Flat-tappet cams work well in many applications, but they create much more friction than their roller brethren. And as shown here, flat-tappet profiles can only be so aggressive before reaching their limits. This diagram also illustrates why flat-tappet lifters can't be reused when installing a new cam, since the two mate during break-in. Roller cams create much less friction and can reach higher velocities, allowing for more aggressive profiles that can achieve higher lift levels within a given duration.
Camshafts only look simple; in reality, they're complex devices that control all four valve events (intake open, intake close, exhaust open, exhaust close), as well as how far the valve opens, how long they stay open, and how long they're both open at the same time. These factors, along with the distance between the intake and exhaust centerlines (LSA), determine how much power an engine will make and where in the rpm range it will make it.
In most cases, the benefits of running a roller cam are hard to argue with, since it can achieve higher velocities than a flat-tappet cam with the same amount of lift. This creates more area under the curve, which translates to improved power at lower valve lifts. On the other hand, a flat-tappet cam's initial acceleration is faster so it actually achieves higher lift levels more quickly than a roller cam until the latter reaches its maximum velocity. If you're running a short-duration cam, a flat-tappet lifter may actually be the better choice.
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, computer-aided cam design, however, lessens this effect by enabling designers to create more complex and efficient 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.