The numbers in the 400s are the valve lift numbers. The valve lift is usually specified as lobe lift times rocker arm ratio. Cam specs assume stock ratio rockers. The numbers in the 200s are the duration numbers. Typically specified as either advertised duration or duration at .050" of lobe lift. The duration at .050" lobe lift numbers are usually much smaller than the adv duration numbers. They are also more accurate at describing the cam's profile.
Theres a lot to cam selection than just lift and duration numbers. Theres the Intake Center Line, which is where the intake valve is fully open in relation to piston location. There is Lobe Separation Angle that determines valve overlap. There is also the intake closing timing, where the intake valve closes as the piston comes up on the compression stroke. This has a lot to do with compression bleed-off at low-midrange RPM.
All of these parameters have an effect on how an engine runs. Tighter LSAs have more overlap and thus more lope at idle. Changing Intake Center Lines shift the power up in the RPM range. Typically higher valve lift numbers are accompanied by longer duration numbers, as the cam lobe's ramp has to be lenghtened to increase lift without overaccelrating the valvetrain.
Actually a larger lift cam may appear to be smaller than a stock cam. This is because you can't make the lobes larger than the cam bearings, it wouldn't fit in the engine! So you make the base circle of the lobe smaller so the total diff between base circle and nose of lobe is larger.
The shape of the ramp is a big factor too. Comp Cam has an Xtreme Energy line that uses fast ramps with a larger LSA to provide more cylinder filling and build dynamic compression with low static-compression engines. This helps an 8.5:1 engine behave like a 10:1 engine without sacraficing as much idle quality or idle vacuum. Theres also 'single-pattern' and 'dual-pattern' cams. Single pattern cams are more common, lift and duration are the same for intake and exhaust valve. A dual pattern cam has lift and duration numbers for the exhaust that are larger than the intake. This is to help some cylinder head designs breath better, cuz some heads have small exhaust valves or restrictive exhaust port configurations. Some engine families like single patterns, some run better with a dual pattern cam. Depends on engine, head selection, manifold selection, valve size and piston shape.
For example, Pontiac engines like about 10 degrees more duration and more lift on the exhaust because the exhaust port is restrictive on some iron D port heads.
Much larger than stock cams recommend stall coverters to revv the motor into the new cam's powerband quicker. Your motor must have the compression, manifolding and RPM range to use a "hot" cam as this. Depending on a lot of factors, a cam with more than about 230 degrees of duration at .050" lift usually requires an increase of compression, valvespring changes, aftermarket manifolds and a stall converter to work really well. There is no free lunch. You've got to build compression and shift the powerband up in the RPM range to make big power.
clear as mud?
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