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See the color-coded diagram in post 32 and its red arrows indicating flow direction when the cap is removed. I'm pretty sure the lowest chamber is connected to the main unfiltered oil chamber.
Let's define some names:
C: clean filtered oil above the upper O-ring
D: the middle section between the 2 O-rings
L: the low section below the lower O-ring

1) why the drain 11 of the D section is higher than unfiltered oil trapped in the lower L section? I means they could have the drain hole at the lowest section L, and the supply unfiltered oil enters section D, and this makes more sense for most of the oil to be drained off with the drain hole at the lowest section.
2) If the drain hole connected to the D section, then why the center rod has the hole there for what purpose?
 

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Discussion Starter · #42 · (Edited)
Let's define some names:
C: clean filtered oil above the upper O-ring
D: the middle section between the 2 O-rings
L: the low section below the lower O-ring

1) why the drain 11 of the D section is higher than unfiltered oil trapped in the lower L section? I means they could have the drain hole at the lowest section L, and the supply unfiltered oil enters section D, and this makes more sense for most of the oil to be drained off with the drain hole at the lowest section.
2) If the drain hole connected to the D section, then why the center rod has the hole there for what purpose?
1) Unless you make the cap's stem with o-rings really long (to go way to the very bottom of the OFH where it connects to the block), you will have to have a vertical wall and opening (sealed via stem o-rings with cap closed) going up some height to separate the drain path D (11) from the lower portion of the unfiltered chamber, "L". You are correct that below the height of that wall/transition, you will not completely get the oil out. I observed this when changing my oil; the unfiltered portion didn't completely drain from the OFH -- hence why I used the vacuum pump -- but the vast majority did. My guess is that they didn't want to make the stem longer than necessary or had other geometric constraints.

2) You are assuming the hole must be flow related. I don't know that it is or isn't for flow, but it might not be, e.g. plastic molding considerations, to have dimensional stability on the sealing surfaces as plastic cools coming out of the mold (though it'd be a weird position for such a feature). I honestly have no idea what it does, but regardless, its existence doesn't necessarily contradict the explanation above.

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You are assuming the hole must be flow related. I don't know that it is or isn't for flow, but it might not be, e.g. plastic molding considerations, to have dimensional stability on the sealing surfaces as plastic cools coming out of the mold (though it'd be a weird position for such a feature). I honestly have no idea what it does, but regardless, its existence doesn't necessarily contradict the explanation above.
I believe the rod hole is for oil flow and nothing to do with manufacturing process. If you have the cap outside, then just to to blow some air into the rod hole to see if it's blind or through hole.
 

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Amazing that after more than 22 years with these cars, we still don't know how the oil filter works.
1) how is the dirty oil outside the middle cylinder wall drained, assuming the drain hole 11 is connected to section D between the 2 O-rings? Do you see the drain tube (3:30 position in the pic above) has a hole on the section outside the cylindrical wall?
 

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Discussion Starter · #45 ·
I believe the rod hole is for oil flow and nothing to do with manufacturing process. If you have the cap outside, then just to to blow some air into the rod hole to see if it's blind or through hole.
If the hole goes all the way thru to the top, maybe it's so that there is no pressure differential between any air at the top of the OFH and the drain area. A high pressure differential might inhibit sealing function of the o-rings in some situations.

Amazing that after more than 22 years with these cars, we still don't know how the oil filter works.
1) how is the dirty oil outside the middle cylinder wall drained, assuming the drain hole 11 is connected to section D between the 2 O-rings? Do you see the drain tube (3:30 position in the pic above) has a hole on the section outside the cylindrical wall?
I don't think that's a hole in the drain tube. It's a post to stabilize the outer edge of the filter (notice the other two at 12:00 and 7:30).

This illustration below is what I've been describing for the past several posts in terms of how oil flows with the cap removed. This is consistent with the experience I just had changing oil and with the explanation of the patent. Once the dirty/unfiltered oil level gets to the height of the drain tube, it will no longer flow through the drain tube, hence the little bit leftover.

Blue (3) = clean
Brown = dirty
Green (11) = drain for both

Map Slope Font Parallel Diagram
 

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If the hole goes all the way thru to the top, maybe it's so that there is no pressure differential between any air at the top of the OFH and the drain area. A high pressure differential might inhibit sealing function of the o-rings in some situations.
If the hole on the rod section D goes all the way to the top, then your theory of the drain tube 11 at section D is wrong, because the pressurized dirty oil will gush down the said rod hole and down the drain tube 11 and to the oil pan.
 

