RE: Oil Filters and Anti-drain back valves

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

... guess it's that time of year again ... :-)

> In the Audi 2.3, 5 cylinder (Audi 80 and 90 anyway) the oil filter is
> located at the absolute bottom of the engine in an "end down" orientation.
> It is virtually level with the oil pan.
> 
... as is true for most of the Audis I've worked on (all of the 5-cyls) ...
what difference does it make where the filter is physically located?  It is
located in the "circuit after the pump and before all of the things that
receive a supply of pressurized oil ... if anything, having the filter
located there should provide the WORST situation for allowing oil to drain
from the passages ... the force of gravity acting on the oil up in the
passages should actually be highest for the filter in this location ...

> How would an anti-drain back oil filter keep any significant quantity of
> oil
> in the upper portions of the engine? (as noted by listers and others, this
> anti-drainback valve would help keep oil from draining away from vital
> engine components during start up).  Whenever I've changed my oil, a small
> quantity of oil does come out of the filter feed tube - is this a function
> of the anti-drain back oil filter?  If so, that doesn't seem like enough
> to
> make a huge difference, and in fact, it doesn't seem like any more than
> when
> I've used a non-valved filter.
> 
Yes, but how much oil drains out when you break the outer seal as you first
spin the filter off?  The "feed tube" is on the other side of the
anti-drainback valve.  Chances are that the only oil that is going to be
left in there is that from the small section of the supply line which is
angled down.  It really shouldn't matter whether you have a valve or not ...
if the oil is backflowing through the filter it will drain back into the
sump so the amount in the feed line should be about the same independent of
the type of filter you use.  

How long has it been since the engine has been run when you change the
filter?  I usually like to have the oil hot when I change it so that it will
flow better, only having shut the engine down a minute or so before draining
the oil and removing the filter.  If you do this as well you certainly have
not allowed a lot of time for gravity to force oil back through the filter.
Try removing the filter after the car has been sitting idle for a few days
... I'll bet you'll see a difference in the amount of oil that drains with
the valved vs. non-valved filter.  

> It's my understanding that ALL filters only "filter" small portions of the
> oil at any given time, the full quantity of oil is not passed through the
> filter in continuous flow... so how much advantage are you getting from an
> anti-drain back filter in an engine with a bottom mounted oil filter?  I'm
> not suggesting that the technology is not useful, I'm just curious if
> others
> have contemplated the logic.  I can see where on my old Toyota, with a
> very
> high, block mounted filter you'd stop quite a quantity of oil from
> draining
> out of the block, but does this hold on the Audi configuration?
> 
... Actually you're thinking about a bypass oil filter system ... very few
production cars are designed with that sort of system.  Most cars use a
"full flow" oil filtering system where all oil that is supplied to the
engine is forced to pass through the filter.  Most full flow systems have
some sort of pressure relief valve built into them so that if the amount of
pressure required to push oil through the filter is too high (due to a
plugged filter or low oil temperature) that the engine is supplied with oil
... a flow of unfiltered oil is better than none at all.  The advantage of
the full flow system is that it will keep those big chunks of phlegm in the
pan out of the little oil passages, but at the price of having to have larer
pores in the filter because it has to support the full flow to the engine.
The advantage of the bypass system is that since it only filters some of the
oil it can have tiny pores to do a better job of filtering fine particles
that can abrade bearing surfaces ... at the risk of perhaps passing a kidney
stone at some point in time ...

As I stated above, when I consider oil filter location it sure seems to me
that a system that had the filter mounted up at the valve cover would be the
one that would need the anti drainback valve less than the one with the
filter mounted near the oil pan ... gravity isn't going to try to force the
oil UP to the filter is it?

HTH!
Steve Buchholz
San Jose, CA (USA)