Unkle

Hi

Just changed my oil out today and noticed something, the oil cold (it is a warm day though) sits at 2 bar at start up, and drops to 1 bar when warm like normal. I remember my previous thicker oil was 3 bar or more i think when cold. So the cold viscosity is far better for flow and lubrication at start up (when most wear occurs).

Knowing that i have lowered the cold viscosity index, i think i got what i wanted. But at what point is increased flow better than high pressure? I always understood that high pressure is good, but thinking about this more, pressure doesn't mean flow of oil? Does it? If my poor explaination is right, high pressure when cold and at idle is not actually more flowing oil for lubrication? Therefore a negative effect? and higher than 1 bar at idle when hot also reduces cooling as flow is reduced?

I have put question marks on these sentences as i wondered if people could help with my understanding of flow and pressure, as it may be completely different on dry sump applications.


Quote for reference
The purpose of the oil pressure is to move oil into the bearing when rotation of the shaft causes an oil loss secondary to the clearance. There is also a need to spray oil into several locations. To keep the bearing supplied with oil there must be oil movement. In order to have movement there must be pressure from the pump or a vacuum at the bearing site. In actuality a vacuum is present in many bearings. Although this may provide the necessary fluid for a film thickness the other function of oil is inadequate. The other function is to cool bearings. More flow equals more cooling.

The false thinking I am trying to dispel is that many people say:

"I have very good, high pressure, therefore I have the best lubrication and wear protection"

The thinking is that there will be less wear because there is more pressure. Wear and pressure are unrelated. It is the flow that counts.

aehaas


No oil brand talk please, just discussion/assistance on pressure and flow...

wilko

a conundrum indeed mr unkle , throwing petrol on the fire here , breaking down your post i read , , , , is high pressure on dash gauge better for the engine as higher viscosity means thicker oil so more of a reading or does lower pressure mean more cooling oil travelling around therefore better for the engine ?
i may have to search my car handbook here , as my car ticks over from cold with almost full pressure i may have a sleepless night wondering ????

Unkle

Yeah thats kind of it Wilko.

And to add: higher viscosity= thicker oil but doesn't translate to lubrication improvement or cooling, however slightly lower pressure will lead to flow and cooling improvement and without any negative impact or lubrication. (thats kind of how i read the info)

It has puzzled me for a while after i obtained the A40 type approval list produced by Porsche (all aircooled and water cooled after 1984). And although i have banter on here with a few people i never really could understand the low viscosity of all oils list in the many brands they approve. But I think they seem to value flow over pressure. Although i understand that you can't have one without the other, I assume they consider it a balance.

Reading some info on pressure for an engine, 10 PSI per 1000rpm is/seems acceptable. So perhaps thats why they don't really give a reading for idle pressure in the manuals, and say its ok if the light comes on for low pressure at idle (.6bar would be 10 psi falling into the 10psi at 1000rpm range for example), as long as you get 5 bar at 5000rpm (in excess of 70PSI). This would indicate that perhaps they also lean slightly towards the low pressure low viscosity theory?

I wouldn't have thought much about it until i stumbled on a study on oil pressure (which many, me included kind of assumed is the a sign of a well functioning oil system) and how this is different to flow.

Interesting none the less, but i'm not sure if i'm reading between the lines right...

KaiB

I'm supprised there aren't many posts here.

I am curious about the subject myself and haven't yet researched it. Who could guide me (us) to good resources on-line?

Unkle

Yeah i am too

Perhaps its a discussion had many times before, but i am interested in this pressure/flow characteristic and how its different to popular belief. (me included)

Try a forum called bob is the oil guy.com and read the interesting info on the front page before searching the forum for flow/pressure etc. not saying i agree with it all but it makes you think. Grab a tea, coffee or a beer as its a lot of info but worth it.

I don't believe everything i read, but i think some of this corrolates to the low viscosity on the Porsche type approval list, perhaps giving it some validity not sure? I would be interested to read alternatives to "why thicker is better"
It doesn't change anything as we will all do as we wish, but may helps us/me to be a bit more informed.


Research Report 2002-24-04.pdf

evoderby

Oh my god, not that dr.AEHaas fellow again.......his 'expert' (he's a medical doctor) oil advice has resulted in at least two catastrophic engine blow ups over on Ferrarichat if I remember correctly.

Let me try and give a pragmatic explanation on the subject of flow and pressure:

• Indeed oil flow is needed to lubricate and cool the moving parts in your engine.
• The oil properties (shear strength, viscosity vs temp etc.) are responsible for how good of a job the oil actually does in preventing wear given ample flow rates.
• There is a correlation between flow and pressure, given the bandwith of oil viscosities used in modern engines one needs about 0.7Bar pressure per 1000RPM to provide sufficient oil flow. Any more wastes power, any less is considered unsafe.
• In a known engine configuration such as the 964's oil pressure can be a good initial indicator of engine health (especially bearing clearances).

