[MagicRat]

Overhead cams give 3 important advantages over pushrod engines.

1. It is an effective way to get the intake and exhaust valves facing each other at a canted angle. This allows for a hemispherical combustion chamber, which is more thermdynamically efficient. It allows for a more efficient burning of the fuel/air mixture, to increase power and fuel economy and reduce emissions.

2. This valve arangement produces greater volumetic efficiencies, allowing more air in and out of the combustion chamber for more power and fuel efficiency.

3. Overhead cam designs allow for less valvetrain mass, allowing for a higher-revving engine and the potential for more power. The engine can rev higher, with softer valvesprings, before valve float occurs.

You should note that some engine designs allow for some, but not all of these advantages. Some engines, like the Chrysler Hemi use pushrods, but get a hemispherical chamber shape. But they cannot rev as high as many OHC engines.

Also, some OHC engines , like the older Chrysler 2.2 ad 2.5 liter engines do not use the opposed-valve arrangement. They use valves lined up in a row and a resulting "wedge" shape chamber, like most pushrod engines, This allows for a higher rev potential, but does not allow the efficient hemispherical shape.

As noted above, all things being equal, pushrod engines are cheaper to manufacture, and are more compact.

Finally, an OHC engine allows for the potential of these advantages. But all components must have a coordinated, optimal design to gain them. It is quite easy to have a real dog of an OHC engine (the older Ford 2.3 4 cyl engine comes to mind) but some pushrod, wedge engines can be awesome, such as many recent Corvette engines.