Here's a diagram of the airflow path. Air is coming in through the two openings at the front and through the hole at the top of the doghouse for the rotormast.
The Pall particle separator and FDC IBF work similarly by filtering the air going to the inlets at the front and sealing up the rotormast. In addition FDC has the bypass doors at the top. Both systems require due diligence on making sure all the new seals on the cowling are properly installed for the systems to be effective.
Airbus moved the inlets outboard when they designed their own IBF thereby eliminating the competition from installing the particle separator or IBF on the newer aircrafts. The IBF kit is about 400 man hours to install, which is why the P3/T3 are built that way from the factory. The filters are mounted outboard, while the bypass doors mounted inboard (original air path).