Kevin in Atlanta

I would like to understand more about where oil pressure comes from.

I have a 87 S4 with 160k and while it had a rod knock and jettisoned mass quantities of oil smoke at startup the oil pressure was always great. Took it apart and the rod bearings look ok. Have not split the girdle yet.

On my 90S4 who's engine came to me in a pile of parts along with new mains and rod end bearings has the most amazing oil pressure - I can actually see the effect of the oil thermostat opening as the engine comes up to temp.

Then there's an 89 I had in here recently and while the oil pressure starts out great after the engine gets hot the oil pressure does not go above 3.5 bars. Same with fresh oil.

Then there's an 87 S4 engine I know of that a fella bought off ebay that never got any oil pressure. Turns out a thrust bearing shell was missing on teardown. Don't ask.....

So, I just just need to get schooled on what contributes to good oil pressure and what tolerances in which bearings make that good or bad.

There's main bearings, rod bearings and cam journal bearing - as well as oil pressure relief, oil relief valve in the heads and oil thermostat. Anything else?

Help me understand.

FredR

Kevin,

The simple answer is "hydraulics".

The engine has to cope with a wide range of conditions from cold start to full load on hot days. The oil pump is a positive displacement pump- what this means is that it will literally displace a given amount of oil for a given engine speed. Double the speed of the engine and the oil flow will double. Oil is non compressible and the pump [theoretically] will generate whatever pressure is needed to move that amount of oil until either the drive shaft snaps, the engine has not enough power to drive it or something goes pop to relieve the pressure and that is where the built in safety valve comes in. In the case of the 928 the safety valve pops at 8 barg and by extension that means that at full relief flow the pressure will be about 9 barg. There is a condition specified by Porsche that indicates when things are working normally and it involves having the engine up to temperature, a certain rpm point and achieving a certain pressure- from memory it is something like 4 barg at 3k rpms give or take a bit. The pressure gauge maxxes out at 5 barg after that the oil pressure is somewhat irrelevant and when warm my engine hits 5 barg at 3k rpms and 2 barg at idle- these figures seem pretty typical. If everything is working as it should once the motor is fully warm the safety valve should never need to lift at all but some will disagree with that statement

For the main and big end bearings to work correctly the bearing clearances are critical to achieving optimal performance and just as importantly, so is the viscosity of the oil. The engine designers will have considered two flow streams as being critical- that going to the mains/big ends and that going to the top of the engine- the sum of the two being the oil pump capacity. If the viscosity of the oil is too low then more oil will flow and the back pressure will drop. If such happens and the flow regime favours one branch over the other then the engine may be in trouble. If the oil overheats this will also reduce viscosity with potential to impact oil distribution within the engine. If a bearing does not get sufficient oil flow to it the bearing temperature will increase and at some point failure will occur. So indicated oil pressure is probably the most important performance indicator of engine health closely followed by engine temperature.

If the indicated oil pressure is not as it should be then something is not working correctly. If a big end bearing is worn more oil will flow through that bearing and that of course means that less oil is flowing somewhere else. If a bearing clearance is too tight the bearing will overheat because of that and worse, the oil flow through the bearing will likely be reduced and thus not be enough to cool the bearing correctly and that just adds to the grief. Once the oil flow is not enough and overheats a plain bearing will wipe in no time.

On major capital machines we have thermocouples embedded in the bearings and together with vibration probes if anything untoward is detected the machine automatically shuts down- we do not have such luxury! However if the behaviour of the pressure gauge changes suddenly one needs to watch out for such carefully. Not sure about earlier models but on the S4 and later we have an alarm annunciator to indicate low pressure but if that goes off it is probably too late to stop damage unless it is a simple case of general overheating or low oil viscosity.

Hopefully the above helps a little.

Kevin in Atlanta

Nice write up. Thank you.

So, is there a way to determine where oil pressure is being disappearing once the car is warmed up?

It is striking that the 89's oil pressure would not go above 3.5 bar even at higher RPMs. Gets me wondering about oil pressure relief valve or thermostat.

I've had more than a few 928s and even hot there was a relationship between RPMs and oil pressure.

Billu

You don’t say what the oil pressure is at idle on the 89, but my guess might be thrust bearings.

FredR

First thing to do on the 89 is try to prove what the oil pressure actually is. A calibrated mechanical oil pressure gauge is very useful in this regard but tricky to set up given you need a lift and the kit etc.

On all 928's you should be able to see a clear relationship between revs and oil pressure along the lines indicated in my note until the dash panel gauge pegs out. Anything different and you should verify the readings before stripping the engine! If the oil pressure is genuinely not present trying to find out why is more problematical. The more obvious starting point would be to check the relief valve out and fit a new spring[s].

On the 89, assuming it is an auto, one assumes that you will have checked the crank end play and it is in order.

Bigfoot928

Having the correct viscosity oil is critical.