a SOHC or single overhead camshaft engine uses a single camshaft on the headto lift all the valves, whereas in a DOHC or double overhead camshaft head, two camshafts are used to lift the valves (a camshaft each for intake and exhaust valves) ; normally, SOHCs are used in both16 valve or 8 valve configurations (for a four cylinder engine), but DOHCs are used in 16 valve configurations (or more) only. some exceptions are there.a DOHC layout is better than a SOHC layout in many ways- in DOHCs, the valve timing is more precise, and better valve lift is achieved. also, a DOHC setup eliminates the need for rocker arms, hence creating better timing, and valve contact is more direct too. but of course, these are all in stock form. a SOHC layout, when tuned, can be as efficient as a DOHC,but under proper tuning, DOHCs always achieve more power than SOHCs..