Marv, previous mailing ended up with a space in the URL

Angier,
I used a K&N filter (33-2006) in my air box and also used the K&N equation, quoted by a few others here (A=CID x RPM/20839), to get an approximate size.   A few additional items to consider in their estimate are:   Their recommendation is probably based on a filter with some level of contamination (from normal automotive road use) in sea level atmospheric conditions.  I would guess that, in general, aircraft filters are not exposed to the same level of dust and dirt and are much easier to keep on the clean end of the spectrum.  (I clean and/or replace mine every year.)  Also, as we go up in altitude, the reduction in air density will reduce the pressure drop across the filter.  Finally, we generally don't do all of our flying at full rpm, further reducing the pressure drop across the filter.
While I could only fit 80% of their recommended size, performance is excellent  -even for the few seconds I spend at seal level.  
http://www.n91cz.com/airbox/bot-with-filter.jpg

good luck,
Chris Zavatson
N91CZ
360 std
www.N91CZ.com