Another solution.

The back pressure through the filter and out the alternate air box can be
stopped with a flapper hinge that closes by gravity.  It is hinged from the
top and is sealed closed by the ram air; but opens when you open the
alternate air valve, engine vacume sucks throught the filter and opens the
flapper.  The flapper is normally closed by gravity and is sealed closed
with ram air pressure. I used aluminum hinge stock, attached an aluminum
plate with rubber gap seal material rivited to the edge.  The mated surface
was created by using a micro flox mixture and the seal works well.  There is
leakage through the hinge line but if you make two small hinges instead of a
hinge the full length of the flapper you can significantly reduce that
leakage.  I show a 1.4 inch differential between ram and alternate air.  Oh
yes, my intake is right under the prop spinner and the opening is cut-a-way
on the right side and the left side extends under the prop spinner.  This
allows for some prop turbulance and additional boost from the propeller.
According to Don Rivera this is helpful.  Works great.

Bob Smiley
N94RJ