Oil Temp vs. Pressure: Discuss ...

[IanT]

Have just had my car back from a lengthy repair session. The main problem that was being fixed was very low oil temperature at high temperatures.

 

Early results are encouraging but I'm still seeing some pressure drop-off above 95 deg. C. In order not to prejudice any replies, I won't bother mentioning what engine my car has as I'm just after a theory lesson.

 

When the oil is cold (below 65 deg. C) , I'm seeing some very high momentary pressures (sometimes as high as 6.5 - 7bar) under medium acceleration.

 

At normal road operating temperatures (circa 85 deg. C) I'm seeing good even pressure of 3-4 bar (sometimes more) under medium/hard acceleration.

 

At high temperatures (95-100 deg. C after some very spirited driving) I'm seeing the pressures of around 1.9 - 2.3bar under hard acceleration at high revs. Based on previous results I would expect this to drop as low as 1.5 bar once the oil temp. gets well over 100 deg. C.

 

So, given people's knowledge of the relationships between oil temperature, viscosity and pressure, would these results seem consistent with how an oil system would normally behave?

 

(Or is it just a sender that reads lower and lower the more it heats up? )

 

Cheers,

 

Ian.

 

[mav]

When I had high temp & reduced pressure, I decided to fit a Laminova (after discussing with one club memebr in particular about the decision).

 

The result post laminova is extremely satisfying - my oil / water temps remain within 5 - 10 deg of each other & the oil pressure on track etc is a constant 4 bar.

 

I'm a happy chappie now......

[Tony C]

Indded the pressure will reduce as the temperature rises.

 

Pressure in a system such as the lubricating system of your engine is largely due to resistance to flow. Cold oil has a high resistance to flow through the various clearances on its engine lubricating journey. As the oil temperature increases, its viscosity decreases and there is less resistance to its passage and lower oil pressure - in a nutshell 😬

 

BRG Brooklands Aero Screen SV 😬 It is only with the heart that one can see rightly; what is essential is invisible to the eye. (Antoine de Saint-Exupery)

[IanT]

As the oil temperature increases, its viscosity decreases and there is less resistance to its passage and lower oil pressure - in a nutshell

 

... so I basically need to find a nice expensive racing oil that stays thicker at higher temps?

 

Ian.

[V7 SLR]

Nope. You need your oil to be thin. All the best oils (Mobile 1 etc) lack the "gloopiness" of old fashioned oils.

 

Any oil pressure it a good thing. Low oil pressure may be indicative of a problem but as long as you are reading any pressure at all, you are winning. The crank acts like a pump of sorts. You (most likely) measure pressure between the pump and the crank. If you have net pressure then you have flow which is a good thing.

 

What you want is an oil which does not thicken when cold. Then you set your oil pressure up for your comfort and it should remain stable as the engine heats up.

 

Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours

 

[IanT]

... but I assume that an overly-thin oil at high temperature (i.e. with the lubricating properties of water) is a very bad thing too?

 

Ian.

[Peter Carmichael]

Fluctuations of the sort you are describing are not normal. Suspect the gauge first and a sticking pressure relief valve second.

[IanT]

Fluctuations of the sort you are describing are not normal. Suspect the gauge first and a sticking pressure relief valve second.

 

By gauge I assume you mean either the sender or the gauge? (I have a Stack dash so gauge obviously not a problem).

 

Good idea about the pressure relief valve ... I'll ask if they checked that.

 

Ian.

[Simos]

[pedantic] water is an excellent lubricant in most situations but has some problem attributes in the presence of metal [/pedantic].

 

As noted, the thinness or even lack of pressure isn't necessarily a problem per se, the lubricating film is microns thick. The attributes of oil that make it suited to lubricating your engine are to do with film strength which keeps the moving metal parts of your engine apart - oil pressure doesn't.

 

That said it should be relatively stable and vary closely with revs as this is the variable which is changing by changing the pumping rate into your oil system.

 

Edited by - Simos on 15 Sep 2003 01:49:54

[Nigel Riches]

Ian, you may like a read of this here, look in "The Oil Bible", and I hope you find what your'e looking for, Nigel.

 

1982. 5 speed, clamshells. B.R.G / Ali. The True Colours.

[warrent]

So whats basically being said here is that, with a synthetic oil, it doesn't really matter that the pressure is always above 4 bar, but more that the pressure varies with the revs being pulled, and is not erratic but predictable.

 

My SL goes down to around 2bar on left hand corners, and will go slightly below 2bar on real hard long left handers. Does this seem reasonable and safe for the enigne?

 

Long term plan is for a Pace dry sump 😬

[SLR No.77]

Are you sure you mean left-handers

 

Stu.

[Peter Carmichael]

When your oil pressure disappears in corners it means the pickup is sucking air not oil. This has the potential to cause the biggest sorts of problems. Traditional cures for a k-series are an Apollo tank (that still means you have dodgy oil temperature) or a dry sump conversion( )

[warrent]

Yes I do mean right handers

[Bob Corb]

Oil pressure will fall at high temperatures for two reasons, 1 the oil viscosity is lower, 2 the bearing clearances increase. On an aluminum engine like a K series the aluminium swells around the main bearings leaving a larger gap for the oil to flow through => pressure drops. Steel bottom ends (VX and Crossflow) wont suffer so much as the steel block only swells by the same amount as the steel crank.

 

Bob

 

[Frying Pan]

Blimey Bob - you're looking way back tonight aren't you?

 

😬

 

Guy

 

(Less than a week to go now!)

 

See some pictures of the build here. PBC and SVA complete - waiting for March 1st!

[scooby dooby doo]

to put a few numbers to all this - when very cold at idle and just started up I get about 80 psi. Once its all warmed up I get about 30 psi at idle. So basically you'll see a BIG variation with temperature.

 

You'll also see variation with revs. When hot I get 30psi at idle as stated above. I get 80psi by about 3500 rpm. Above that the relief valve opens and the pressue is pretty much constant with revs.

 

If you see sudden fluctuations then it suggests something is up. Are the fluctuations dependant on rpm (ie do they happen in neutral 🤔) or on acceleration 🤔.

 

he former suggests the relief valve to me although the std gauges are cr4p so it could be them.

 

The latter suggests its probably the gauge or some dodgy wiring that is moving under acceleration. So if you can trace it then try giving all the wires a bit of a wiggle and a tug.

 

Do any of the other gauges fluctuate? If so it could be a dodgy earth under the dash.

 

Ideal diagnostic so to borrow a gauge and sender off somewhere and wire it in independantly so you've eliminated as many sources of trouble as possible.

 

PS Peter - Are you back from skiing now 🤔

 

HOOPY R706KGU Hoopylight R

[moomin]

Hi Ian,

 

I vaguely remember your previous problems.

If I've correctly understood what you've said in your post, something is still not right.

First thing I'd check is to swap the oil pressure sender for a _known_ quantity, i.e. one which you know works ok (Hoopy speaks sense here for once! 😬 i.e. check the wiring etc).

Secondly, as you said, I'd be checking the PRV because it should be fully open at ~5bar@~5500rpm (as stated by the engine manufacturer) and should therefore be steady above that at all temperatures (obviously oil viscosity may affect this a little). Even with hot oil (100c) mine has not failed to drop below 4bar at engine speed. At idle it drops right off, but that is perfectly normal.

After this I guess you are looking at the oil pump/pickup situation?

 

Hope this helps.

 

Dave

 

Edited by - moomin on 24 Feb 2004 10:07:30