Apologies if this has already been covered before but I couldn't find anything after a quick search. Cos I don't read Japanese I thought I'd better confirm with others who have already installed this filter.
The dense hard layer faces the intake side (unfiltered air) and the less dense soft sponge is at the rear towards the MAF sensor?
This pic seems to indicate that the above assumption is correct but it just seems so counter intuitive to me
Taken from http://www.hks-power.co.jp/db/product_chart.aspx?MODE=ENU&LN=EN&N_ID=449
Would've been so much easier if they had coloured the sides differently
Cheers