Since this question(s) has/have been asked a number of times in many places, I thought that I'd put together a post to discuss the things that I've learned from reviewing the threads I've seen over the past couple of years. This will hopefully condense the info and save others - especially newbies - from having to reinvent the wheel and do all the research themselves...
First, lets identify the two 'breather' pipe locations where an OCC could be installed:
) The braided hose that attaches to the *mid-point* of the valve cover and terminates on the top of the cold air intake at the, easily broken, plastic nipple of a stock LNF air intake.
) The braided hose that attaches to the *front* of the valve cover and terminates at the metal bracket, down directly in front of the turbo.
Now, the LNF engine is designed by GM not to require an external OCC. So let's first discuss why not... Hose (A
) is there to allow cold air INTO the top of the engine. There is a one-way plastic check valve at the end of the hose on the CAI nipple that only allows air into the valve cover and blocks air and oil vapor from getting out
of the engine into the air intake. Thus, with a working valve, there's no need for an OCC as there won't be
any oily air flow.
On the other hand, hose (B
) is there to allow/pull air-flow OUT of the top of the engine. In this case however, to avoid oil vapor getting ingested into the turbo and burned in the cylinders, the valve cover contains a set of internal baffles designed to condense out any oil vapor in the air and thus keep it in the top of the engine. This effectively forms an internal OCC and thus should not require an external OCC.
OKAY, so far so good. So then why would
anyone install an OCC? The argument for putting an OCC in path (A
) is that the the one-way check valve could malfunction and stick open, thus allowing oil into the cylinders. Thus an OCC in this position acts as insurance against that happening. [Note: Some folks have put an OCC in here and then also removed the check valve. DDM Dave has written that removing the valve can lead to weird noises (bad breathing?) from the engine and is not recommended. Apologies, Dave, if I've misquoted ] UPDATE
: Per GSStage1, old gummy oil can accumulate in the check valve and stick it open. Probably a good idea to clean the valve out occasionally with MAF cleaner, or similar, to protect against this.
The argument for putting an OCC in path (B
) is that maybe the internal baffling does not do a good enough job and so an OCC would be a backup to capture any remaining oil vapor coming through that path. By the way, IIRC, path (B) is where DDM recommends installing their catch can.
Couple of closing points... oil vapor in the cylinder is BAD. It causes premature/uncontrolled detonation (knock) which can destroy your pistons/rings/cylinders. By all means add an OCC (or two) if you're scared of this. Secondly, any OCC you add will NOT protect against two other possible paths for oil getting into the cylinders: past worn piston rings; and more importantly, via worn turbo bearings.