Fastest928

Doug,
Really??

Are you stating the only reason oil pressure drops from 8+ bar at cold start down to a tick over 2 bar at operating temps with a temp rise of only 100 degrees F is only due to a change in oil viscosity?

Doug, viscosity matters, but oil pressure is also determined by the back pressure of the engine oil flow paths. The oil flow paths are the "gaps" present in the bearings and othe lubricated areas. Just forget to put in place a rear oil cam plug and watch the oil pressure

Maintaining consistant bearing Tols over temp are one of the basis for using steel main caps in aluminum engines, the steel to steel maintains the bearing clearances over temp better than an all aluminun case and steel crank. And this method, steel on steel, allows for use of lower viscosity oils with quicker fill times and yet maintain excellent oil pressure and tols.

Gretch

"smartest guy in the room" is back................

SharkSkin

Doesn't the pressure also drop to some degree due to the oil t-stat opening and diverting some flow to the oil cooler?

Jim bailey - 928 International

The oil thermostat actually closes not opens and directs oil to pass through the heat exchanger and not simply bypass the oil cooler lines plus there is a bypass valve (pre 1990 )set at about 1/2 bar which helps assure that cold thick oil in the heat exchanger does not build up too much pressure and rupture. The main pressure relief blows off at about 9 bar. As our learned friend from Downunder has pointed out before the key is oil volume not pressure. The difference between 30lbs and 90lbs when compared to the pressure working on the oil film between a crank bearing and a crank is actually pretty insignificant. Since oil pressure is far easier to measure than oil volume and no oil pressure is a pretty good indication that things are not as intended that is what our gauges read..pressure.

SharkSkin

Thanks Jim...

Mark87AT

Wow! thanks to everyone! I had no idea such a simple question would generate such delightfully involved discussions!! This forum really rocks!!!!!
Thank goodness for the intarwetneb!

samk

Did you ever wonder what is inside yuur oil filter?

Check this link.

http://minimopar.knizefamily.net/oilfilterstudy.html

Doug Hillary

Hi,
Marc - I do not wish to get into a "stoush" here like we have done in the past but it may pay you to re-read my earlier post to "get the gist". To assist this process I will go over it again;

You said:
"Are you stating the only reason oil pressure drops from 8+ bar at cold start down to a tick over 2 bar at operating temps with a temp rise of only 100 degrees F is only due to a change in oil viscosity?"

NO! - I said:
"ALL engines usually lose pressure as the oil (and engine) warms to normal core operating temperature regardless of their metallurgy! That's why we have a by-pass in the lubrication system and a differential by-pass valve in the oil filter. The pressure loss has more to do with the oil's viscosity, structure and formulation than the metallurgy of the engine!"

The word "ONLY" does not appear anywhere in this phrase! The word I used was MORE!!!
And this is so.

You said:
"Doug, viscosity matters, but oil pressure is also determined by the back pressure of the engine oil flow paths. The oil flow paths are the "gaps" present in the bearings and othe lubricated areas. Just forget to put in place a rear oil cam plug and watch the oil pressure"

Marc - we are not discussing the loss of pressure caused by the failure to insert a part - whatever it may be.
Pressure is caused by the resistance to flow. Excessive gaps are a great concern but these would manifest themselves in a variety of ways. The operating gaps (clearances) and their limits are predetermined and allowed for in Porsche's tolerances for oil pressure and the engine's well-being

You said:
"Maintaining consistant bearing Tols over temp are one of the basis for using steel main caps in aluminum engines, the steel to steel maintains the bearing clearances over temp better than an all aluminun case and steel crank. And this method, steel on steel, allows for use of lower viscosity oils with quicker fill times and yet maintain excellent oil pressure and tols"

I said:
"The actual pressure is not as important as the flow which is of course constant per engine rev and these factors are integral at the engine's preliminary design stage and well factored in. It takes on board then the metallurgy used along with all the other components within the lubrication system and other "hardware"."

Marc - Strength is also a factor with the steel caps of course??
Many types of bearing materials are used in aluminium engines. These include aluminium(alloys and silicates), bi and tri-metals and etc. It is all a design and application issue and many modern bearings can be retrofitted of course. Journal bearings must be either able to absorb minute particles in circulation that are larger than the HTHS viscosity of the oil and be able to "polish" the crank's surface too. Newer aluminium bearings can tolerate temperatures up to 1000f, have lessor embeddability (than bi/tri metal types) and their finish enables better oil flow(and cooling) and oil retention. They usually have a longer life when used in the correct application

I suspect that in many 928 engines the actual oil pressure at 3000+rpm is NOT 5bar at all!

Marc - what do you mean by quicker fill times?? I presume you mean the time taken to establish OP!

Perhaps you can tell us what the real OP is on a 32v 928 engine when using a Porsche Approved oil (say a synthetic 0w-40) and as it warms up on a dyno at say 10C increments in oil temperature. And also tell us what the OP is on the engine when running at 3000rpm with a stabilised core temperature - to make it interesting log the oil's temperature and idle pressure after 10 minutes and then repeat the process using the following oils:
0w-40 Approved synthetic
5w-40 Approved synthetic
15w-50 Approved synthetic
10w-40 Mineral
20w-50 Mineral

Now that would be a real contribution from Devek!

Regards
Doug

Fastest928

Here is a 85/86 oil pressure test perfromed in intervals of 2 minutes starting at 1 minute after cold start warmup, oil temp at beginning of test was 75 degrees F, with a 1 minute hold at each test point. Ending oil temp was 183 F

On this test we measured actual pan oil oil temp at beginning and end of test. Oil was mobil 1 15-50.

