Hello;  When I was 18 I had a Triumph (motorcycle) TR6 for my main
transportation.  The engine was a real vibrator due to engine design, both
pistons rose and fell together.  I went 60,000 miles on that shaker, a lot
of them while thinking how smooth a horizontally opposed engine would be.
I finally bummed a ride on a BMW which had an opposed twin engine.  The
engine was very smooth and shakeless, but the ride was dog slow and not at
all like the TR6 I has used to. I had other twins, some triples, a couple
of V twins, all shook, some so much that you couldn't hold on tight enough
at speed and so had to go slow.

Got married and bought some cars, in line engines, V engines, all rear
drive, all of them had some engine vibration intruding at some point of
their rev range.  Bought a new beetle (68) this had (has) a horizontally
opposed 4 cylinder rear driver.  The engine was  silky smooth.  The
designer of that engine must have had engine vibration and balance in mind.
 The opposing pistons move in and out in lock step, canceling each others
motions, the other move in when the first ones are moving out, beautiful.

I know modern engines are rubber mounted to reduce vibration, and it takes
a sensitive butt to feel these vibrations in modern cars.  So to most
people engine configuration may be a  moot point, but there are those out
there who notice things like that.

I'm into aviation lately, and have noticed that airplane engines (piston
ones) are horizontally opposed.  They are very smooth and reliable.  Part
of the charm of my 87GL is the opposed engine.  My choice, for a swap,
would be another opposed engine.               Larry Carter