You will need some sort of breather for fresh make-up air to replace the fumes that get sucked from the crankcase. The best solution is a breather cap that has a hose leading to the clean side of the air cleaner so the make-up air is filtered prior to entering your engine. Short of that, just a breather open to atmosphere will work. Without a breather the engine may start sucking contaminated air past the crank seals etc. to make up for whats being sucked thru the PCV valve.
