Honda I-4 smoothness

Horizontally opposed engines like in Subaru, old VW beetles, Porsche are
very well balanced and have a lower center of gravity. I don't know why
more cars don't use these.

Many small aircraft engines are horizontally opposed and are made by
Mitsubishi, Saab, Kohler, Kawasaki, Bombardier, Fuji and others. The Subaru
ones are actually made by parent company Fuji Heavy Industries come to think
of it and are very durable in aircraft or in Subarus.

TL

 

lower c.g. may be good [but minor - and in the case of a subaru,
somewhat academic]. from what i understand however, they're not
necessarily better balanced than straight 4's - they're still subject to
second order free moments. only 6's are inherently balanced, but that's
the same for straight or flat. no real advantage plus higher cost means
they're not worth bothering with.