A warm idle 1 Bar reading in your engine is definately on the low side, only 2 Bar when cold however is very strange....this should move the meter right off the scale. It makes me suspect your oil pump is losing pressure somewhere.

Unkle

Very interesting Evoderby, do you have any links i would love to read them. As i said i don't fully subscribe to some of the info but its interesting that it bears some resemblance to the low viscosity of manufactures (in Europe anyway) and the Porsche oil approval list.

Laker

No brand talk, but what was your before and after oil weights?

Unkle

Hi Laker

I previously had a 10w40 for normal work and in variable climates, what the indys in the UK suggest and use. but i thought i would try one from the oil approval list so trying a "how to make you car like a sieve" i went for 0w40. My train of thought is that i have kept the operating viscosity for normal running the same, so when up to temp i should be approx exactly the same for flow, lubrication, pressure and performance, but improved flow when cold. Not sure if it works (but sounds plausable) and only time will tell, and i will possibly never tell unless i get oil analysis done (beyond me).

I was going to go for another oil that is 5w40 instead but 1 is yet to be released, and 1 was very hard to source due to the company i work for changing supplier.

I have read all the info on that thread that Evoderby has also mentioned may be wrong and many others, and looked at the Porsche type approval list for 1984 on cars and i would never change the working temp viscosity like mentioned by that thread, but its the cold flow i am interesting in understanding and i found that part informative. It seems so at odds with what we seem to do to our cars. Again i don't call for experimentation of hot weights but cold weigths are something that could be improved with more understanding, by me anyway...

I will look at the pressure gauge again today now i have fully sorted the oil level after the oil change, topped up and left the car overnight.

Henry964

i did exactly the opposite, previous owner used a local brand in 5w-40, and I switched to 10w-40 from a german brand (Liqui Moly) and I am happy with change - lower oil consumption and better pressure - the cold start lubrication is taken care by the oil high Mos2 level and in Buenos Aires winter wheather is always in the 8-10 celsius degrees minimum temperatures

Unkle

Hi Henry, its interesting that you went the other way. If you don't mind can i ask how you got lower consumption if the operating temp for the oils viscosity is the same as the previous oil? Were you leaking oil when cold, because from what i am reading you haven't changed the working temp viscosity so shouldn't leak any more or less with a 5w40 or 10w40 when hot. If that makes sense.

Perhaps the additives in that oil are working better with your seals? (don't name the oil )

The 5w will certainly be thinner when cold, but at operating temp the thickness should be the same - i assume. Did you leak a little when standing or did you find it burnt through on initial start up until warm as it leaks into the cylinders? Just curious about the change in consumption, its something i am sure i will also notice with the change of oil i have made, but just really trying to get a better understanding as i also notice you mention the word pressure as being better when high (the very point of the thread).

Thanks

Laker

perhaps when cold, more 5w gets past the rings and burns off?
10w: less oil gets past the rings when cold, less consumption. (higher compression) And as the manual states some consumptions is normal, I am by no means hinting that Henry has a consumption problem.

warmfuzzies

The Mobil guys who did a presentation at the local OPC said it was something to do with misting off, almost like vapours were being sent through the system and thats why their synthetic had a tendancy to get used more than the old dyno oils, not saying it's true, but thats the info.

K

Unkle

Thanks Laker and WF

Misting off is something i will look into as it sounds interesting. I use 1ltr to 2000 miles approx, the book says 1.5 ltrs to 600 miles approx (it really does sound like a lot to me to burn off) so i am starting well within the limit. So i will see how i get on.

Just to clarify my oil pressure after a proper 24hour cold morning start.

Cold: idle 3bar+
Cold: 3000rpm 4bar+

Hot: idle 1.5bar
Hot: 3000rpm 4+bar
Hot: 5000rpm 4.5+ bar (taking age of car, oil pressure and gauge reading accuracy, it seems to work out about where i thought, and where the manufacturer intended it).

On thing that is curious though, the oil takes longer to get to thermo opening temp. This i guess would be consistent with a lower viscosity oil? And would also possibly indicate the increased flow and not pressure helping the cooling? Not sure, but it also drops away quite a lot after the thermo opens (i think more than previously)

But if it results in burn off or misting or blue smoke at start up after a week in the garage i will add it here. If i find a puddle under the car i will put it down along with any problems with oil pressure at idle when hot. I have never looked at the oil pressure gauge so much in the last 24 hours and am starting to see every little fluctuation of the needle. TBH it could have been doing that before but i can't say i looked as much...

Unkle

Does that mean you're indicating that your car has over 5bar pressure at idle when cold? That really does seem like a lot of pressure to try and flow quickly to lubricate parts. If when hot your car idle drops to 2bar at idle, (which is more than acceptable) what is all that pressure doing when cold? apart from slowing down the flow for lubrication?

Just trying to understand the balance you strike with high pressure and flow and the reasons for it...

P.S Do you have the links to the Ferrari's detonating? Would be interesting to read, although probably not too uncommon in a Ferrari i guess...