This test was to identify an unusual problem associated with a poor running 85/86 during the first 3-8 minutes of cold startup ...california cold that is

hopefully the upload took


Marc

Doug Hillary

Hi Marcc,
thank you this is good data

It does demonstate what I meant - OP may well be at or near system by-pass for some time. With some very viscous (say 25w-50) mineral oils this will mean a lack of full filtering and a dumping of excess back oil to the pump (not flowing around the engine) for quite some time

a) What version of M1 15w-50 was used - TriSynthetic or SuperSyn? (or what year was the test done)
b) Was the engine under load - if so what load?
c) Depending on b) above, it may show that the oil cooler thermostat's crack point is quite high in actual fact and is not open for long periods during normal running (this would confirm my own research)
d) It shows why a 0w-?? or 5w-?? oil will come off by-pass (and filter differential filter by-pass) quicker and be more beneficial
e) It shows that the 5bar we take as "the oil pressure" (and mentioned as the only OP test parameter by Porsche) may well be lower than the systems "core" pressure in the 3000-4000rpm range. It may be closer to 7bar or so. This is also a case for using Porsche Approved engine oils - and a reason why a 5w-40 oil may be the best choice for our engines as Porsche suggests

North Queensland's "cold" is like Californias' as I recall - it has been near 100f for the last few days around here

Thank you Marc
Regards
Doug

Glenn Evans

Hi Marc,

I currently run a 10W-50 oil, and it takes about 8 minutes for the pressure at idle to drop below 5 bar at about 70F ambient temperature. (Mind you, that is driving in congested traffic, and most of that time would be spent either stopped or under 25mph. I think that it takes about 20min for the idle pressure to drop to 2.5bar, which indicates that the oil is up to its operating temperature.

No doubt that your much faster warm up was due to the engine being revved (and under load?).

What was the ambient temperature when you conducted your test? Do you know whether 183F was the oil's full operating temperature? (If you didn't measure the temp, was the car's oil pressure at idle as high as 3.5 bar?)

If the pressure relief valve opens at 8 bar (116psi), as the Chilton manual states for the early 928s, your test engine's filter will be being bypassed above 4000rpm in your 9-minute test. Hopefully, Jim was correct when he said that the bypass valve operates at 9 bar (130psi).

Whatever, your chart should remind us all not to rev the 928 engine hard until the oil is up to operating temperature, as the oil probably will be bypassing the filter!

Jim bailey - 928 International

Glenn the early service booklet does show pressure relief at 8 bar BUT this just RETURNS the oil to the suction side of the pump. In addition, the correct oil filter has a safety valve designed to "open when the filter is plugged" which when it activates allows unfiltered oil to circulate but I have not read what pressure is required to open that valve. The oil cooler thermostat starts to cool the oil at 87 C. The Porsche tech spec books call for approx 5 bar at 3000 RPM oil temp at 80 C engines m28/43..45,47, and 5 bar at 4000 RPM on a M28/21/22... So I can document the 8 bar number but at the moment do not recall where I saw the 9 Bar ......... which is interesting because if the pressure relief piston blows open at about 8 bar ,116 lbs why how would the measured oil pressure exceed that number ??

Doug Hillary

Hi,
As you can see I am still awaiting some answers from Marc and your (Glenn & Jim) input has pre-empted the same "issues" I had too!

The oil filter's differential by-pass valve activates at 2.2bar as mentioned in an earlier post. This activates when the filter is plugged (relatively uncommon) and more importantly when the pressure differential on each side of the filtration media exceeds that figure. This enables any "excess to media flow rate" to return unfiltered. It is more likely to occur and for a longer period with cold mineral oil - and oils with a low Viscosity Index and a high "w" viscosity (eg. 0w-40 will be better than a 15w-40). In some situations this will occur until the oil's temperature is well beyond 70f

There are at least two lubrication system by-pass settings - 8bar and 8.5bar (the engine Marc is discussing is at 8.5bar). Jim like you I think some engines had a by-pass setting at 9bar

Glenn - the oil temp at start up was 75f (24C)

Perhaps Marc will answer the (now many) previous questions - especially why the oil pressure exceeded 8.5bar (125psi) (is this for instance resistance through the by-pass?) and, what is meant by "no shim start"?

Regards
Doug

Fastest928

a) What version of M1 15w-50 was used - TriSynthetic or SuperSyn? (or what year was the test done)

2001 I believe it was the tri syn

b) Was the engine under load - if so what load?

No load

c) Depending on b) above, it may show that the oil cooler thermostat's crack point is quite high in actual fact and is not open for long periods during normal running (this would confirm my own research)

It is very high - we continued exp to lower that point by shimming the plug. It worked as expected.

d) It shows why a 0w-?? or 5w-?? oil will come off by-pass (and filter differential filter by-pass) quicker and be more beneficial

At start up ... i suspect that a 0-50 would be ideal

e) It shows that the 5bar we take as "the oil pressure" (and mentioned as the only OP test parameter by Porsche) may well be lower than the systems "core" pressure in the 3000-4000rpm range. It may be closer to 7bar or so. This is also a case for using Porsche Approved engine oils - and a reason why a 5w-40 oil may be the best choice for our engines as Porsche suggests.

Often, the pressure is much higher than 5 bar...85/86 have the highest oil pressures I have recorded, except those from experimental sroker engines tht i was varying engine bearing, side to side and other tols. Ambiant was about 76 F, same s the oil.

The best oil control is with later blocks where the lower girdle is also 390 vs 356 alloy. ... smallest reduction of oil pressure per degree of all engines.

ALL tests perfromed on engine without oil squirters.

G'day