two outa three ain't bad...
You've got most of it figured out already. OHV stands for OverHead Valve. This is the standard configuration for the vast majority of automotive engines manufactured for since the 1940's or 50's.
An OHV engine has a single camshaft situated in the block a few inches above the crankshaft, and is generally driven by a timing chain. The camshaft actuates lifters, which drive the pushrods, which lever the rocker arms, which open and close the valves.
In a SOHC engine, this single camshaft is replaced by a camshaft located on the top of each cylinder head. This camshaft actuates the valves, I believe by way of rocker arms. (I've never actually peeked inside a SOHC valve cover...) This arrangement reduces the valve-train "mass" by eliminating the pushrods and lifters, and probably allows for more agressive camshaft profiles. Valve-train "lash" is also reduced by this arrangement.
In a DOHC, the single camshaft is replaced by two camshafts on top of each head - one for the exhaust valves, and one for the intakes. Some of these engines eliminate the rocker arms, further reducing valve-train mass and lash, with the camshaft directly actuating the valves. Other engines retain rocker arms, but they're small. The performance gains coming in part by reduced torsional stresses on the camshafts and even more aggressive profile possibilities.
On both the SOHC and DOHC engines, the camshafts are generally driven by a long serpentine timing belt. (And Heaven help you if it fails...) I'm not an engineer, so I can't comment on the trade-offs with authority, but it's clearly evident that the additional frictional losses (due to multiple camshaft bearing surfaces) and the belt parasitic loss of the multiple cams is more than made up for in power gains of the OHC configurations. What I do know for sure is that the DOHC in my 3000GT's little 3.0L pumps out 222HP. That's comparable to the 5.0L OHV V8s, at only 60% of the size. (And the twin turbo version runs 300-400HP! Woohooo!)
As for converting your OHV to SOHC, I'd advise against it. Maximum work for moderate gains. I'm not sure, but I think that even the blocks are different between the OHC and OHV engines, so you'd be looking at a complete engine swap (plus computer swaps, wiring harness, etc.) Plus from what I've read here on the board, there are some reliability issues with the SOHC. If I were intent on maximizing HP, I'd do the 5.0 swap and go from there...
Just my 50 cents worth. Others may have additional insight and opinions...
Michael
