Combustion pressure is kept out of the crankcase by the upper piston rings.
Of course there are ring gaps and cylinder wear so this is never 100%.
A leakdown tester measures this, so you will find a number like 5% is very good, 10% is ...ok, 15% on way out, 20% is not good at all.
In multi cylinder engines (except for oddities like the boxer or twingle) one piston is going up while another is going down so the internal crankcase volume doesn't change very much.
What the PCV valve allows out is pressure that gets past the piston rings (blow by) and oil vapor churned up by crankshaft windage.
So the PCV valve pulls from one corner and the breather in the other corner allows filtered air into the engine.
If your house was full of smoke you could put a fan blowing out one window, but if you opened a window on the other side of the house the smoke would clear much quicker.
....anyway, back to puffing.
Noticeable crankcase pressure often comes in pulses.
I've had beat engines that would actually blow smoke rings out the breather!
If there is excess pressure in the crankcase it will push oil out the seals and gaskets.
Some people have gone so far as to put a second pcv in the place of the breather.
Constant suction from both sides creates vacuum in the engine, pulling air (and dust, engine & road grime) in through the gaskets and seals.
Slurps the oil back in but also accelerates wear.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.
Locked
