Chevy Camshafts - Got Lobes?

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 partthrottle 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-rpmperformance at the expense of low-endgrunt. 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.

Lift
In general, increasing valve lift allows more air/fuel mixture to enter the cylinders and more exhaust to exit, which leads to an increase in engine performance-but only to a certain extent. The first thing to consider is the cylinder heads you're using. "You want to try and utilize your heads as much as possible," says Humphrey. Accordingly, you should always try to get your hand on flow figures for your lungs of choice when selecting a camshaft. On the other hand, there's no point in adding lift when it can't be used. This is especially true considering that an increase in lift almost always leads to at least a slight increase in duration, since the cam needs longer opening and closing ramps to accommodate the extra lift. This increased duration is not always good, since it can alter the engine's powerband. Modern, computeraided cam design, however, lessens this effect by enabling designers to create more complex and effi cient lobe designs that aim for lots of high-lift duration without low-lift duration... In camspeak, it's called creating more area under the curve, as shown when the valve lift is plotted on a graph. "The more quickly the valve opens and dwells in that position," says Godbold, "the greater the space from opening to closing, or beneath the curve, which makes for better performance at all rpm."

Even so, as the numbers climb, so do the cam maker's engine recommendations concerning compression ratio, converter stall, and intended usage. Comp's Xtreme Energy line, for instance, starts out at 200/206 degrees duration at 0.050 (intake/exhaust) and 0.472/ 0.480 inch lift, a combo that's "good for mileage and for towing" and has a "smooth idle." Just four cams away, its cousin that runs 224/230 degrees duration at 0.050 and 0.502/0.510 lift is for "high-performance street machines" and calls for a 2,000-plus stall converter plus deeper rear gears and warns of a choppy idle. It doesn't take all that much to significantly alter an engine's characteristics, so again, keep your combination and goals in mind when making a pick.