Choosing a camshaft should be a simple process, but people insist on making it difficult. Decide what RPM range you want the engine to have its best performance, and choose a cam, carburetion, exhaust, and gear ratio to make the engine do its best there.
Low-RPM torque- - - -short duration, high lift, with moderate sized valves and ports and a relatively small-diameter exhaust system. calibrate the distributor to get its centrifugal timing advance in a slow, steady curve that maxes out about 2200-2500 RPM.
Medium-range horsepower increase with a slight loss of low end torque- - - - -Increase valve and port size slightly, open up the exhaust a little, and choose a cam somewhere in the 250-265 degree duration range. DO NOT OVERDO IT ON CARBURETION! You'll lose midrange torque in huge quantities!
High RPM horsepower- - - -Increase compression as much as possible to offset the compression loss caused by a long-duration camshaft. Cam duration needs to be in the 270-290 degree range, depending on combustion chamber design and volumetric efficiency. Big valves and open ports would help, but their effect is wasted on a stovebolt engine. Stovebolt engine head design sucks like a Hoover upright, so a radical cam is a total waste of effort. Domed pistons and/or radical head milling helps compression pressure, but not breathing. Multiple carburetors and big exhaust pipes are also only moderately effective due to the lousy breathing capabilities of the head design. Low end torque goes away almost completely due to reversion of exhaust gas into the intake at low RPM. At this point you've got an engine that only does one thing- - - -make pretty good horsepower at full throttle- - -high RPM. It has no use whatsoever on a street-driven vehicle.
The more radical a cam grind gets, the less suitable the engine is for street operation. There's no free lunch.
Jerry
