The idea is to reduce the velocity of the vapour in the vent line by letting it breathe into a larger area, ie the catch can. If the low velocity vapour can then pass through a coalescing filter mesh the microscopic oil particles will gather on the mesh as the vapour passes slowly through it. Eventually they will find each other and start to form heavier droplets that will seep under their own weight against the low velocity vapour flow and drop into the catch tank while the less oily vapours exit the catch tank vent for re breathing if required. Thats the basic theory I think. I'm sure I could build one if I had the time and the space in my engine bay. Until that time arrives I'll stick with my twin 40mm bore X 50mm long mesh breathers that dont let anything drip out
I'm sure there must be a formula to calculate the area's of the relevant pipes and vents based on engine size and blow by? But as I dont know this I just fitted two of the biggest vents I could and it seems to have paid off.
But then my car is race only and my main concern is not having oil seeping out onto my engine
If I were to make a catch tank system for my car. The first thing I would be designing is a system where the vent pipes all rise gently to the point where they enter the catch tank. This would aid natural oil drain back to the engine. A lot of the systems I've seen on cars have awful dips and low bends that just act as oil traps that fill with oil. This means the discharge vapour has to force it way past an obstruction which never clears??