A little knowledge...

[JJDraper]

As part of my rebuild, I opted for an oil temp gauge instead of refurbing the ambient air temp unit. The sensor sits in the sump plug and works pretty well, but... any ideas what 'normal' oil temperature is?! I have been running the car for some months now and have the following observations which may be interesting. First, the oil takes an age to warm up! 10miles or more in cold weather. Normal running, at 50-60mph the oil temp sits at around 90-100DegC, but sitting in traffic and high speed running makes the temp rise up to 130-140DegC - close to the max on the gauge.

Now, the engine has not been touched from before the resto as it was running perfectly and the water temp behaves as before, but now I have something more to feed the paranoia! On the plus side, it means I am now more respectful of engine speeds until the oil is warm..

Does anyone have experience of oil temperatures and what is considered normal? Any other ideas for a location for a sensor?

Jeremy

[Frank Howard]

Jeremy,

One thing that most gearheads don't realize is that in addition to lubricating the engine, the oil serves as a coolant. If your oil is hot, one way to look it is that it is removing a lot of heat from the engine. Another way is that your engine is running hot and the oil and coolant can't remove the heat fast enough.

[bill308]

On the other hand, the fact that you know the enigne oil is cold or hot provides motivation to do something about it.

Maybe the best thing to do is to add a thermostatically controlled oil cooler. Presumably, during warm up, oil circulates as before but when the oil reaches a particular threshold temperature, oil is routed to the cooler. The combined heat rejection of the radiator and oil cooler is greater than the heat rejection of only a water radiator.

Bill

[69S4]

I fitted an oil temperature gauge into my S4 about 25yrs ago together with an oil cooler. I drilled the sump and brazed in a spigot for the sensor fairly close to the drain plug so not too far from where yours is. I do get very different temperature readings though. Normal running - say cruising at 60-70mph it's reading 55C, with the water temp at 85C. That's with the cooler covered up although uncovering it doesn't seem to make much difference - it just takes a bit longer to get there. Cruising faster makes very little difference although "spirited driving" on normal roads will put the oil reading up to 75C or so.

Driving in traffic is different though. The electric fan will normally hold the water temp at around 90C but the oil temp continues to rise. When it gets to 90C also (about 15 mins if the car isn't moving) both oil and water go up together slowly and the fan is overwhelmed. By about 45mins it's time to switch it off as both gauges will be reading around 110C. I've never seen the oil temp reading over about 110C.

Those numbers come from the respective dials and I've no idea how accurate they are - the oil temp in particular could be way out as I've never cross checked it. You're right about a little knowledge though. I can't say that having the oil temp information has been of much help at all.

[gerrym]

"69S4", One method of achieving better oil temperature control (faster warmup and lower temperatures at max load and hot low speed conditions) is to install an oil-to-water heat exchanger. These can be fairly compact. The downsides are you will be increasing the total maximum load on the radiator so this and the fan will need to be in good condition. The heat exchanger is of course subject to a possible failure mode which would allow water and oil to mix.

Another completely different approach is to use an oil which is thinner at lower temperatures and still holds it viscosity under high temperature high shear conditions (HTHS). Modern oils meeting ACEA specifications such as A3/B4 grades should be able to help here.

Regards

Gerry

Note, continuous operation with the bulk temperature in the sump at true 55 degrees C would indicate long term severe engine wear. Has the calibration of the gauge/sender been checked

[69S4]

Note, continuous operation with the bulk temperature in the sump at true 55 degrees C would indicate long term severe engine wear. Has the calibration of the gauge/sender been checked

Like Jeremy in the original post I've wondered what the optimal oil temperature should be but I seem to have the opposite problem, with the oil too cold. Whether it actually too cold or not is another matter as the act of adding the gauge doesn't actually do anything to the oil, it just gives me a reading on a dial. I've not checked the dial calibration so I've no idea what an indication of 55c actually is but other than the addition of the gauge the engine is close to std. I would expect therefore that the oil is doing something fairly close to what the designers intended, irrespective of what the gauge indicates, so I haven't panicked too much.

I've done somewhere around 150,000 miles in the car over the decades, with the gauge fitted for probably 100,000 of those and while the engine has been rebuilt a couple of times it's been for things like dropping a valve seat etc, never for wear beyond what is normal for a tc.

Which oil to put in it though, particularly these days, now there's a question to bring me out in a cold sweat.

[GrUmPyBoDgEr]

