When I flight tested my old system, I found that the open filter was indeed allowing a loss of ram pressure due to back-flow. After repeated flights, I got about .75" mp boost at 7500 ft. With the filter system replaced with a straight piece of tubing, I got 1.2" mp boost.

Back to the drawing board. I built this filter can with one-way valves built into the outside of it. This is similar to what Rod Bower did last year, but more compact to fit in my James cowl.

You can see in the picture below that the fit is very tight in the SJ cowl...(no prop extension).

Here are some pictures showing how I built the flaps in the can...

Flight testing shows a full ram mp boost of 1.2". However, the slots are not enough for full power, and loose about 2" of mp with the ram air inlet closed. This is good enough for me for taxi in dusty conditions, and even takeoff if the runway is long enough and the conditions are dusty.

If a guy had more room in his cowl, he could cut more than three slots, and get most of the mp back.