Oil flow and Vehicle start with oil cap off.

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I wouldn't think so... When one piston is going down (causing positive pressure) one is going up (negative pressure) so I don't think there is a net change in airflow in the engine. [At least when there is an even number of pistons, err at least in most conventional automotive engines.]
Likewise, when the engine lifts oil up and fills the galleries, the oil level drops in the pan.

Ultimately there is a ventilation system for the crankcase. This is to deal with blowby that is created while running.

Lastly, whenever I have removed the filler cap while running I've always had some amount of oil spray coming out and all over. Not a geyser mind you just enough to realize it's making a mess. Maybe that was just on the mills I've dealt with, maybe a BMW has better baffles.

In this case I don't think it will matter at all if you are just priming it, but that's my wild guess of the morning. Will be curious what the BMW mechanics have to say.
 
Missed the OP...but read the second...

The swept volume of an automotive cylinder is not a pure sinusoidal volume change.

For exactly the same reasons that you can't balance a 4 cylinder, there is a significant change in crankcase volume every 90 degrees, and a change in pressure...leave the oil cap off a running 4 cylinder and have a look...

And there's not enough to change any sort of priming mechanism post oil change.
 
Originally Posted by supton
I wouldn't think so... When one piston is going down (causing positive pressure) one is going up (negative pressure) so I don't think there is a net change in airflow in the engine. [At least when there is an even number of pistons, err at least in most conventional automotive engines.]
Likewise, when the engine lifts oil up and fills the galleries, the oil level drops in the pan.

Ultimately there is a ventilation system for the crankcase. This is to deal with blowby that is created while running.

Lastly, whenever I have removed the filler cap while running I've always had some amount of oil spray coming out and all over. Not a geyser mind you just enough to realize it's making a mess. Maybe that was just on the mills I've dealt with, maybe a BMW has better baffles.

In this case I don't think it will matter at all if you are just priming it, but that's my wild guess of the morning. Will be curious what the BMW mechanics have to say.



Originally Posted by Shannow
Missed the OP...but read the second...

The swept volume of an automotive cylinder is not a pure sinusoidal volume change.

For exactly the same reasons that you can't balance a 4 cylinder, there is a significant change in crankcase volume every 90 degrees, and a change in pressure...leave the oil cap off a running 4 cylinder and have a look...

And there's not enough to change any sort of priming mechanism post oil change.






Thanks for the replies. I deleted it because I figured it wasn't thread worthy and somewhat of a stupid question.

Shannow: Here is the SIB I had originally mentioned.

SIB
 
Originally Posted by Shannow
Missed the OP...but read the second...

The swept volume of an automotive cylinder is not a pure sinusoidal volume change.

For exactly the same reasons that you can't balance a 4 cylinder, there is a significant change in crankcase volume every 90 degrees, and a change in pressure...leave the oil cap off a running 4 cylinder and have a look...

I stand corrected then.

I have pulled the cap off my 4 bangers and figured that airflow was just air into the engine... hmm.
 
Originally Posted by talest
You should feel air sucking IN and not pushing OUT is this correct

It can be both, the pistons displace air in the crankcase as well as the cylinder.
 
Don't forget the effect of blow-by, which positively pressurises the crankcase and is the reason a PCV is needed. If you run an engine with the cap off, the crankcase won't pressurise and you will get a flow of blow-by gases out of the oil filler.
 
I did this lovely little feat the other week by accident
lol.gif



I was impressed how quiet my motor was running with the cap off.
 
Not a stupid question at all...

There's zero doubt in my mind that you are very smart.

We all have areas where we don't know everything about everything... And asking questions is how we learn new things.
 
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