mike77

I suspect they went with 10w50 because it covers a greater range of temperature conditions. As such they can produce a single product and can recommend it to the majority of owners who live in either hot or cold climates. Although 10w50 isn't specifically listed in the owners manual it's not out of keeping with their original advice which of the 50s was either 5w50 or 20w50 according to my owners manual.

pdxmotorhead

Motrex 0W60 Covers all the bases.. LOL at least for my 950 KTM..

GregBBRD

I'm going to assume that there was a slight language barrier and what they meant to say is:

"Recent Standard Production Vehicles".....after 1998.

Putting 0-40 anything into your 356 would be downright stupid, right?

That being said, I'm thinking that a 5-50 would also work fine in a 928, especially anywhere with a mild climate. Our oil pumps are really "active" with cold oil.

FredR

10W50 oils to my knowledge did not exist because the additives that modulate viscosity at higher temperatures did not exist in those days to cover the range required.

Even if they did, it is a pretty safe bet that a 20W50 will have higher film strength compared to the 10w50 at 100C. The multigrade will exhibit viscosity characteristics of a more viscous oil at the specified temperature [100C] but that does not mean it will have the film strength of a straight 50 at that temperature. Same principle refers to 10w50 compared to a 20w50- The thicker base oil will always win that shoot out but at the end of the day all that matters is that the film strength has to be adequate..

GregBBRD

No question.
I spent a few years with a Nascar team. Many things learned there. Those boys know a bunch....

ptuomov

I am wondering if a completely stock 928 engine ever runs close to not having enough oil film strength anywhere in the engine, provided that the oil supply works. The bearing areas are massive, piston skirt areas, too. The pressures in camshaft lobes are minor, too, compared to push-rod V8s.

At low rpms, the required oil pressure for a completely stock 928 should also be really low. Only at high rpms, does the engine need a very high oil pressure.

I'm thinking that by far the biggest issue is keeping the oil pickup submerged. The oil viscosity may change the rate at which oil is pumped out of the sump, as well as the time that it takes for oil to return. This in turn would change the equilibrium amount of oil somewhere else than in the sump.

FredR

Keeping the oil pump pick up submerged is absolutely of paramount importance and all things being equal, an oil of higher viscosity will for sure reduce the amount of oil in the sump by a notional increment due to the amount that sticks to the walls of the engine. Needless to say Porsche would have well understood this and have allowed for such in their specifications or so I would hope!

The oil pump is a positive displacement machine and viscosity does not change the amount of oil being pumped unless and until the oil is so viscous that the pump cavitates due to excessive suction losses and that is a whole different ball game. That is why Porsche prepared charts to advise the allowable temperature range for different grades and thus why at some low temperature of around minus 10C they recommend a thinner oil as the lesser of two evils- the engine being designed around a 20W50 grade or so my engineering instincts tell me. Back in the 70's and 80's there was little else available and 20W50 was the motor oil of choice.

When we rebuilt my late 90S4 motor to put in my current GTS chassis, I prepared a detailed quality control plan itemising what I required inspecting and by whom. I gave them strict instructions that the motor was not to be fired up under any circumstances unless I was present and gave the go ahead. When the time came I checked the oil level and we then fired it up. I immediately took the car for a test drive [around this hot time of year] and after about 10 minutes I had to stop at some lights - horror of horrors the idle pressure was 1 barg. Took the car back to the workshop gingerly and asked them what grade of oil they had used. I knew they used Mobil 1 but was astonished when they told me that Porsche had recommended their stock 0W40 for my motor in this ridiculously hot climate- aaargh! Told them to remove it, they had no other alternative so purchased a couple of jugs of Shell Helix 20W50 API grade SJ mineral oil- problem solved. Nice idle at 2 barg on hot days and oil pressure at 5 barg by about 2800 rpms. I have a feeling the under radiator oil cooler of the later models is a bit on the feeble side but reasoned that was done to get heat load out of the radiator- fortunately traffic jams are quite a rarity over here these days and having the luxury of deciding what car I use and when thus I avoid using the 928 in rush hours during the hot season.

