http://www.turnology.com/news/video-the-importance-of-using-a-catch-can-system/

Its why on a hi comp desmodue I like running a filter off the breather box/catch tank. I did a simple dyno test on this years ago. Of course I accept that a high revving desmoquattro can have benefits from a race closed system, but the dyno test showed otherwise on a two valve slogger. I also like the added virtual crankcase volume of the oem breather oil seperator boxes.

Another interesting bit of knowledge,

http://www.enginelabs.com/news/detonation-what-causes-it-and-how-to-control-it-using-efi/