Removing heat from the exhaust flow increases back-pressure, slows the
   flow, so has less exhaust velocity, less thrust. Most designers go to a
   lot of work, upflow or downflow, to expand the air in a
   smooth, well-designed plenum, then just let the air try to find its way
   out of the cowl past wires and tubes and sharp edges. tsk! tsk! Tom
   Aberle's Reno Gold-Champion Biplane racer has upflow, but he exits the
   cooling flow from the cowling immediately above each cylinder in
   rearward-facing outlets and the same for the oil cooler. Good design,
   lots of speed. Klaus gets rid of his air efficiently, too! Ducted inlet
   with ducted outlet can't be beat for cooling efficiency and hence less
   drag! Tightly wrapping the cylinders with fiberglass or carbonfiber to
   keep the cooling air in intimate contact with the fins, preventing any
   air from escaping, helps loads too! Rolled edges on the lower baffles
   where the air leaves the cylinders keeps the flow from separating there
   alloing it to leave smoothly. Up or down, no difference with good
   design!

   Paul Lipps

Thanks for the comments ideas and analysis, Paul.

One more question..  Though more total heat may be transfered to the
airstream by downdraft cooling the bottom exhaust HO engine, I wonder if
there are longevity and horsepower benefits from blowing the coldest air
on the hot side of the cylinder.  The side of the cylinder with the
exhaust port seems like it would run hotter.  Flowing the cold air on
the cold side first seems like it would increase this temperature
differential.  I know that O-200 powered airplanes often have the
baffling modified to expose the exhaust port region of the end cylinder
to more cooling flow.


Regards,

Matt-



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