There are so many differing points of view on the optimum Oil operating Temperature in Lotus Twincams & the types of Oils to use in them all of which are debatable.
If you feel happier with your Engine running "cool" oil temperatures then fit an Oil Cooler; I don't know of any cases where doing so has harmed an Engine.
Due to the additional Cooler & associated Hoses there will probably be a measurable pressure drop but mostly insignificant.
Generally speaking Engine Oil Temperature can be directly related to Engine Speed i.e. the longer you keep the Engine running at high speeds the higher the Engine Oil temperature will rise.
Any drop in Engine speed will immediately drop the Oil Temperature.
On the other hand if a hot Engine is left to Idle for a long time there will be a heat transfer from the Coolant to the Oil, generally via the large contact Areas in the Cylinder Head.
It could possible under some circumstances that the Coolant & Oil temperatures could equal one another. (Heat Soak)
The usually accepted maximum in modern Cars is 120?C Coolant temperature; depending on the Coolant system Layout boiling, coolant loss & possible Engine damage could occur when that limit is exceeded over longer periods.
The higher Temperatures mentioned here seem to be below the upper limits normally considered acceptable for non-synthetic Oils.
The general rule of thumb is that long period running over 150?C should be avoided as those Oils are known to Oxidise around that point i.e. the Oil starts to permanently break down.
Synthetic Oils have a slightly higher threshold but only marginally i.e. 155?C extended or 165?C for short blasts.
The modern take on Engine Oil temperature is to get the Oil hot as quickly as possible & keep it hot.
This is in order to reduce internal friction & subsequently reduce fuel consumption & emissions.
In order to achieve this some manufacturers include a Coolant/Oil Heat exchanger (as mentioned by Gerry), which helps heat transfer both in the warm up phase & high speed running.
Running close to threshold temperatures is however something best left to modern Engines with intelligent ECU's

FWIW, Cheers
John

[rgh0]

In terms of temperature resistance I would class oils in 3 categories

1. Low - normal mineral oil - will oxidise and foul a normal road engine over time
2. Medium - normal poly alpha olefin synthetic oil - will keep an normal road engine clean by not oxidising in normal use in a road engine. Price double a normal mineral oil
3. High - Redline ( and gas turbine synthetic oils from the major companies) polyol ester synthetic oil - will maintain viscosity / pressure and resist oxidation in a race engine at extreme temperatures. Price double a normal synthetic oil

I have never actually correllated these against actual sump temperatures but i would say a normal road engine would be seeing around 100 to max 140 C in the sump with no cooler and a race engine around 130 to 160 C depending on the amount of oil cooling fitted.

just my 2 cents worth
Rohan

[GrUmPyBoDgEr]

It's me back again.
Rohan's posting reminded me of a small point about Sump temperatures.
There can be minor variations in the temperature measured depending on the placement if the thermocouple.
There are some areas in the Sump where the oil is not 100% in circulation "Oil Hiding" & will not show the "operating" temperature.
Putting the thermocouple close to the Suction Pipe as previously mentioned is as about a good a place as any.
The best place is to put it in the oil flow in the main Oil Gallery. There you will be measuring the Oil temperature that the Engine is receiving.

Cheers
John

[JJDraper]

Thanks for the replies. John, I now better understand the mechanism for the behaviour of the temp in various engine run situations. To be clear, I was happy with the water temperature behaviour of the engine before fitting the oil temp gauge. I was trying to see if there was any additional useful information from the reported temperatures from the new gauge. Certainly, the most important information I have gleaned is the long warm up time of the oil. The increase in Oil temp during high speed runs and when the engine is working hard now makes sense. As for fitting an oil cooler, we will see how the summer holiday run works out.

Primary worry is water temperature, although that is not normally an issue.

Jeremy

[GrUmPyBoDgEr]

Thanks for the replies. John, I now better understand the mechanism for the behaviour of the temp in various engine run situations. To be clear, I was happy with the water temperature behaviour of the engine before fitting the oil temp gauge. I was trying to see if there was any additional useful information from the reported temperatures from the new gauge. Certainly, the most important information I have gleaned is the long warm up time of the oil. The increase in Oil temp during high speed runs and when the engine is working hard now makes sense. As for fitting an oil cooler, we will see how the summer holiday run works out.

Primary worry is water temperature, although that is not normally an issue.

Jeremy

My pleasure, one of my Pet subjects.
Payments may be made via paygal
A basic rule of thumb for Engine Fluid temperatures is:-
Load increases Coolant temperature (slogging up a Mountain)
Revs increase Oil temperature (hairing along an Autobahn)
Ambient temperature having an influence on both

Hope you have an enjoyable trip
Cheers
John

[Galwaylotus]

One other thing to know: ignition timing affects coolant temperature. I recently found my Mini running much hotter than normal and as I had just had it serviced, I checked the timing and found it about 8? retarded! As soon as I reset it to spec, I had no more temperature issues.

[GrUmPyBoDgEr]

One other thing to know: ignition timing affects coolant temperature. I recently found my Mini running much hotter than normal and as I had just had it serviced, I checked the timing and found it about 8? retarded! As soon as I reset it to spec, I had no more temperature issues.

How very true!
If the Engine is not set up properly or not running correctly it can have an adverse affect on the Cooling System.
Any increase in Compression Ratio or increase to power output will also have an effect.
In one of my previous Job's I was responsible for Cooling Systems & every time the Engine tune Guys decided to change things I had to further testing to ensure that the Cooling was up to the job.

Cheers
John

[Frank Howard]

Same thing goes for fuel mixture. A lean running engine will run hot. If you've ever run out of fuel, the engine runs lean at the end. I've run out of fuel and witnessed the headers glowing.

[alaric]

Hi all. I saw a big difference in oil pressure on slowing down from high speed cruising when I removed my oil cooler from my old mini just before I sold it. I attributed this to the difference in oil temperture. That was running with standard 20/50 oil. I still have the oil cooler, and was considering fitting it to the elan. I wasn't going to bother with thermostatic control though.

All the best.

Sean.