Quote:
Originally Posted by jr_ss
You have positive pressure in your crankcase, hence the factory catch cans... Pull your oil fill cap and tell me your hand gets sucked in and I'll call you a liar. Healthy engines have minimal to no air coming out, as engines age it increases over time.
Pulling a vacuum in your crankcase helps seal rings to the cyl walls and remove oily air from it. There are so many ways to argue this though.
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good info here, I would like to add:
anything that leaks past the piston rings, into the crank case, is a potential combustion byproduct that should not be allowed to become part of the engine oil, which it may dilute and render less effective. Simply providing a way out (PCV) will keep the oil cleaner, longer. That is the first major benefit of PCV.
The next benefit is piston ring seal. A long time favorite for blown V8 engines is the vacuum pump- Drawing as much vacuum as possible from the crankcase may yield between 50 and 100 horsepower on a roots-blower style 700 horsepower V8 engine. The additional horsepower speaks for itself, having a vacuum in the crankcase is a benefit to power production, regardless of the arguable reasoning for it,
It works.
So we have additional power, and cleaner oil. Now lets talk placement and functioning.
On an OEM SR20DET engine, if you notice, there is always a slight vacuum during idle coming from the T-shape valve on the valvecover. Thats because there is a direct connection from the crankcase to the intake manifold, that, during engine vacuum situations, pulls a vacuum from the crankcase.
But during a boost situation- that is, when power production is expected, that valve shuts off tight. Why? because if it remained open, the boost pressure in the intake manifold would ADD pressure to the crankcase, instead of pull a vacuum (we want a vacuum, not additional pressure). So where does PCV come from during boost situations? How can the engine have a vacuum in the crankcase during boost, if there is boost pressure in the intake manifold?
The only source inlet vacuum on the SR20 engine during a boost situation is: The pre-turbocharger inlet pipe. That is where
and why the OEM PCV valve connects to the valvecover from the turbocharger inlet- During boost, the turbocharger is sucking air from the inlet pipe, and some of that suction is applied to the crank case!
The negative of this situation, is of course, oil vapor from the valve cover may enter the turbocharger. And indeed, some of it does. The factory has a decent baffle and catchcan designed to somewhat deal with this situation, but it may be improved upon with an additional catch can of proper design, placed between the valvecover and turbocharger inlet. This is perhaps the safest design, however, some catch cans do not provide an adequate vacuum source to be applied through them, and there is very little testing and development done this way, so choose carefully and route carefully when adding a catch can between the valvecover and turbocharger inlet.
The only "safest possible way" of keeping a PCV system during boost functioning without utilizing the turbocharger as a vacuum source is: an external vacuum pump. If you have an external pump (say, belt driven) Pulling air from the crankcase- you no longer need to worry about oil getting into the turbocharger inlet and fouling your precious plumbing.
