The NASCAR Technical Institute's worksheet for determining duration. Courtesy of NTI
On the other hand, if the camshaft is retarded, the intake valve will close later (usually sometime during the compression stroke). As you might expect, this drops cranking compression and hurts low-rpm power. But as rpm increases and cylinder filling is aided by the extreme velocity of the air/fuel charge in the ports, a retarded camshaft will help power. Usually, the change-either advancing or retarding-should be less than eight degrees. If you need to go more than that you should consider using a different cam.
Duration
Duration is the amount of time the valve (either the intake or exhaust) is open. Like centerline, it is measured in terms of degrees of crankshaft rotation. This number is fixed once the crankshaft is ground and will not change if it is advanced or retarded.
There are two types of durations, typically called "advertised" and "at 0.050." The difference is a matter of measurement, specifically when you consider a valve to be open. Advertised duration is the number that the manufacturers supply. But you cannot compare advertised duration numbers because almost all manufacturers use different valve lift numbers to begin measuring duration. One manufacturer may begin measuring duration at 0.003 of valve lift while another may use 0.006. Also, because the lifter is still moving relatively slowly at such low lift numbers, it is quite hard to accurately measure.
The tactic most engine builders take is to mainly concern themselves with duration at 0.050. This means the number of crankshaft degrees the cam holds the valve open from the point that it raises 0.050-inch off the seat until it is lowered to within 0.050 from the seat. Most cam cards will provide duration numbers at 0.050, and it is easy to measure at this amount of lift.
The NASCAR Technical Institute's worksheet for determining lobe separation angle. Courtesy of NTI
Begin as you did before with a dial indicator on the lobe of the cam. This time, however, position the cam so the dial indicator is sitting on the base circle (zero lift) and zero out the dial. Rotate the engine in the normal direction of rotation until you have 0.050 lift and look at your degree wheel. Count your degrees from either TDC or BDC (whichever is closer) and make a note of it. This is your opening number.
Next, continue rotating the engine through camshaft max lift and stop when the lifter is 0.050-inch from the base circle. Again, count the degrees from either TDC or BDC to the pointer and note it. This is your closing number. Both the opening and closing numbers should never be more than 90 degrees. To find duration, add your opening number, plus your closing number, plus 180 degrees. For example, if your opening number is 38.25 and your closing number is 67, your duration for that lobe would be 285.25.
When checking duration, you should check both for the intake and exhaust. More duration in a camshaft, both for the intake and the exhaust lobes, is usually useful for racing. Just like retarding a cam, a longer duration will delay the valve closing, which can lower cranking compression but will allow for greater cylinder filling at high rpm levels.
Lobe Separation Angle
The lobe separation angle is a measure of the relationship between the intake and exhaust lobes for the same cylinder and the only measurement discussed in this article that is measured in camshaft degrees of rotation. Lobe separation angle (LSA) is found by adding the intake and exhaust centerlines and then dividing by two. Typically, a camshaft with a wider LSA (higher numerically) will have less overlap than a camshaft with a narrower angle. But don't be fooled into thinking that LSA always goes hand-in-hand with overlap. A race cam with a 112 LSA may have more overlap than a street cam with the same LSA. That is because the race cam will almost certainly have lobes with more duration, which also creates more overlap.