In a message dated 96-07-31 01:27:39 EDT, VWnut@smus.bc.ca writes:

<< Dennis,
 
 I thought the bypass valve also served to prevent overly high oil 
 presure when cold? Is this not the case? Just wondering - in cold 
 weather (30-40f) using 20w50 I will get 100-120 psi of oil pressure 
 during the first few minutes of driving. This is with a VW filter - I 
 have not used anything else since adding the gauge, so I don't know 
 if the bypass valve has any effect.
 
  >>
The bypass or actually relief valve in the engine controls the oil pressure.
 It does this by allowing oil to escape out of the pressure system when it
exceeds a certain level.  As an oil filter only has an inlet and an outlet,
both connected to the "pressure" part of the system  there is no way it can
control system pressure.  It would actually need a third port to do that.
 The pressure relief or bypass is used to bypass the filter element in case
of clogging or during some unusual conditions.  For example, during extremely
cold weather, the oil may not be avble to flow through the element fast
enough so the bypas would open so that the bearings would get oil.  Also, a
good filter will trap some water.  If enough water gets trapped and freezes,
it would block the filter until it melted.  This is why the sump screens on
type 1 engines have that little spring set up that lets the screen go down.
 Those early engines ran cool and condensation would build up and collect at
the bottom of the block, especially if car was run for short distances in
winter.  I hope this gives some insigt to filter bypases.  BTW, most filter
bypasses are set at somwhere between 12 and 18 psi.  If you have this
pressure drop in any VW, you have a real dirt problem.

Dennis.