Posted for "George Braly" <gwbraly@gami.com>:

 Mark,
 
 The larger plenum volume is doing something useful.
 
 Did you come to Ada?   If so you saw the wild and chaotic air flow conditions
under the cowl on the video with the tuft testing done inside the cowling.
 
From what you wrote,  I think your understanding of the physics is a bit
mis-placed.   The air does not “expand” and cool as it enters the larger
volume plenum,  it actually compresses and the pressure and temperature RISE -
- just the opposite of the process you describe.   The temperature rise is
rather negligible and is recovered as the air accelerates down through the
cylinders.
 
 The problem with the low volume plenum  design arrangement is that the
airflow in and around and down and through the cylinders becomes much less
predictable - - with the real risk of there being complete “zero flow” areas.
 That is exactly what we found last week on the RV-8 that was here with the
high tech – low volume – (nearly impossible to work on the engine) plenum
coupled to the  “hi-tech”  style cowl inlets.
 
 Regards,  George


"""
 The air in this chamber isn't doing anything useful.  The volume as the air
is passing the cylinder cooling fins may be what's important.  If the air
expands and cools above the engine but speeds up and heats up as it goes past
the cylinders, have we gained anything?
"""