That being said, we'll tackle duration here and consider the other two elsewhere. In general, shorter-duration camshafts produce power lower in the rpm range and lend themselves toward a smooth idle and good part-throttle response. Longer-duration cams produce their power at a higher rpm but sacrifice lower-end grunt as the powerband is moved up. So what constitutes shorter and longer duration, at least when it comes to a traditional small-block Chevy? "The lower 200s to 220 degree range (measured at 0.050) is more street-performance oriented," says Lunati's James Humphrey. "Once you're in the 230-250 degree range, you're getting into more of a street/strip application." At this level, the powerband comes in later, after 2,500 rpm, and if you're running an automatic transmission, you'll need a converter with a higher-than-stock stall speed. Once you're running duration longer than 250 degrees at 0.050, according to Humphrey, you're dealing with a hi-po setup more suited to the track than the street.
It's also important to remember that increasing duration without changing the cam's lobe separation angle increases overlap, which also furthers high-rpm performance at the expense of low-end grunt. So what duration level is right for you? Again, it depends on what you're trying to accomplish with the combination you have. "A guy in a street car sees more benefit in torque," observes Humphrey. "That's what carries a guy." But as with most things, choosing duration is a compromise, and leaning toward the conservative side with this spec helps create a wider, user-friendly torque band.
Hint:
According to Crane, each 10-degree change in duration causes the engine's powerband to move up or down approximately 500 rpm.
LOBE SEPARATION ANGLE EFFECTS
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance
WIDE LSA
Reduced overlap
Improved top-end power
Wider Powerband
Improved idle quality
Reduced cranking comparession
Increased piston-to-valve clearance
It's one thing to talk about cam specs and throw out terms like "area under the curve," and quite another to see those plotted out on a graph. Note the difference between advertised duration and duration at 0.050. Remember, it takes two turns of the camshaft (720 degrees) to complete the four-stroke cycle for every one crankshaft turn.