mpenner49
+1y
That smaller "air cleaner" is for emissions sorta... The hole is for a PCV valve that safly releases gases from your bottom end. When combustion happens a small amount of the vapor goes by your rings. This vapor is full of unburned fuel mixture which contains very high levels of Hydro Carbons (HC). Well without that PCV or in this case small filter, the pressure could build up and make your oil pan leak from pressure build-up or mix with oil and leave milky residue on your oil fill cap. The PCV on a factory system emits this HC vapor back into intake which then gets reburnt. But with this small filter it just emits the HC into the atmosphere.