This diagram shows many of...
This diagram shows many of the critical areas on the cam lobe as well as the relationship between the intake and exhaust lobes. Courtesy of Comp Cams
Overlap is the point in crank rotation when both the intake and exhaust valves are open simultaneously. This happens at the end of the exhaust stroke when the exhaust valve is closing and the intake is opening. During the period of overlap, the intake and exhaust ports can communicate with each other. Ideally, you want the scavenge effect from the exhaust port to pull the air/fuel mixture from the intake port into the combustion chamber to achieve more efficient cylinder filling. A poorly designed cam and port combination, however, can cause reversion, where exhaust gases push their way past the intake valve and into the intake tract.
Several factors influence how much overlap is ideal for your engine. Small combustion chambers typically require minimal overlap, as do engines designed to maximize low-rpm torque. Most current stock car racing engines depend on high rpm to take advantage of better gear ratios, so more overlap is normally helpful. When the revolutions per minute increase, the intake valve is open for a shorter period of time. The same amount of air and fuel must be pulled into the combustion chamber in less time, and the engine can use all the help it can get to fill the chamber. Increasing the overlap can help here.
Long rod/stroke packages, which are becoming increasingly popular in circle track racing, also have an effect here just as with the lobe separation. Because the piston dwells near TDC longer, it makes the combustion chamber appear smaller to the incoming air/fuel charge. Because of this, less overlap is needed to properly fill the chamber. Along with reduced vacuum and potential reversion problems, running too much overlap in your race engine sends unburned fuel out of the exhaust pipes, reducing fuel efficiency. For most short track racers, this isn't a problem. But if you run into a fuel-mileage situation to cut out pit stops, it can be helpful.
When degreeing your cam, always...
When degreeing your cam, always use the duration provided for when the lifters are at 0.050 inch lift. At this much lift, the lifters have a greater velocity than the normal advertised duration (anywhere between 0.004 and 0.020 inch) which allows you to be much more accurate.
Duration is the amount of time, measured in degrees of crankshaft rotation, that the valve-either intake or exhaust-is open. Most camshaft manufacturers list both an advertised duration and duration at 0.050 inch. We'll discuss this in more detail later.
As engine rpm increases, the engine eventually reaches a point at which it has trouble effectively filling the cylinders with the air/fuel charge in the short amount of time the intake valve is open. The same thing holds true with the spent exhaust gases. The simple answer here is to increase the amount of time the valve is open, which is referred to as increasing its duration. For example, to maximize flow during the exhaust stroke, many extreme performance cam designs begin opening the exhaust valve near the midpoint of the power stroke. This may seem harmful to power production, but the idea is to have the exhaust valve fully open when the exhaust stroke begins. During the power stroke, the burning fuel has used about 80 percent of its available force on the piston by the time the crank has turned 90 degrees. The bottom half of the power stroke actually provides very little in terms of engine power, and it can be better used to help exhaust the combustion chamber so that there is more efficient cylinder filling on the intake stroke.
Here's a statement that you already know: The valve is most efficient at allowing air (either intake or exhaust) to flow past it when it is fully open. Not to insult your intelligence, but we needed to get that out of the way. What that statement tells us is that in terms of achieving maximum engine performance, the amount of time the camshaft is either raising or lowering the valve is effectively wasted. In a perfect world, the valve would be completely seated to seal the chamber, then it would fully open instantly at the appropriate time to allow maximum flow.
To get as close to this as possible, maximum race cams use extreme lobe profiles that open and close the valve ridiculously quickly. This requires stronger valvesprings and lightweight valvetrain components to maintain valve control, and engine builders and cam designers alike are still researching ways to open the valves even faster.
A more aggressive cam with high lift velocities allows you to shorten the duration in certain situations, which can help power. "Aggressive ramps allow the valve to reach maximum velocity sooner, allowing more area for a given duration," says Godbold. "Engines with significant airflow or compression restrictions [often seen in Street Stock classes or other classes with small carburetors] seem to love aggressive profiles. This is likely due to the increased signal to get more of the charge through the restriction. The decreased seat timing also results in earlier intake closing and more cylinder pressure."