Sorry to contradict Jim 'SyncroHead' Davis but the flat Porsche 6s
are NOT '180 degree Vs'. A Boxer and a 180 degree V have different
crank-shafts. By convention, a V engine has two (or more ;) ) connecting
rods on each crank throw, which is why the V was revolutionary when
first introduced- for the number of cylinders they're very short.
(Just 4 con-rod big ends more length for a V8 versus a straight 4)
 And cheap since they obviously don't have a main bearing between each
con-rod. 
	A boxer is by convention horizontally opposed AND has separate
crank throws for each conecting rod- so the cylinders 'box' ie go back
and forth 180 degrees out of phase.
	A 180 degree V can and has been built- Ferarri has done it for
race cars I believe- those lovely F156s at the beginning of the 60s? 
Unlike a boxer, a 180 V will have the two pistons move back and forth
together, sorta slushing back and forth.

	Since its Friday, I'll add that there have been (this is true!)
'Arrow' shaped engines with three banks of cylinders (almost certainly
three con-rods per throw) and H engines with two horizontally opposed
engines geared together (BRM made one for F1 and Naiper made an airplane
version in WWII) The H could be a boxer or V crank. Daimler-Benz made a 
'W' engine using two V's geared together in WWII also.

	Bill