Installed oil pressure gauge -- sludge problem

[Arthur Marks]

  ----- Original Message ----- 
  From: Lyle 
  To: quattro at audifans.com 
  Sent: Sunday, July 24, 2005 9:30 AM
  Subject: RE: Installed oil pressure gauge -- sludge problem


  I always thought that oil filters are supposed to allow dirty oil to flow
  when they get clogged up.  IOW, they stop filtering, but they don't starve
  the engine of lube - so a drop in oil pressure must be caused by something
  else.  I suppose you could verify that if you could install an oil pressure
  gauge "upstream" of the filter.


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You may be right.

I'm confused by the physics of pressure in the system. The more I read the less I can make statements about what's going on, but I'll try anyway. I've read since my last post. I'm not sure if there's much pressure difference in and out of a clean oil filter. When the pressure diff increases due to clogging, the valve in the filter will begin to open at some pressure differential. Then I assume the differential remains constant and more flow goes through the bypass as the filter is even more clogged? 

So I would guess upstream oil pressure could drop by the amount required to open the valve?

I installed my gauge in place of the plug labeled #12 seen here:
http://server5.theimagehosting.com/image.php?img=A4%20(B6)%201.8T%20oil%20filter%20housing.gif
(image from Bentley EBahn CD)

Without knowing the internal channels in the housing it's hard to know whether this is upstream or downstream from the filter. If this plug does access the channel to the center of the filter (as it looks like it might based on its position) then it is upstream (oil leaves the filter through the center hole -- I originally mistakenly thought the reverse of filter design). Does anyone know whether the stock pressure warning light is installed upstream or downstream of the filter?

There is some interesting reading on oil and to some extent pressure (though more in relation to oil weight than throughout the system) here:
http://members.rennlist.com/oil/Motor%20Oil%20101.htm

Haven't found a good physics description of pressure and flow through the lube system.

Thanks.
--Art