quiet and don't require periodic adjustment. Mechanical lifters are solid metal and require scheduled adjustment for proper valve clearance. These are used in high-rpm applications. Roller lifters use a roller device at one end and can be hydraulic or mechanical. They are used in applications where a very fast rate of valve lift is required.

So Much For Hardware, How Does It Work?

There are things you must know to understand valve train operation, so here goes:

Lash: The clearance between the end of the rocker arm and the tip of the valve stem that allows for thermal expansion of the components. Mechanical lifters typically have lash adjustments between 0.10 and 0.30 inches.

Lift: The maximum distance the valve's head is lifted off its seat in the cylinder head. Since lift is determined by cam lobe lift multiplied by the rocker-arm ratio, maximum lift can be carefully controlled or adversely affected by the choice of components. Total lift is the product of lobe lift times the rocker arm ratio. Manufacturers consider a rocker arm ratio of 1.5 to be standard. For example, a cam lobe lift of .500 in a normal engine would result in a total valve lift of .750 (.500 X 1.5 rocker ratio).

Valve Overlap: The condition in which, during rotation of the crankhaft, both the intake and exhaust valve in a given cylinder are off their seats. This exists to some extent in all engines because it has been determined that the exhaust valve should open before the piston reaches bottom-dead-center (BDC). Residual pressure in the cylinder pushes exhaust gases into the ports, helping to create negative pressure (vacuum) as the piston continues to push the gases out. This vacuum acts to help draw a little bit of the intake mixture into the exhaust port (due to overlapping valve openings) to scavenge the cylinder. The amount of valve overlap dictates how a specific engine will perform.

When valve overlap in increased engine vacuum and cylinder pressure is lower, both of which are undesirable for low end power and driveability.

Lobe Separation Angle: The geometric angle between the centerlines of the intake and exhaust lobes. Once this angle is determined it directly affects the amount of valve overlap. If the lobes are moved closer together (lower LSA) the valve overlap increases.

Duration: The amount of time (in degrees of rotation of the camshaft) that the lobe holds the valve off its seat. Duration also affects the total lift of the valve because of the inherent limitations to the rate-of-lift of the lifter itself. Duration is generally the most important thing to consider when choosing a camshaft.

Rate of Lift: The lobe shape that determines how fast the valve is opened, or how many inches of lift are achieved per degree of crankshaft rotation.

Profile: The overall combination of lift, duration and lobe separation angle provided by the camshaft. This is commonly called the cam "grind."

Start 'Er Up!

With the camshaft and valve train in place we can start the engine. As each lobe touches the lifter, its end rides up the lobe's leading surface. This action "lifts" the lifter and the pushrod, which presses up on the rocker arm. The rocker arm "seesaws" over its pivot mount and its other end pushes on the valve stem, compressing its spring and opening the valve. The valve is fully open with the lifter reaches the tip of the cam lobe, after which it starts closing as the lifter slides down the lobe's trailing edge, pushed by the valve's spring tension. The valve stays closed until the lobe comes back around as the camshaft turns.

Here's where things get tricky, because there's no such thing as the "perfect" cam grind or profile. Each engine configuration requires its own ideal camshaft profile. Racing engines differ from run-of-the-mill passenger car engines, as these do from truck or heavy-duty engines. It's the work an engine is expected to perform that dictates which cam profile is best, and here's why:

The point where the intake valve opens is critical to an engine's running properly. If it opens too early, exhaust gases can get forced into the intake manifold (remember valve overlap?) This causes soot buildup on the intake runners, low engine vacuum and low power. If the valve opens too late, less of the fuel/air mixture gets into the combustion chamber and exhaust gases won't be as efficiently removed.

If the exhaust valve closes too early the desired "scavenging effect" will be less and some exhaust gases can get trapped in the cylinder. If the valve closes too late an excessive amount of fuel/air mixture will escape into the exhaust port and the combustion chamber will not be optimized.