There are a lot of myths when it comes to oil pressure and as I am concerned oil pressure in reality has only an indirect connection to lubrication in that oil pressure is required to flow the right amount of oil into the bearing to cool it correctly. Logic should tell us this is the case given that oil film strengths of the day for a mineral 20W50 were typically in the region of 80,000 psig- no contest with oil pressure of 80 psig that is required to get the flow required into the bearing at higher rpms. When using a lower viscosity oil there is a danger that too much oil will flow through some bearings and maybe not enough to others. Remember there are two lubrication mechanisms in the engine- force feed and splash. The 5 barg we typically see on the dash panel gauge is controlled by the oil pressure relief mechanism bleeding off excess pressure, if too much oil is flowing through the journals due to reduced viscosity there will come a point when the relief device is fully closed and after that the gauge pressure will start drop and then [I suspect] oil flow to the cams will be reduced. Thus cam wear may be a result of too little oil viscosity- that or too little ZDDP. That GT cams commonly show signs of distress may be the result of one or [more likely] both these conditions taking place or so I suspect.

ptuomov

I disagree. With higher viscosity oil, a much greater fraction of the pumped oil gets returned back to the suction side of the pump via the bypass. The effect of this is that the rate at which the pickup sucks oil is much greater with low viscosity oil than with high viscosity oil.

Thicker oil also drains somewhat slower, but my SWAG would be that this is a smaller effect.

The reason why the factory specs a higher [edit for typo, not lower] viscosity oil for hotter climates is to keep the oil flow rate (net of what is bypassed back to the oil pump suction side) within a spec window. They need enough oil flow to cool the bearings and not to overheat the oil in the bearings.

(For this reason, if one wants to run thinner oil to make more net power, one also needs to reduce bearing clearances, at least in principle. Symmetrically, if one wants to run higher viscosity oil with greater oil film strength, then one should in principle open up the bearing clearances to keep the oil flow rate up.)

FredR

By all mean feel free to disagree, however no one in their right mind designs a positive displacement pump to by pass all the time. The pressure relief device is there to protect the system from over pressure during transient unsteady state conditions- the obvious one being initial warm up. Upon starting from cold, the pressure gauge on the dash goes full span and the reason for this is the set pressure of the spring is 8 barg [or so I understand] and that logically is the pressure that will be initially generated. During this time for sure some oil will be recycled within the pump body and I doubt anyone knows just how much is recycled without access to an appropriate test rig but probably not much at all, thus a good reason to go lightly on the gas pedal during initial operation. Within a few minutes the motor warms up, the back pressure reduces, the relief valve closes and stays that way. At this point the pressure stabilises around 5 barg at revs over 3k rpm and that pressure will ensure that the correct amount of oil flows to the bearings to cool them and to also provide the correct amount of oil to the cam gallery for splash lubrication. At idle the pressure should be 2 barg. Using a lower viscosity oil the pump capacity will be delivered at a lower pressure due to the lower frictional losses. If the flow distribution remains the same to each bearing then lubrication should be fine but that is a big gamble for something with potentially huge ramifications.

I have no idea where you got the notion that Porsche specifies lower viscosity oil for higher ambient temperatures given the documents I have seen clearly indicated lower viscosity oils for lower ambient temperatures but feel free to educate me on such matter.

ptuomov

Thanks for the noticing the typo. Of course, hotter the climate more viscous the oil.

The oil pump system including the pressure regulator is only a positive displacement device when the pressure regulator valve is completely closed. When it is open at all, then it’s no longer a positive displacement device.

It is my understanding that the 928 valve cracks open at relatively low rpms in most practical situations and is set to fully bypass a large flow rate of oil at 8 bar pressure such that pressure can never exceed 8 bar.

If I am correct, the oil viscosity only makes a difference to oil pressure at low rpms. It makes no difference to oil pressure at high rpms, but makes a big difference to oil flow rates at high rpms. Since at low rpms the engine requires only a low oil pressure, let’s say 1.5 bar still making it to the heads, I’m not too worried about the oil pressure gauge not pegging the 5 bar at low rpms. I am worried about the pickup uncovering at high engine speeds and cornering.

