wrote in message
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good point, Veeduber. Wonder how much this effect/penalty is.
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In quantitative terms, I've no idea. At a guess, it appeared to be
about 25%.

The main reason for this is that the cooling air expands as it picks
up heat. The casting draft takes this into account with the
passageways being progressively larger on the 'down-wind' direction.
Reverse the direction of the air-flow, you end up trying to force the
heated air into a passageway that is steadily decreasing in cross-
section.

I went through this phase yearz & years ago, was surprised when it
didn't work as well up vs down, went back to doing it the other way.
Every few years I read about another instant expert who thinks they've
discovered the Silver Bullet, citing all sorts of benefits. Best I
can say is try it both ways... then think for yourself.

-R.S.Hoover


Indeed the breakthrough that led to the development of high power air cooled
engines for WWII aircraft was advances in cooling fin design. The ability
to cast and machine extremely fine fins combined with high octane fuel
allowed Allied aircraft to develop far more HP per pound than Axis aircraft
engines. Fins aren't trivial.

Bill Daniels