Sport Pilot wrote:
Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions


True, but this doesn introduce a "rocking couple" vibration mode.
There's no free lunch WRT to engine design. :-)

Although some configurations come close (I-6, V-8, etc.)


Matt