Intake Stroke
The next stroke is the intake stroke. As the piston is getting closer and closer to TDC, the exhaust valve hasn't completely shut and the intake valve begins to open. This point is labeled valve "overlap," and it is a critical governor of engine performance. If the intake valve opens too early, the intake charge can travel back up into the intake manifold (diluting the incoming air/fuel charge) and can limit power at low- and mid-range engine rpm. If the intake is opened too late, it can lean out the cylinder and reduce engine performance.

Concurrently, if the exhaust valve closes too early, it will trap burnt combustion mixture in the cylinder, which is beneficial for emissions control, but not for power. But if the exhaust valve closes late (the piston is farther down the cylinder on the intake stroke), low-end rpm performance is severely hampered. The fresh air/fuel charge just passes through the combustion chamber without an opportunity to be fully compressed and perform work. This helps cool the exhaust valve as it closes, but that is a poor trade at the expense of power.

When the piston passes TDC and begins traveling back to BDC, the intake valve continues to open. A pressure difference occurs-nature abhors a vacuum-and atmospheric pressure forces the fresh air/fuel charge past the fully opened intake valve. The piston is not sucking in the air/fuel mixture; atmospheric pressure is pushing it into the cylinder.

Compression Stroke
The piston once again changes direction and starts up to TDC on the compression stroke. The intake valve begins to close, and the piston continues toward TDC. Both valves eventually fully close, the air/fuel mixture is compressed by the piston, and cylinder volume is decreased as the piston reaches TDC. A spark is applied, ignition occurs, and the engine cycle starts again on the power stroke.

Is it any wonder the camshaft has sometimes been called the "brain" of the engine because it has to mechanically control all these valve events with precision within a few degrees of rotation?

Camshaft Terms
We'll overview some of the basic camshaft terminology here and explain why it is important to your engine's peak performance.

Lift: The distance a valve travels. In our valve lift curve plots, lift is specified in inches along the left vertical axis. The horizontal axis is the number of degrees of crankshaft rotation, and each of the engine strokes are specified below it to reference where the valve events are taking place in the engine cycle.

The gross valve lift in this example is 0.470-inch maximum (the peak of each valve travel plot). Camshaft lobe lift, or lobe lift, is the gross valve lift value divided by the rocker arm ratio: in this example, 0.470/1.5 = 0.313-inch lobe lift. Valve lift amount, and the rate the valves open and close, are critical to how much engine torque is produced.

The camshaft lobe lift is fixed by the camshaft grind, but you can easily and relatively inexpensively increase gross valve lift, and improve the performance of an existing camshaft, by changing the rocker arm ratio. For example, if we take the 0.313-inch lobe lift in our example and change the rocker arm ratio to 1.6, the gross valve lift is now 0.5 inch (0.313 x 1.6 = 0.5 inch). The engine responds as if a larger lift cam has been installed, and improved torque can result. Keep in mind, though, that you've changed mechanical relationships with the increased rocker arm ratio. Small increases in rocker arm ratio can usually pass, but major increased rocker ratio can cause coil spring binding or piston/valve and other mechanical interference. Check for this interference before firing up an engine when you've changed the rocker arm ratio, or expensive noises can result.