Don, I ran high horsepower door slammer cars for a few decades with the system you described. It works like, and does exactly what the down draft tube on our truck engines does. It uses a type of Venturi effect to draw out crankcase vapor/pressure. In the case of our trucks road speed creates the pressure differential for it to work and in race cars it is the speed of the exhaust gas doing it.
In the race cars you have an oil separator/breather up on the valve cover that functions like the the one in the down draft tube. The race system used one way valves down on the header with a small angled pipe welded into the collector. It is angle cut on the ends just like our breather tubes for the same reasons. Race cars, at least those without zoomie headers, need to be in great mechanical shape for obvious reasons. They just want pretty clean air coming out this system. Last thing you want is any oil mist coming out the exhaust and hitting the slicks.
Chuck
