Cold start oil pressure - the flow discussion

I'm going to start this with a video, please ignore the dust. This is a true cold start, vehicle has been sitting 24hrs, it was -18C overnight (-0.5F), was -14C at the time of the video (7F). Interesting artifact in that oil temperature doesn't register at this temperature, but did on a subsequent start after I drove a couple of blocks.

Pertinent information:
- 0w-40 Ravenol in the pan
- OG FRAM Ultra filter
- Pump relief pressure is 65psi

Video:


Picture from about 10 minutes later after it had run for about 5 minutes and was parked while I ran in and did an errand, this was ~1,200RPM:

View attachment 88827

You can see in the video that once the engine is full enveloped, the pump does hit the relief pressure briefly. You can also see that oil pressure is immediate, and high, but below the relief, so all volume is going through the engine.

Once oil temp hits about 35C, 2,000RPM is 60psi, so driving around does not engage the relief. I was quite surprised at how quickly coolant temperature (and thus oil temperature, it has a coolant/oil heat exchanger) came up.

On to the flow discussion.

So, with a positive displacement oil pump, unless the pump is on the relief, every rotation of the pump displaces the same volume of oil; forces the same volume of oil through the engine, regardless of the viscosity. You can see that on a true cold start that it's possible to engage the pump relief, thus a percentage of that volume is being shunted back to the feed side of the pump while that is occurring, this means that the flow through the engine is capped at that point; the relationship between pump RPM and volume is decoupled; ceases being direct. @4WD posted a nice chart from Mobil in another thread that shows what this looks like with the pump relief engaging at 2,000RPM. There are various flow paths in the engine that vary in size, which dictates how much volume is bled off through each section. Some engines also have piston cooling jets (like this one), whose performance will be dramatically different at this temperature than at operating. But all of this is factored into the design.

The Winter rating of the lubricant tells us that the oil has passed the requisite testing (CCS and MRV) for impact on cranking speed and being able to pump. This allows a consumer to choose an oil with the appropriate Winter rating for their climate. The OEM also factors this into their blanket recommendations, which is why 5w-xx has become the standard and now 0w-xx is becoming even more common, being appropriate for all anticipated operating conditions.

However, if we look at this specific example, had I had a 5w-40 in the pan, which would have been about twice as thick (using the doubling rule) than the 0w-40, I'd have expected it to be on the relief quicker, and longer, which would have meant that, at this temperature, flow through the engine would have been a bit less. Does that matter? No. There is still plenty of volume being pushed through the engine, but once you've engaged the pump relief, you are wasting power heating the oil (bypassing it back to the feed side) and if you were somebody who was inclined to modify their oil selection based on this, it may be an indication to look if this can be further optimized.

Now, this engine doesn't allow for any grade other than 0w-40, but a 5w-40 would have been well within its Winter rating at this temperature, so I think this does yield the potential for some productive dialogue based on individual examples if your vehicle provides detailed data and you can find out what the relief pressure for your pump is.

In a more temperate climate, there would be no meaningful difference in behavior between this 0w-40 and a 5w-40. I've started at -32C this year, which is a key reason for my oil selection in both vehicles, as that's very close to the 0w-xx CCS temperature. PAO dominates at these temperatures and I don't use the block heaters.

Stopped reading at Fram. Cardboard construction filter. 🤮
 
machinery lubrication notes oil viscosity spec for cold is at 40C aka 104 F BUT unless its a REAL synthetic PAO or Ester it thickens VERY quickly when temps drop!! so depending on how much REAL synthetic is in the base oil they can behave a LOT differently!!
 
On my John Deere mower I put in a Hayden ATX cooler. The header kind that restricts flow when cold. I have an user installed oil pressure gauge and an OE oil pressure light. Never had an issue until I started using it below 40F. I'm using 0w40 Mobil 1 in it. But below 40F the ATX cooler restricts the oil flow enough to show 0 psi on the pressure gauge.

I was contemplating using a 5w30 but today realized it was the ATX cooler. So I need to bypass it for cold weather. I was blaming the Fram filter.
 
machinery lubrication notes oil viscosity spec for cold is at 40C aka 104 F BUT unless its a REAL synthetic PAO or Ester it thickens VERY quickly when temps drop!! so depending on how much REAL synthetic is in the base oil they can behave a LOT differently!!
The Winter grade for this oil is qualified/measured at -35C (CCS) and -40C (MRV) because it's a 0W-40.

And yes, it's a "real" synthetic, PAO-based.
 
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