How does a racing cam work?

Answer 1

The purpose of a racing cam (sometimes called off-road cam because they frequently do not meet emission standards) is generally to get more air into the combustion chamber. More air equals more fuel which means more power. Contrary to popular belief, air is not "sucked" into an engine but it is "pushed" in by the air pressure in the intake manifold. If the engine is naturally aspirated, then that air pressure is the atmospheric pressure around the engine which is less than 15 pounds of force per square inch. The primary goal of a racing cam is to minimize obstructions to the air that is pushed through the engine. To do this, cam designers generally look at three areas: Lift, duration, and overlap.

Lift determines how high off the seat the valve is lifted by the cam lobe. Very generally, higher valve lift means more air flow into the engine. It is very much like a window in your house; the higher you open it, the more air can flow through the window. But there are issues with high valve lift. The valve has mass and is moving. That means the valve must accelerate and decelerate. Higher valve lift means that the valve must accelerate more quickly to move the greater distance in the same amount of time. This acceleration can become so large that the valve's momentum (inertia) keeps it moving even when it has reached its designed height. This is called valve float and very bad things can happen when valves float - if they strike the piston mechanical failure is almost certain. To prevent valve float, more powerful springs must be used. These stronger springs exert greater pressure on the valve train and can cause premature wear. The greater speed of the valve and the greater height of the valve can also cause the springs to fail prematurely. So the cam with the biggest lift is not always the best choice.

Valve lift for intake and exhaust valves are frequently different. Becuase the exhaust gases are being pushed out by the piston and the rapidly expanding flame front, exhaust valve lifts are generally not as high as intake valve lifts.

Duration determines how long the valve is open. The longer the valve is open, the more time that is available for the air (or exhaust gases) to be pushed into or out of the cylinder. More time generally equals more air. But just as with lift, bigger is not always better. To open a valve longer means that it has to open sooner or close later, or both. But opening an intake valve too soon may allow combustion gasses to escape into the intake, robbing power and disturbing the incoming charge. Closing an exhaust valve to late can also rob power by allowing unburned fuel to escape the engine. And in general, the longer the duration the lower the effective compression ratio at most RPMs. So, like lift, the longest duration is not always the best.

Overlap is the amount of time that both valves are open at the same time. There are many good reasons to have both valves open at the same time. As mentioned, this can allow the incoming air/fuel charge fuel to escape the cylinder unburned. But consider the overall movement of the air/fuel charge through the engine. The greater the velocity that it moves, the greater the momentum it has (the air and the fuel have mass, thus they exhibit the tendency to stay in motion). If the movement into the cylinder through the open intake valve can be accelerated by allowing some of the charge to escape through the open exhaust valve, the higher air flow velocity may actually fill the cylinder with more air/fuel than if the exhaust valve closed sooner. This is somewhat like a "ram air" effect. Also, the exiting unburned air/fuel may also have a cooling effect on the exhaust valve, preventing the valve from overheating and burning. So the loss of unburned fuel may actually help make more power at certain RPMs.

The higher the overlap of the intake and exhaust valves, the more unstable the engine operation is at low RPM speeds. This accounts for the "lopey" sound of a racing engine. This results in pushing up the RPM where peak power is produced. In general, the greater the overlap, the higher the RPM where peak torque is produced. So like the other two, the cam with the greatest overlap is not always best.