This is from Michael at FP.
Quote:
If you're not pulling a vacuum on your catch can you are doing it wrong.
The stock PCV system other than the size of the lines is by far the best way to setup a crankcase ventilation system. If your motor is tight and you have very little crankcase issue more than likely a catch can does nothing for you other than removing the oil from the intake system.
If you have a some crankcase issue enough to where you see some light smoke on decel you're producing more than the stock PCV system can handle and it's pushing enough oil into the intake to make it smoke. Putting a catch can that doesn't pull a vacuum and is vented will make the smoke go away but won't really alleviate your crankcase pressure. It just gives the oil some place to go while the pressure persists, all you've effectively done is increase your crankcase pressure.
Now if you're on a freshly built motor, a motor with loose tolerances, or just a worn out motor you might have excessive crankcase pressure. This can cause all sorts of fun issues, smoking at idle/cruise, smoking under load, smoking on decel, and even pressurizing the bearing housing of the turbocharger and forcing oil out of it. In this instance to truly alleviate your crankcase issue you need to IMPROVE the stock system which means it needs to retain at least the vacuum under load. So the best way to do this is to increase the size of the lines and either run them directly to vacuum sources or run them to a baffled catch can and then run the vacuum sources directly off that.
We have done the testing and so had Garrett and Holset as little as .1PSI in a bearing housing can cause leakage. Let that sink in a for a minute just that little bit of pressure can cause this to happen. A catch can vented to atmosphere with stock sized lines is useless and if you measured pressure in your crankcase vs. the stock system you would see a rise in pressure with removing the vacuum and adding a catch can alone. So if you want to do it properly and get the best of both words, run a baffled catch can with it pulling a vacuum, no oily mess in your intake or engine bay and you still get the benefit of having your crankcase scavenged of excessive pressure.
-Michael
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