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Thoughts on oil pressure and temperature

Z

zcacogp

New member
Chaps,

A bit of a Monday morning musing. (Maybe I'm bored, which is a bit naughty as I've just told Mr Empson off for a very similar admission!)

My S2 is standard. When I start it in the morning, the engine is cold and the oil pressure gauge shows 5 bar (I think it would show more if it could.)

It being a 944, it heats up very quickly. 5 minutes or so of gentle driving later and the temperature gauge is around the first mark, the usual 'running' temperature, and stable. And the oil pressure is around the 4-bar mark. Slightly over, if anything. So, normal running temperature and 4 bar of oil pressure.

If I then drive it as it was designed to be driven, the temperature gauge will rise ever so slightly. Slow down a little, some more sedate pootling, and it will be back to normal. But the oil pressure gauge will have dropped to 3 bar ... less, if it's a hot day. I think I've seen sub-2 bar on a really hot day in a long traffic jam (manual says over 0.5 bar is OK.) BUT the temperature gauge was still around the 'normal' point. The oil pressure will rise again, with some more gentle driving, but will never get back above about 3.5 bar unless it is switched off and left to cool.

So, what's happening? There isn't a direct relationship between the temperature gauge reading and the oil pressure gauge reading. 'Normal' temperature can equate to anything between about 1.7 bar and 4.1 bar on the pressure gauge.

I know that the temperature gauge is showing water (not oil) temperature, which probably goes some way to explaining this, but I have two questions:

1. What is the relationship between water temperature and oil temperature? Does the water temperature lead the oil temperature? Always? Or isn't it this simple?

2. What other factors affect the oil pressure at a given revs? (Assuming it's the same car, at the same time - i.e. engine wear hasn't changed.)


Oli.
 
Oil takes longer to heat up than water and there is an oil/water heat exchanger on the 944.

What you have described is exactly what my S2 does (or did before the current overheating problem).

The water is circulated around the cylinders and head where the combustion is and so heats up quicker than the oil. Oil is pumped to areas that need lubrication and on the 944 is not the primary coolant. I would suggest that the 944 engine is not fully warmed up till the oil pressure drops to 2-3bat at idle. then you know ALL of the engine moving parts are up to temperature as is the oil. Personally, I never thrash the engine (6000+rpm etc) till the oil is fully warmed up.

BTW this is my personal theory.
 
My oil pressure behaves very similarly to yours, although I've never seen it drop much below 3 bar.

My temperature gauge rises a bit when sat in traffic though then the fans kick in and put it firmly back where it should be - that seemed normal to me but from what others have said those on other cars seem to stay pretty static. I'm really hoping this isn't another issue that needs sorting!!
 
Sounds similar to mine too. When properly, open-the-bonnet-and-singe-your-eyebrows hot [:D], mine can drop to slightly below 3 bar (that's on 10w60 fully synth oil).

The water temp definitely climbs a lot faster than the oil pressure drops - it will stay at just under 4 bar at idle, even though according to the water temp gauge, the car is up to temperature.

My water temp gauge will quickly rise after a run at speed and then coming to a stop in traffic, until the fans kick in and cool everything down.
 
Sounds OK. The water is obviously cooled and the oil isn't (at least, not to anywhere near the same extent. The oil heat exchanger is "cooled" with hot water....) Anyway, oil has a higher specific heat capacity than water so once made very hot with a good thrape will cool by such a tiny amount with the engine still running as to virtually remain at its hottest temparature. Switching off is the only way. The oil probably gets to about 120oC with the coolant at about 100oC.
 
I have been using 10-60 in mine for a while and my oil pressure never, even on track days dropped below
3 bar, normal running lower than 4 bar. I have just changed for 5-50 and instently noticed the car running 1/2 bar lower unless cold or with the throttle open. I will keep this oil in for a while, do a track day with it and then decide. I think 10-60 is great but started to worry about it being a little thick for our oil pumps.
 
Making the assumption that all our gauges are 100% accurate and our senders are in perfect condition :ROFLMAO: then the readings should be roughly

When Cold

Idle - 5 bar
Driving - 5 bar

Fully Hot

Idle - 3 bar
Driving - 5 bar

Track Hot

Idle 2-3 bar
Driving 4-5 bar

Too much pressure is almost as bad as no pressure and the handbook does say over 0.5 bar is fine. There is also an oil pressure relief valve which will open if the pressure is too much. This can sometimes stick and/or wear out and cause funny noises if the pressure gets too high. Mine went just after I bought the S2 and I had it changed to get rid of the groaning and stop seals/gaskets being blown
 
Chaps,

Interesting comments. Particularly about the specific heat capacity of iol being greater than that of water (didn't know that, but it stands to reason I guess), and that the water is the primary coolant of the S2 engine (did know that, didn't join up the dots so to speak.)

It sounds like the behaviour of S2's reported on this thread are remarkably similar, when the oil type is taken into account. PJS917 - that's pretty thick oil you have there, hence the high pressures. I run 10W40 semi, and always have done, hence my lower pressures.

Matt - 'another issue that needs sorting'? No - all sounds as ticketty and boo as every other S2 on here!

Thanks for your input.


Oli.
 
My 2.7 lux never gets above 4.5 bar when driving cold or hot or ever below 3 bar at idle unless I really give it some on a hot day, then maybe just below 3 bar.
My 2.5 used to move round much like others have mentioned. same oil 10w40
I think I may try and get hold of a mechanical guage and see what the pressure really is
 
A lux should be able to sustain higher oil pressure than an S2 as it only has 8 valves to feed - the S2 has 16.

Oli.
 
Fair enough, just not sure if I should be concerned about not reaching 5 bar cold or if its just the sensor or grounds/wiring.
On my ealy dash car the oil pressure "indication" dropped to 1 bar at idle and temp "indication" read high, until I sorted assorted bad grounds/ poor wiring.
Since then I'm not always inclined to believe what the gauges tell me.
 
I think your skepticism is wise ... the original point of the thread was musings on the relationship between temperature and oil pressure; but you have a good point.

I read a story about a guy who once paid to have an oil pressure gauge into his car (Vaux Calibra, I seem to remember.) The advice from the garage that fitted it was "Don't - you'll spend all your time watching it and worrying about it."

He pressed ahead and had it done ... then found that he spent all his time watching it and worrying about it!


Oli.
 
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