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Discussion Starter · #47 ·
If the hole on the rod section D goes all the way to the top, then your theory of the drain tube 11 at section D is wrong, because the pressurized dirty oil will gush down the said rod hole and down the drain tube 11 and to the oil pan.
Let's apply basic fluid mechanics before making this assertion. If there is a hole in the rod, and if the oil level in the OFH is completely filled, then yes, a little might go down the tube. As with electric current, fluid flow prefers the path of least resistance. Look at the smallest cross section of the theoretical hole in the stem (tiny) compared to the cross section of the filtered/clean zone. Not sure "gushing" is an accurate term for the amount that'd go through the hole vs the clean outlet. You'd likely still have plenty of filtered oil.

You yourself quoted the patent earlier that the discharge duct is 11:
Font Circle Rectangle Number Magenta


Last two images of OP here shows the discharge duct 11 aka "drain" back to the crankcase being ABOVE the unfiltered oil: DIY: M54 Oil Filter Housing

Is it possible that they twist and switch places? Maybe, but I have yet to see any evidence contrary to the position indicated in the patent.
Font Bicycle part Metal Jewellery Wood
 

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Let's apply basic fluid mechanics before making this assertion. If there is a hole in the rod, and if the oil level in the OFH is completely filled, then yes, a little might go down the tube. As with electric current, fluid flow prefers the path of least resistance. Look at the smallest cross section of the theoretical hole in the stem (tiny) compared to the cross section of the filtered/clean zone. Not sure "gushing" is an accurate term for the amount that'd go through the hole vs the clean outlet. You'd likely still have plenty of filtered oil.
You misunderstood me. I was talking pressurized dirty oil during engine running, not when the cap is removing.
During engine running, pressure at drain pipe 11 is below atmosphere, and pressure of dirty oil at the top of the rod hole is around 50 psi. It should gush literally.
 

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Discussion Starter · #49 ·
You misunderstood me. I was talking pressurized dirty oil during engine running, not when the cap is removing.
During engine running, pressure at drain pipe 11 is below atmosphere, and pressure of dirty oil at the top of the rod hole is around 50 psi. It should gush literally.
I was also talking about with the cap on. OK so,
  • 50psi/3.4 bar into the OFH
  • <1 bar in the pan
  • oil gallery: probably also a little less than 1 bar?
ballpark numbers to establish cross section area:
clean oil OD: maybe around 40mm
max stem OD aka inner diameter where clean oil flows around stem: maybe around 20mm
min diameter of hole in stem: maybe a few mm, say 4mm.

Clean oil min cross section would be 942 mm^2 = pi*( (40/2)^2 - (20/2)^2) )
Hole in stem 12.6 mm^2 = pi* (4/2)^2

Yes, the velocity thru the hole would be high, but the cross section is much greater for the clean section, so therefore the volumetric flow rate, which is what matters (m^3/s) for the engine to function, would be much higher given similar pressure in pan and gallery. Additionally, if you look at the cap, there are plastic features that would further create obstacles to the 50psi OFH oil going to the small hole.
 

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given similar pressure in pan and gallery
This is the big problem of the assumptions: filtered oil in the galleries is high pressure as the bearings clearance is very small versus the pan and the drain 11 is negative pressure (suction pressure). It is wrong to think the 50 psi unfiltered oil under the cap is allowed to gush back to the pan, for no good reason. Think about a 4mm hole on the pipe of 50 psi oil, and how far the stream shooting out. Btw, the flow rate of 50 psi oil through the drain hole 11 has nothing to do with the filtered oil passing the 942mm^2 cross section, as the bearings have very small clearance, 0.001" gap or smaller.
if you look at the cap, there are plastic features that would further create obstacles to the 50psi OFH oil going to the small hole.
We know the center rod has a bit of axial clearance with the cap, and the 50 psi dirty oil is pushed the rod upper flange down so the gap between the rod top and the cap is there. The cap also has a barb-like boss with 4 radial cut (for the barbs radially expanded when inserting the rod top end), and so there is good clearance for dirty oil to flow into the rod.
 

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I just torqued the cap to 18.4 lb-ft (25 Nm) and marked the cap's position relative to the housing. Even backed it off and torqued again several times to make sure the cap landed in the same place. I then took off the cap, put the washer in the erroneous position, and torqued the cap back down again. The cap was in the same rotational position each time. I then removed the washer forever.

