It has been said:
<<Water cooling an aircraft engine makes about as much sense as air cooling
a submarine.>>

And there is a germ of truth, but then why use liquid-cooling in cars?  The
analogy doesn't, shall we say, hold much water.

The liquid vs. air cooling is a fairly close call.  Air cooling requires
less coolant weight (about 20 pounds less), but it requires that the engine
be much more bulky and consequently heavier than an equivalent liquid-cooled
engine.  Note that I said "equivalent" as even the really good automotive
designs haven't been done with the emphasis on weight reduction that would
be appropriate for an aircraft application.  Liquid cooling allows a V8 to
be built with 4 crank throws and a main bearing between each.  The
conventional aircraft engine has 6 throws and only 4 main bearings.  Yes,
the piston temperatures are much more consistent, allowing reduced
clearances.  That doesn't really do anything for efficiency, but does reduce
oil consumption and increases life.  The reduced combustion chamber surface
temperature allows for higher compression ratios.  If there really is such a
thing as shock cooling, the liquid cooled engine doesn't suffer from it,
allowing rapid descents with a closed throttle, eliminating the need for
speed brakes.  The bottom line, I think is that liquid cooling makes for a
better engine design, but requires another fluid to be carried on board.
Which is more important?

My recollection is that the Navy, even though they were "closer to the
water" than the Army Air Force, was worried about the cooling of engines on
the deck as the aircraft couldn't always be turned into the wind.  The Air
Force operations on the ground were more controlled and therefore could take
into account the potential overheating of the engines.  The air cooled
engines still overheated, but the effect was not so catastrophic.

After all, we put oil in the engines even when we are a long way away from
an oil well.  Same thing for the oil in the brakes.  Cars have a lot of
liquid systems.  Oh, and turbine engines DO suffer from the increased
clearances caused by large temperature variations from air cooling - that's
what the tip clearances on the turbine blades are for.

The liquid-cooled V8 is a very compact and has a very rigid structure,
including the crankshaft.  I think for a high-speed aircraft it has a slight
advantage.  Is it worth the extra build time?  Only the owner can answer
that.

Gary Casey