Oil & Coolant Temp Correlation; Oil Sump vs. Bearing Temp Coorelation?

[NewbVetteGuy]

Does anyone have any good data or rules-of-thumb covering typical oil and coolant temp coorelation or engine sump vs. bearing temperature correlation?

Engine Coolant temps and engine oil temps on their own as independent subjects get covered plenty all the time, IMO.
What I'd like to understand better is how coolant temps and oil temps tend to coorelate with each other in different situations (highway cruise vs. WOT acceleration and heavy loads, maybe).


I think it was MadBill who's posted a few times that engine coolant temps in a car are highly coorelated to thermostat temps, ambient air temp, and radiator efficiency and oil temps are highly coorelated to load and HP developed, but there definitely still seems to be plenty of coorelation between coolant temps and oil temps.


I've also heard estimates that oil bearing temps will typically run 40F hotter than sump temps and in other times I've heard up to 75F degrees hotter in bearings. -Are the variations simply related to the load? (Say 40F hotter at the bearings vs the sump at idle or highway cruise situations and up to 75F under heavy loads?)


Just looking for an old V8 engine, wet sump engine, no dedicated oil cooler type situation, if the problem needs to be constrained.



Adam

[RDY4WAR]

It varies from engine to engine. Engine load has a big effect on oil temp, regardless of the coolant temp. There's differences in specific heat capacity between oils that cause them to run at different temperatures. In general, conventional oils and group III synthetic oils have a tendency to run slightly hotter than group IV/V synthetic oils. Higher viscosity oils tend to run hotter than lower viscosity oils. Aeration will make the oil temperature spike as well. There are some general trends though.

Cruising down the highway, the oil will usually be 10-30°F above coolant temp. If coolant is 200°F, the oil should be 210-230°F. For the temp delta at the bearings, it's about the same so 230°F sump temp can expect ~250°F bearing temp at light load / cruise condition.

At WOT, the oil can be 25-75°F higher at the bearing than in the sump. The highest I've seen was in a NASCAR engine at Daytona. Huge sump capacity, 0W-20 oil, sump temp at 281°F and oil temp at main bearing exit was 359°F, a 78°F temp rise across the bearing. The cause of that is hydrodynamic friction. The higher the viscosity, the higher that friction. The more aromatic the base oil and lower the specific heat, the higher the friction and temperature. For a spirited driven street engine or mild race engine, 40-50°F over sump temp is a good estimate.

[PackardV8]

As mentioned each engine, each application, each oil is its own universe. For example, oil temps vs bearing temps are load dependent, RPM dependent and clearance dependent. If there are piston oilers, or valve spring oilers, those adds heat to the oil not coming from the bearings, so in those cases, there's be higher oil temp for the same bearing temp.

I had a well-instrumented SBF application. The coolant temperature, with a thermostat, would quickly get up to an even 100 degrees above ambient, if the ambient was above the 180-degree thermostat opening. Around town or at cruise, the oil temperature would take forever to get up to an operating temperature, because it had a small oil cooler without a thermostat. In normal driving, the oil would stay 20 degrees below the coolant; it was being cooled too much. However, start adding load and the oil temperature would rise. Put it at a high cruise, 4,000 RPM, 100 MPH on a 100-degree day and the oil temp would slowly continue to rise. The cooler wasn't quite large enough to hold the oil temp at the 220-degree range I'd have preferred. This was back in the bad old days when I thought 30W Kendall was the good stuff. When I switched to 10W-40 Mobil 1, the oil warmed up more quickly. I could actually feel the straight-30W had been dragging on the oil pump and thus the distributor timing. The oil temp rise also slowed. Switching to Mobil 1 5w-30 was better still. That engine was put together with everything on the snug side and it didn't appreciate thick oil.

Another example, a big block Mopar, never had oil temperature issues until the owner freshened the engine and gave it a half-fill of block-crete. From then on, the oil temp would get too high at cruise; had to add a large oil cooler.

In the last days of the SBC, when they went to Mobil 1, the Corvette coolant fans didn't even come on until 230-degrees. The old guys who were always trying to keep things cool were convinced they'd burn down, but they didn't.

[jsgarage]

Sometime later in the last century, a major oil company published a paper on that. They were working on documenting the value to customers of synthetic oils, so they instrumented a small block Chev with temp sensors in at least two cam bearings (easy) two main bearings (almost as easy) and two rod bearings (NOT easy!)

The numbers they got on an engine dyno showed fully stabilized main and rod bearing oil temps were at least 30 degrees F above oil pan temps under heavy load. When the pan oil temp was 280F. main and rod oil temps were at least 310F and slowly climbing. But that was two computers ago and sadly, I never found that white paper again.

My application was DeTomaso Panteras running the 92 mile long Silver State Open Road series in Nevada where we have to regulate top speeds to oil temps, not available power.

[Rick!]

Cruising down the highway, the oil will usually be 10-30°F above coolant temp. If coolant is 200°F, the oil should be 210-230°F. For the temp delta at the bearings, it's about the same so 230°F sump temp can expect ~250°F bearing temp at light load / cruise condition.

This is in line with my experience, from dry sump turbo 4 stroke sleds with a plate cooler to late model diesel engines. Last summer, I learned about oil temp regulation with the throttle as a 4000ft altitude pull up a pass out west yielded 240*F coolant temp with the oil reaching 265*F. I guess that's why a 20psi radiator cap is used and a 265-270* oil temp derate.