Date:         Fri, 2 Aug 1996 12:05:48 -0400
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Re: FRAM specualtion re pressure

In a message dated 96-08-02 00:03:14 EDT, DHaynes57@aol.com writes:

>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.

I think the above may be a bit misleading.  There is a pressure drop across
the natural restriction of the filter element.  The inlet side of the filter
is at a higher pressure than the outlet side.  It the differential pressure
across the filter element gets too high (clogging), the bypass valve will
open thereby allowing some (or all) of the oil to bypass the filter and
reduce the differential pressure drop.  Oil might enter the filter at 50 psi
(for instance) and normally drop 5 psi (for instance) across the filter,
causing the downstream oil to be at 45 psi.  If the filter's pressure drop
became too high, say 15psi (for instance) and exceeded the filter's bypass
valve limit, the bypass valve will open and allow the downstream pressure to
come back up to within the normal range, thus "controlling" the oil pressure
downstream of the filter.  No third port necessary for this operation.

Jim Davis
87 GL Syncro
88 GL Wolfsburg