This likely implies that the critical dimension for cap closure at spec'd torque is either the filter height or the cap threaded section/large o-ring, and that the bottom inner edge of the filter bottoms out on the housing (near item "13" on drawing) before or independently of the stem/washer. In other words, as unbelievable as this unforced error was, it may not have caused oil to bypass the filter or affected proper OF/OFH function. Maybe I'm wrong but would be curious to know others' thoughts on this assessment.
Good observations, however where the upper/outer threads of the cap come to seat (final torque spot) bears no relevance as to if if/not the center stem and the sealing ring boss's have migrated or have been compromised in some way.

I've seen 2 cars over the years that the entire center of the oil filler cap was missing. I'd didn't change the position of the cap.
 

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Discussion Starter · #52 · (Edited)
This is the big problem of the assumptions: filtered oil in the galleries is high pressure as the bearings clearance is very small versus the pan and the drain 11 is negative pressure (suction pressure). It is wrong to think the 50 psi unfiltered oil under the cap is allowed to gush back to the pan, for no good reason. Think about a 4mm hole on the pipe of 50 psi oil, and how far the stream shooting out. Btw, the flow rate of 50 psi oil through the drain hole 11 has nothing to do with the filtered oil passing the 942mm^2 cross section, as the bearings have very small clearance, 0.001" gap or smaller.

We know the center rod has a bit of axial clearance with the cap, and the 50 psi dirty oil is pushed the rod upper flange down so the gap between the rod top and the cap is there. The cap also has a barb-like boss with 4 radial cut (for the barbs radially expanded when inserting the rod top end), and so there is good clearance for dirty oil to flow into the rod.
Definitely good points you bring up. I was thinking more that b/c the oil flows to so many places in parallel (e.g. VANOS, cams, valves, crank) there would be some pressure drop from the OFH's 50psi. Also was thinking oil flowing directly onto rotating components would create a solid-to-fluid shearing effect / "pulling" the oil out of the gallery, although I'm not 100% if that's accurate assumption / how the design works. Yes, I can visualize that a cross section of 4mm hole with 50psi would shoot out a jet of oil, but that still tells us nothing about actual mass flow rate relative to what'd be going to the gallery -- without actual pressure and velocity measurements at multiple points in the flow path and/or solving Bernoulli's + continuity equations, it's educated guessing/speculation. Regardless, I agree with you that it would be a nonsensical design move on BMW's part.

The next time I open the cap I'll take a look at the stem, see if that hole connects or not, and post back here.

Good observations, however where the upper/outer threads of the cap come to seat (final torque spot) bears no relevance as to if if/not the center stem and the sealing ring boss's have migrated or have been compromised in some way.

I've seen 2 cars over the years that the entire center of the oil filler cap was missing. I'd didn't change the position of the cap.
Thanks for the feedback and good point about that. Yeah indeed, if there was interference/bottoming out at the stem, one could still torque the cap down enough to fracture the plastic boss either on the cap or stem perhaps without realizing it. My thinking was that if originally flush, an extra 1.2mm axial compression on the stem would most certainly fracture one of the cylindrical bosses at the top, such that you'd have lose plastic rattling around in the top. Probably the safest thing like you advised would be to purchase a new one; when I do, I'll measure some cap and OFH dimensions to determine whether or not there's a gap by default.
 

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I was thinking more that b/c the oil flows to so many places in parallel (e.g. VANOS, cams, valves, crank) there would be some pressure drop from the OFH's 50psi.
The oil pressure switch or sensor is located in the filtered oil path, not the pump's output dirty oil which is much higher than 50 psi.
Yes, I can visualize that a cross section of 4mm hole with 50psi would shoot out a jet of oil, but that still tells us nothing about actual mass flow rate relative to what'd be going to the gallery
No normal engineer would design the system such that to waste the valuable pressurized oil going down the drain via the 4mm hole. so by logical reasoning, the assumption cannot hold even with measured data.
 

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The oil pressure switch or sensor is located in the filtered oil path, not the pump's output dirty oil which is much higher than 50 psi.

Huh?
 

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The oil pressure switch or sensor is located in the filtered oil path, not the pump's output dirty oil which is much higher than 50 psi.

Huh?
Oil pressure switch is located down stream of the oil filter where the pressure is around 50 psi average.
Oil output of the pump is up stream of the filter and has higher pressure than the oil at the pressure switch.
 

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If there's a disparity, there's an issue.
"Dirty oil"???
 

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Dirty is easy to type than unfiltered.
So I'm curious, what do you think the delta is before/after filter if the car is maintained and the filter is NOT clogging?
 
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