Although I was joking when I said "Blockage = Crap / Area" ... There is truth in this. If the surface AREA of the filter is too small (ie ie same or similar Area to the cross section area of the air flow, then air flow will be restricted. This may cause problems: A) Lower amount of cooling air reaching motor and PEM !! and B) Higher current draw from the fan motors - which is bad for the connectors into the PEM, which are known to have marginal current capacity.