As siaokao mentioned, the OHC design is pretty much standard
practice these days, as opposed to the older OHV (overhead valve)
engines in which the camshaft drives a set of pushrods which in
turn operate the valves.
The advantage of the OHC over the OHV is that, because of the
longer train of components driving the valves, flexion of the metal
causes OHV engines to have less precise operation of the valves at
higher revs.
As an extension of that, I would expect that the DOHC may have a
slight advantage over the SOHC because with each camshaft being
simpler, it may twist less when the torsion increases at high revs.
Perhaps Heng could confirm if my suspicions are correct.
As for compression, the only inherently weaker design was the
L-head, which was completely phased out in the 1950's by the OHV
design.
