NefMoto

Hi,

had a conversation with a friend lately.
The crankcase ventilation on my 1.8t build is going to the outside into a catch can.
It is not connected to my intake anymore. This is what you see very often on the internet.

His argument was the following:
Lets say there are 10% of boost going by the pistons as so called blow by gas.
Theses 10% were measured and would recirculate into the intake "post" maf on a stock engine.
I miss these 10% air mass and my engine should run too rich.

My trims are unter 5% so i was under the impression every thing is fine.

 

He is correct to an extent. Un-metered air is indeed bad. Whether it be from the PCV or a leak.

Commenting to subscribe, I'm curious.

I had my PCV venting to atmos, but got bored of having a constant oily smell in the cabin, so it's not vented into the inlet again (via a catch can).

Will wait 5/10k and pull a hose to see the colour.

Is it really a fact that noticeable amount of air will leak past piston rings during intake and compression stroke on a healthy engine? Blow-by is usually understood to be exhaust gasses. After ignition, what ever oxygen is left in the exhaust gasses, does not account much for anything (other than for oxygen sensor) and can be ignored as a "measured air". I think it's nonsense to talk about metered air in crank ventilation but if you know better, I would love to know.

I want to hear a discussion about this, rather than argue, but why do you think this?

Surely blowby will occur all the time - so during compression the air forced in (especially during FI) will be pushed past the rings into the crankcase.

I understand that exhaust gases will be under much more pressure, but a common test for blow-by is to pressurise the inlet and watch for excessive crankcase venting - hence the blow-by..

Just a hunch really :-) It makes sense to me that pressure of a magnitude greater would account most of the leak. Of course, I understand that piston ring design can affect this and actually seal better under power stroke. I also would like to hear facts about this. There are issues here where common sense can fail when comparing high pressure peak during the ignition and longer lasting lower pressure during charge/exhaust.

I was always under the impression the crankcase liked a small amount of vac on it. On all the big turbo stuff we've done, we almost always set it up like the factory has it.

You should really see a graph from an in-cylinder pressure sensor.
Then you would understand how nonsensical this claim is.

If your engine has blowby on the compression stroke, it's dead, finito, done for.

On the power stroke you will have more than 10 times the pressure and that is where the blowby occurs. So it does not matter at all if you vent to atmosphere or not and your afr will not be affected.

Guess that's this thread done then.

/endthread.