Here's some information applicable to the BA-4106 Brackett foam air filter assembly you might be running, including the 14 CFR Part 91.403(d) manufacturer's permission letter, foam air filter manual (with I-194 document) and applicability chart with STC numbers:
http://www.brackettaerofilters.com/documents/far403dpermissionletter.pdf
http://www.brackettaerofilters.com/documents/homebld.pdf
http://www.brackettaerofilters.com/documents/arl.pdf
I found the description of the properties of foam air filters to be particularly interesting, especially the part about how they become more efficient as air flow increases, rather than less efficient like paper filters.
http://www.brackettaerofilters.com/documents/far403dpermissionletter.pdf
http://www.brackettaerofilters.com/documents/homebld.pdf
http://www.brackettaerofilters.com/documents/arl.pdf
I found the description of the properties of foam air filters to be particularly interesting, especially the part about how they become more efficient as air flow increases, rather than less efficient like paper filters.
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