Boxer engines of up to six cylinders have proved highly
successful in both automobiles and motorcycles, and continue to be
popular for light aircraft engines.
One benefit of using a boxer engine versus a V engine is that the
design provides good balance because each piston's momentum is
exactly counterbalanced by the corresponding piston movement of the
opposite side. These engines can run very smoothly and free of
vibrations with a four-stroke cycle, regardless of number of
cylinders, and do not require a balance shaft or counterweights on
the crankshaft to balance the weight of the reciprocating parts
which are required in other engine configurations. But boxer
engines tend to produce more noise than inline and V-engines
because valve clatter is not so well dampened due to lack of
covering by air-filters and other components, and produce a larger
torsional vibration than a V engine, and so tend to require a
larger flywheel.
http://en.wikipedia.org/wiki/Boxer_engine
