it's a way to allow better breathing of the crankcase without spilling oil all over or letting contaminants in.


The crankcase is under constant pressure pulses from the pistons when the engine is running.
We don't want positive pressure inside the case, it would try to push oil out (leaks are common).
In high performance engines, the rpms can get much higher than normal; and the air volume displaced by the pistons may be much bigger than stock, and then stock breathing will be inadequate. The case builds pressure, and you get oil leaks.

The breather box usually introduces an extra hole for crankcase ventilation, and a hose would run out of the case and lead into a breather box that is located somewhere higher than the case. The pressure pulses and any airflow would then move in the hose and into the box. The top of the box is open to the athmosphere to make breathing possible. There is some sort of filter media inside, a piece of foam or stainless wire "sponge" to eliminate dirt particles from getting IN. Inside the box, there are air baffles, that force the air flowing inside to "hit a wall". Think of a complicated exhaust muffler with several walls built in. The idea is, that AIR will travel freely, but any ( heavier than air ) oil mist carried by the air would hit the baffles and just run back down again and collect at the bottom of the breather box, instead of being blown out with the air. From the bottom of the box the oil would then drain back into the case via another hose.

In many cases the valve covers are modified to accept a hose fitting for breathing, and the oil return form the box back to the case goes where the original breather hose went - on the side of the oil filler neck. This is not the only way to do it but it's probably the most common.

The cheap "empi" or whatever breather box kits come with an aluminum box that is small and has no baffling inside. Just a piece of foam. It may work ok in many applications.

I should add that you earn street credibility points and about 3 horsepower per dollar spent, on any shiny aftermarket add-ons you manage to clutter your engine bay with. If it is not shiny, it wont' be as effective. Unless it's "anodized" to some "kewl" color of course


Edit: if your breather kit comes with an "EMPI" sticker, you may want to save it for your scrapbook instead of gluing it on the fan shroud. EMPI quit being "cool" in 1971.