Originally posted by ebbfolls
Both DOHC and SOHC should be the same, all things being equal (same # of valves, same valve actuation, displacement, etc.).

Theoretically, DOHC's should have an inertial disadvantage by reason of the greater moment of inertia resulting from 2 instead of one camshaft/gear. But the difference is very slight because the crankshaft has a 2:1 mechanical advantage over the camshaft. However, since combustion is so much better with a centrally located spark plug (consider Dodge/Chrysler's "Hemi" design), a DOHC will usually be used for engines designed more for Horsepower than Torque. Another reason is with DOHC, there no longer is a need for tappets as the valves are directly acted upon by the cams. Less moving parts, less inertia and inertial loads, the higher the rpm capacity of the engine, the greater the power output. Thus, more powerful engines are usually found with DOHC rather than SOHC, but that's not to say a DOHC setup is more superior to the SOHC design.

There's more to an engine's power characteristics than the valve train, though that may be a crucial part.
good analysis.

in practical terms, DOHC is superior to SOHC because it's easier to design and package a 4 or 5 valve-per-cylinder setup with an intake and an exhaust cam than a single cam doing double duty. this is why people tend to think DOHC > SOHC...because most DOHC engines are 4v/cylinder and SOHC are 2v/cylinder. the advantage really comes from 4v/cyl > 2v/cyl, not the camshaft design.

it's also more practical to have a DOHC engine if you want to do variable valve timing (VVT-i, VTEC, Vanos) because you can dynamically and independently adjust intake and exhaust timing with two cams. so chalk that up as another advantage of DOHC.