Crankcase pressure control

[ac427]

I know the CPV will release if pressure exceeeds 1 bar in the crankcase. However, what stops a oil from being forced in the air box via the vent pipes shown below?

[sapote]

I know the CPV will release if pressure exceeeds 1 bar in the crankcase. However, what stops a oil from being forced in the air box via the vent pipes shown below?

Normal operation should have vacuum inside the crankcase, should be around 16 inches of water column.
Are you talking 1bar of positive pressure or vacuum? It's a huge number as 1bar = 400" water column as this should cause crank seals leaking.
I don't know if there is such CPV for releasing P.

The so-called vent hose/pipe #3, 4, 5 are for draining the collected oil in the air box to the dipstick, and #2 is the bypass gas returning to the intake after #1 separated the oil and drained it to the pan. 2 not supposed to have liquid oil in it.

[Shonky]

The CPV on the rear exhaust side of the head keeps oil pressure in the head itself as I understood it.

The PCV relieves positive pressure from the crankcase. Is that what you mean?

Just because there's positive pressure in the crankcase doesn't mean oil is instantly forced out via the PCV or any of the drains.

Sapote how do you end up with negative pressure in the crank case?

[ac427]

The CPV on the rear exhaust side of the head keeps oil pressure in the head itself as I understood it.

The PCV relieves positive pressure from the crankcase. Is that what you mean?

Just because there's positive pressure in the crankcase doesn't mean oil is instantly forced out via the PCV or any of the drains.

Sapote how do you end up with negative pressure in the crank case?

Thanks Shonky. I was mixing up the CPV and PCV.

I think I need a catch can.

[sapote]

Sapote how do you end up with negative pressure in the crank case?

I thought everyone knows this, that the factory setup has negative crankcase -- around 16" water column or more. The non-M is 16" water.

How does it works you asked? Inside the oil separator is a spiral path that the engine oil vapor is pulled in from the outer of the path by a vacuum at the center of the spiral upper part. Oil vapor accelerating faster as it entering the spiral, so the small droplets hit the wall and settle at the bottom which are collected and drained down out. The vacuum at the center is regulated by a diaphragm/spring and the source is from the intake after throttle valves. The spring determines the vacuum level.

I think #2 is the vacuum source from intake and it pulls the bypass gas into the intake for combustion.

[ethan]

There's also a check valve in line #5, but if the CCV/PCV is working right, it's not likely to be an issue.

[ac427]

There's also a check valve in line #5, but if the CCV/PCV is working right, it's not likely to be an issue.

Perhaps it could be a cause of oil vapour in the bottom of the air box though?

[ethan]

Not sure - mine haven't ever failed on me. Easy to replace though.

You'll get some oil vapor coming through into the box no matter what. It'll be less with a catch can, but still present. Filtered open dump off the catch can would solve it, but it stinks. You could also have a gunked up cyclone separator CCV on the valve cover.

If you have too much crankcase pressure because of some fundamental problem, you'll get oil leaks from seals being pressurized and can even blow your oil cap off. I inadequately gapped rings once and had that happen on track when I hit a critical temperature where the ring ends would just touch and cause compression loss - ugly.