The crankcase in any engine creates a positive pressure internally. Most seals, gaskets, and such aren’t designed for a pressurized system. That pressure needs to be vented. Back in our dads day, that’s pressure was vented out of the crankcase via a breather cap of some sorts straight to atmosphere. Along comes the late ‘70’s and the EPA and decides that’s not so good for the environment. So, that pressure is vented through a PCV valve (positive crankcase ventilation) back into the intake track somewhere, into the combustion chaimbsr, and out the exhaust pipe. Cleaner for the environment than venting straight out to atmosphere. Since it’s plummed into the intake, it also has the added benefit of creating a slight vacuum to draw that pressure out of the crankcase. This is a good thing for seals and rings.
The bad part is, along with the venting and the vacuum, that air being drawn from the crankcase contains quite a bit of oil mist suspended in the air. The oil ends up in the intake, coating everything in a layer of oil. Especially bad where and when it gets baked to the top of intake valves. The oil also wreaks total havoc on the octane rating of your fuel since it ends up in the combustion chamber.
These catch cans have a metal plate, screen, or some other clever design that unsuspends the oil from the air traveling through. Designs vary, but the basic function is the same. The component that removes the oil, collects it in this “catch can”. The oil content falls out of suspension from the traveling air and clean air continues on into the intake after passing through the can.
It probably doesn’t remove 100% of the oil, but probably darn near close. Enough to make them a valuable asset, and a mandatory first mod IMO.
