You don't need conventional icing to block up your air filter.  

It will block very nicely with a big batch of cirrus cloud ice crystals if
you fly in them long enough.  

You will see no icing on the airframe, but will get a big buildup of "ice
dust" on the air filter.  Then your manifold pressure starts to sag
inexplicably.    

It begins to happen about the time that the ice crystals put enough
precipitation static on your airframe to make the radios check out one by
one.  Everything slowly turns to crap all at once.  It tends to bend your
mind as it occurs in slow motion.

Been there, done that.  Get an alternate door that will swallow ice
crystals.

Fred Moreno