How to choose a camshaft

*Stock/near stock cam: 0.260" to 0.273” lobe lift (0.390” to 0.410” lift w/1.5 rockers); duration @ 0.050” lift around 200°  

*RV/mild performance cam: 0.300 to 0.307" (0.450” to 0.460” lift w/1.5 rockers); duration @ 0.050” lift around 212° to 222°  

*Hot street performance cam: 0.320" or so lobe lift (around 0.480” lift w/1.5 rockers); duration @ 0.050 lift around 230°  

*Racy street/strip: 0.333" or higher lobe lift (0.500”-plus lift w/1.5 rockers), duration @ 0.050” lift around 232°-up

Increasing duration will tend to shift the power and torque curves upward.  Longer durations lend themselves to higher RPM operation, because at higher RPM the amount of time the valve spends open is smaller than at lower RPM.  Keeping the valves open longer allows the cylinders to fill with more air and fuel. Since the valve may be open considerably longer than the intake stroke, it does tend to reduce power and torque at lower RPM.  At lower RPM the intake valve is open too long for maximum efficiency, so the suffers. and some of the good stuff you just sucked in there gets pushed back out because the valve is open longer than optimal for low-RPM operation. Another important factor to remember is that larger engines tend to "tame down" a cam's duration.  The same duration cam in a small displacement engine will have a higher peak RPM than if you installed it in a larger displacement engine. For example, if a cam provides a 6500 RPM peak hp in a 305 Chevy, the same cam might peak its HP at 5500 in a 400 Chevy.  Here is a comparison between two engines.  The only thing I changed about these two simulations is the duration of the camshaft.  Notice that the engine with the larger cam makes more power, but you would have to rev it 1000 rpms faster to get it.  Notice also the huge loss of torque down low.  This is an extreme example just for comparison.

Lift is a number that is best matched to your heads.  Head flows for common American head castings can be found on the internet. They are flow tested and the numbers published at different lift levels. More lift is generally better provided two things are addressed: the valves, retainers, and springs are capable of the lift you plan without binding, and the heads flow more as you lift more.  If your heads start decreasing flow above .500" lift, there is no reason for .700" lift, but in most cases more is better up to the point where the heads start losing flow.  Since the aftermarket has plenty of rocker ratios available for most engines, the lift that is ground into the cam is usually sufficient, but shopping around between cam brands and product lines might yield slightly different lifts that you can use to fine tune. One more finer point about lift that I like to mention is ramp speed.  For a given duration, more lift means the lobe ramps (the opening and closing faces on the sides of the lobe) are more aggressive.  That is to say, they have to accelerate the lifter faster to get to the peak lift in the given duration.  Faster ramp speeds usually pay off big time because they get the valve lifted higher, faster.  The sooner you can get the heads flowing their peak air, the more air can get sucked in the cylinder. The downside for flat-tappet cams is that the steeper ramps mean they contact the lifter at a stronger angle.  The potential for wiping out a cam lobe or lifter is greater, but manufacturers know that and design ramps to be as fast as they can be without destroying components.