http://bordzbiz.weebly.com/overhead-camshaft-ohc-sohc--dohc.htmlhttp://www.suzukisv1000.info/faq/tlrman/valve_trains.htmthis image shows a DAOHC a SOHC and and pushrod. as you can see both DAOHC and pushrod valves are in the center of the cylinder where as SOHC are angled.

the sohc and the pushrod are both using rocker arms to activate the valves. where as the DAOHC is using the same activation method as a DOHC set up, at least in game.
as far as the game is concerned imagine a DAOHC as half of a DOHC setup there are a lot of different real world setups but lets just stick to in game.
Pushrod is the cheapest, and lowest revving, it's advantage is it's lightweight, minimum space requirement, and durability. it's drawbacks are limited valve space therefore limited power, and it's inherent valvetrain weight limits it's rev range.
SOHC is a setup allowing you to run 2, 3, or 4 valves per cylinder, however the more valves you add the more stress you put on the cam and you lose reliability. It's cheaper and lighter than a dohc but is not as powerful, efficient or durable, nor can it rev as high on larger bore engines due again to larger valves stressing the camshaft.
DAOHC is a cheaper alternative for high revving engines, with less moving parts then any other setup its the most reliable to rev hard, however it's valve placement limits valve size and therefore it is not as powerful as a SOHC, nor as efficient, but it's lighter, cheaper, and more durable then anything aside from a pushrod.
DOHC is the most expensive, heaviest, most efficient, and highest revving of the lot, with options for 2,4, or 5 valves. it is made from two Direct acting camshafts set above valves set at the same angles as in the SOHC, it has all the benefits of the single DAOHC with the added bonus of the SOHC's valve placement which allows both bigger valves than the DAOHC as well as more valves .
For a better understanding select each setup in game, use the right hand vertical slider bar to expose the valves, and zoom in while it's running to get a better idea of how it all works.