Internal combustion engines use a series of valves (like inverted mushrooms) to
inject air and fuel into the cylinders, then expel the burnt gases.
These valves are opened and closed by camshafts that were once located in the
side of the engine but are almost universally located now at the top of the
engine (overhead camshaft, or OHC).
The most efficient way of operating the valves for both economy and performance
is with two camshafts – one for the inlet valves and the second for the exhaust
valves – hence the term twin-cam (or DOHC for double overhead camshaft).
Of course, a V-shaped engine (V6, V8, V10 or V12) has two separate rows of
cylinders and therefore two sets of camshafts, hence the term quad-cam.