From: Jim Bailey [jim@928intl.com]
Sent: Thursday, January 18, 2001 1:00 PM
To: 928
Subject: [928] engine oil flow , rod bearing failure

The flow of oil in the 928 - the oil from the pump is presented with three
paths . The first path is a pressure relief that opens at 8 BAR - which is
why 928s peg the gauge when cold . The excess oil is routed back to the
intake side of the pump . This one is a dead end and no longer is part of
the discussion . The second path available is to a bypass valve which limits
the amount of oil pressure flowing through the oil cooler so that it never
exceeds the design limit of the cooler (This was deleted as of 1991 models)
It opens at a pressure differential of .5 to 1 bar. .The third path of oil
flow for all but 1980-84 USA cars is through the heat exchanger in the
radiator or the external air cooler standard as of 1990  . There is a
thermostat element mounted in the block just above the oil pressure sending
unit ( Porsche chose this location next to the ports for the oil cooler
lines as the best place to monitor oil pressure) . When the oil is cold both
oil cooler line ports are exposed to full system pressure and being in a
state of balance little oil would flow to the cooler but the oil lines and
cooler are always completely FULL of pressurized oil . As engine oil
temperature increases the thermostat expansion element closes the main flow
path and the oil flows out the lower oil line port to the cooler returning
to the upper return port . At this point the flow paths converge . From the
thermostat / relief housing all the oil flows through the oil filter then
into the oil galleys which are made as part of the block /lower cradle
assembly at the point where they are bolted together . So what is the point
of this description of oil flow ?? No matter where you add an outside source
of pressurized oil the oil has the option to flow toward the bearings or to
flow back toward the oil pump . The only time the reserve oil can flow is
when there is low oil pressure in the system which would mean the pump was
trying to pump air . It would make little difference where the oiling "extra
oil pressure" is added to the system you must pressurize all the oil from
the pump to the bearings including the oil cooler lines , oil filter , heat
exchanger. Unless you have added a one way check valve that all the oil must
flow through all the time . (The oil filter mount is a reasonable
alternative) And introduce the pressure above the one way valve the oil has
the option to flow both ways . Additionally most pressure accumulators have
a capacity of 2 or 3 quarts of oil . The accumulator is initially charged
with 5-10 lbs. of air pressure on one side of a diaphragm and when the
engine is started , oil under pressure displaces the diaphragm ( be aware
that there are some "accumulators" being sold that are empty cylinders and
as such do not "store" any pressure) .  The discharge of the pressurized oil
is a result of the compressed air ( liquids do not compress ). If the
accumulator stores 3 quarts @ 5 bar as soon as 1 1/2 quarts have flowed back
into the system the oil pressure remaining in the cylinder should be about
2.5 bar ,.  As the last bit of oil flows it is at the 5-10 lbs. of the
original air charge to the accumulator . In reality the pressure in the
accumulator is what ever the engine oil pressure happens to be every time
you get off the gas , brake, clutch and shift . For the accumulator to add
oil and pressure there must be a lack of pressure in the oiling system .
Those who have lost engines that I have talked with have NOT REPORTED any
loss of pressure (Don Hansen as a recent example)( David Lloyd) no red
warning , no central warning , no fluctuating oil gauge....  If no loss of
oil pressure the accumulator would not add oil pressure ......  It would be
a very interesting real world test to remotely mount the air pressure gauge
from the accumulator and monitor the pressure or video tape it' s
movements --- if oil is flowing back to the engine the fluctuating air
pressure gauge will clearly indicate this ..... if not , then it is another
problem that kills 2 and 6 rod bearings. Which is the position that we have
maintained for many years based upon Mark Anderson's experience using a dry
sump oiling system and still having bearing failures of # 2 and 6 ...until
he had the cranks redrilled .




From: Marc Thomas [mmthomas@ix.netcom.com]
Sent: Friday, January 19, 2001 7:17 AM
To: 928
Subject: [928] engine oil flow , rod bearing failure

Sorry folks but the factory oil pressure gauge is VERY SLOW to react!   I
have two gauges mounted on the development mule...one is the standard
factory oil pressure gauge and the other is at the accusumps output.

during testing at Laguna running at high rpms (6-6500) there were slight
nudges in the factory oil gauge on the corkscrew with larger fluctuations in
the accusumps activity!

I have built and run one engine on the dyno at 8000 rpm about a dozen times
and then taken it apart.....there was no damage to the bearings due to lack
of oiling.   I performed the same test with a drilled crank.....no
difference in bearing wear.  Obviously, at 70 or so psi, the oil does flow
around the corner on the crank!  Remember, no curve or off angle is going to
stop the 70-90  or so psi going to the bearings and bleeding out.   Remember
the oil pressure is a measure of the BACKPRESSURE in the system!

This is what led me to the accusump development effort some years ago.

Adding the accusump to the oil cooler lines is undesirable, adding it to the
oil filter with a sandwich plate (DEVEK standard accusump config) is ok, but
allows oil to flow backwards and forwards.  Adding the DEVEK racing config
only allows oil to flow one direction....into the bearings.

The only thing better is the dry sump system.....the one developed by Bob
DeVore, over 10 years ago, (the DEV in DEVEK) is my favorite and is used on
Mark A racer.  He developed the absolute best combo for using the stock pump
as the pressure pump and an additional sump pump as the
scavenger....expensive, but worth every penny!

Marc
DEVEK