Engines have valves in their cylinders, the valves opening and closing for intake and exhaust strokes is timed by Cams.
Pushrods have one cam in the engine that pushes rods that go to the lifters that push the valves in.
Other technology is to have the cams in the heads -ie OverHeadCams. OHC. The difference between Single Over Head Cams (one cam per head) and Dual Over Head Cams (two cams per head) is pretty obvious. These cams therefore push down on the valves from over head.
See there in the picture. Recognize the front valve cover. In where all them springs are, right above them is the cams. There's also a great shot of one exposed right underneath the springs (drawing wise, not for real like)
I can't be bothered reading the link, so if this is repetition, sorry...
SOHC - single heavy metal camshaft with big cam lobes on it, two big heavy valves per cylinder.
DOHC - two smaller metal camshafts with smaller cam lobes on them, four smaller lighter valves per cylinder.
(1) uses less engine power to make the lighter cams spin up;
(2) uses less engine power to push the lighter valves down;
(3) requires smaller, lighter, springs to push the lighter valves back up.
DOHC revs up quicker, all rotating and reciprocating parts weigh less and can therefore move faster so engine has a higher red-line, therefore generates more power.
Any time you shave weight off of a moving piece of your engine, you can raise the redline and, therefore, increase the horsepower.
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