Postscript: In this post, Greg Gray who is going for high rpms, estimates that 2/3 of the stock oil pump flow would be bypassed at 8,000 rpm for his high-rpm engine: https://rennlist.com/forums/928-foru...ml#post8088414 . Greg estimates that the oil pump flows is 0.0015L per minute per rpm, so at 6,000 rpm that would be 90 liters per minute.

FredR

Tuomo,

There is a reason positive displacement pumps are invariably used in lube systems and that is to ensure the system gets a guaranteed amount of oil flowing- to have an inline device that bypassed the pump would seemingly defeat the object of the exercise thus the notion that it is a pressure regulator I find somewhat strange. Before opening the discussion further there is something in the system that I have struggled to get my head around and that may give a clue as to where we are seeing things differently.

On the side of the block there are two spring loaded plungers with blanking plugs. In section 17 of the WSM, there is a diagram and parts table for the lube system. Part number 33 is called the "piston for pressure release valve" and this is the set pressure I refer as opening at 8 barg. On the other spring loaded device there is part number 29 that it refers to as "plunger for bypass valve". At the end of the parts table there is a footnote stating that the short circuiting valve covering parts 26 to 29 is deleted for MY91 and presumably later models [my motor is a 90 and has both]. What exactly is the function of this component?

I feel as though I am missing something.

ptuomov

The typical engine oil flow demand looks something like this (generic, not specifically about 928):



There's also the pressure requirement curve, that's pretty flat at a low level for low rpms, but then moves at the second power of rpm at high rpms. (This is to overcome the main journal centrifugal force at high rpms.)

In any case, at very high rpms, something like half the oil is bypassed for a typical car engine. Because of this, the pump and bypass system acts nothing like a positive displacement pump at high rpms.

The 928 engine was very overbuilt on many dimensions like oil pump volume, so I'd expect it to bypass more than a typical car engine. Especially when run with high viscosity oil.

ptuomov

FredR -- Can you point the puzzling components in these sheets?

GregBBRD

I believe that the pressure bypass only works when the engines are cold, as regardless of the rpms. Getting over 5 bar of pressure, with hot oil, is not an issue.

According to Bill Dailey (Dailey dry sump fame) our oil pumps are particularly poor at higher roms. He says that the pumps will begin cavitating at ~6000 rpms (crank) and will be a complete "mess" by 7,000 rpms (crank).

He says that if you plan on turning one of our engines at 8,000 rpms (with a stock pump), you need to install a different gear, to slow it down, and then "live" with the reduced volume at lower rpms (which you can not).

FredR

Tuomo,

Thanks for the sheets- confirms that system operation is just as I understood it to be.

The bit I was unsure about was the bypass valve functionality. Seems pretty clear that this was intended to act as a safeguard in case something in the thermostat housing blocked flow- obviously they decided it was not needed and thus dropped it for MY91 onwards. Either way not relevant to our discussion.

The only element that protects against over pressure is item 3 on the diagram- note it is called a "pressure relief valve" and the set pressure is 8 barg. This item does absolutely nothing to oil pressure until 8 barg is reached and then it will open. Relief valves of this kind typically take about 10% over pressure before they are fully opened and thus flow the rated amount the system designers require and that would typically be the pump rated flow capacity at max rpms.

The pressure we see on the dash panel gauge is the pressure at the thermostat housing. The thermostat bypasses the cooler when cold and when the temperature reaches 87C it opens the flow circuit to the cooler whilst closing direct flow to the oil filter. Flow through the cooler creates a small differential pressure and if the bypass valve opens at 0.5 bar differential pressure then the pressure drop across the cooler must be less than that.

Now the puzzling bit- the oil pressure we typically see when the engine is pushing along is about 5 barg. The oil pump is a positive displacement machine and it will generate whatever pressure is needed to displace the flow it generates. As per the generic diagram you submitted the flow will increase directly with engine speed. You suggest that pressure is being controlled by pump recirculation but this just does not happen as there is nothing in the system to make it happen- if it did happen we would have to see 8 barg pressure for the relief valve to open but all we see is circa 5 barg- so how is this conundrum explained?