Cold start oil pressure - the flow discussion

OVERKILL

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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:

IMG_2993.JPEG


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.
 
Interesting, thanks for posting. The pressure in the 3.6L Pentastar in my 2016 Wrangler is a lot higher than that even at 35F with ESP 0w30. It's a different animal though. IIRC it's about 90 psi. 5w20 factory fill was about the same at 35F. Once warm it idles with about 33 psi of oil pressure.
 
Interesting, thanks for posting. The pressure in the 3.6L Pentastar in my 2016 Wrangler is a lot higher than that even at 35F with ESP 0w30. It's a different animal though. IIRC it's about 90 psi. 5w20 factory fill was about the same at 35F. Once warm it idles with about 33 psi of oil pressure.
Yes, that's a different animal, as it uses a chambered vane pump where the volume is manipulated by changing the chamber size and pressure is also altered through an electric solenoid. Sounds like the high pressure limit is 90psi.
 
At the minimum the 0W40 should make it easier on the battery and starter cranking it over at those temps.
I mean, that's the purpose of the CCS limits, but yes, in application, if the 0w-40 is half as heavy as the 5w-40 at these temperatures, it's going to require less effort to roll over.
 
@OVERKILL do you know if the 5.7 HEMI oil pump has the same relief pressure of 65 PSI, or is it less? IIRC the 6.4 has a different model number for the oil pump. Thank you, and thank you for the video.
 
@OVERKILL do you know if the 5.7 HEMI oil pump has the same relief pressure of 65 PSI, or is it less? IIRC the 6.4 has a different model number for the oil pump. Thank you, and thank you for the video.
They take the same pump... In the Jeep, but only 2021+ and it looks like it started with the NBS GC L:
Screen Shot 2022-02-14 at 12.01.56 AM.jpg


Intersting eh?

The older 5.7L pump that my wife's truck and your truck have:
Screen Shot 2022-02-14 at 12.04.15 AM.jpg

was never used on the 6.4L, but the 6.4L pump is now spec'd on some versions of the 5.7L 🤷‍♂️ That 6.4L part number goes right back to 2012.

As far as I know, they use the same relief pressure.

Also, the 6.2L has its own pump.
 
Overkill...

Great, great presentation here... Awesome.

I have noticed in my area when it has gotten down in the low single digits that vehicles run and sound different than normal. Even going down the road they are louder too.

Something about getting close to 0°F and things start changing with vehicle operation.

Way, way different vs when it's warmer.
 
On my Gen Coupe 2L Turbo I have two oil temp senders. One immediately after the filter and the other in the head by the oil solenoids. The oil filter one leads the temp climb until abut 125F then the head is the higher temp. Only with extended idling do they again get near the same.
 
I mean, that's the purpose of the CCS limits, but yes, in application, if the 0w-40 is half as heavy as the 5w-40 at these temperatures, it's going to require less effort to roll over.
So could the same be said for 0w30 and 5w30 or for some reason not? Also, which brand of 0w40 are you using?
 
That is awesome Overkill. You mentioned, I believe, that you thought you were at the nominal relief pressure of 65 and the digital gauge dropped below 65 and so you thought the relief valve was closing once the pressure was dropping. Is that correct? Knowing that the mechanism is simply a metal spring ( inside the right hand side of the pump photograph), I think it was in the relief mode the entire time of the video and that those slight pressure fluctuations might be “ in the noise”. You have an extremely sensitive measuring device monitoring a very simple mechanical device ( a spring sliding inside a cylinder with a friction fit piston all at -18 C). However, you have definitely proven the function of the relief valve in a oil pump, and the positive displacement pump theory in cold starts is subject to the relief feature. Thanks for posting.

I guess Ontario is done with the cold weather for now. Maybe next year you can try the proper oil for that car. Pennzoil Platinum Ultra 0w40 :D.
 
Also, the 6.2L has its own pump.
I wanted to circle back to this and almost forgot. Thanks to @Snagglefoot for replying to this thread, I got a notification and remembered.

I believe the Hellcat oil pump has wider gears within the same housing to move a bit more volume. They did this by milling the inside of the pump to create extra space. These YouTube videos explain it pretty well:



 
On my John Deer F510 w Kawasaki, Fram Tough guard and 0w40 I have the pressure sender in the oil cooler bypass loop.

This late winter I was using it in about 25F temps and when I first started it 0 PSI for a few minutes!!!! I was freaking out as I've never used it that cold. No rattling so I let it idle.

But as it warmed up the pressure slowly rose. Never did get the oil over 100F according to my gauge. But 20 psi at idle.
 
Interesting, thanks for posting. The pressure in the 3.6L Pentastar in my 2016 Wrangler is a lot higher than that even at 35F with ESP 0w30. It's a different animal though. IIRC it's about 90 psi. 5w20 factory fill was about the same at 35F. Once warm it idles with about 33 psi of oil pressure.
I was going to say, pretty sure I’ve seen 75+ psi in more temperate conditions in 3.6L minivans.

I’m not sure that shearing and heating the oil at super cold temperatures is a bad thing.

The question really is at the end of long circuits or small orifices, in an older engine that may be somewhat dirty, what are the ramifications then?
 
I was going to say, pretty sure I’ve seen 75+ psi in more temperate conditions in 3.6L minivans.

I’m not sure that shearing and heating the oil at super cold temperatures is a bad thing.

The question really is at the end of long circuits or small orifices, in an older engine that may be somewhat dirty, what are the ramifications then?
IIRC even during the summer the oil pressure during the first start of the day might have been in the 90 psi range as well. I'll have to remember to take note this summer. It does drop off a lot faster during the summer. Hot idle summer or winter is about 33 psi. Regarding an older dirty engine, good question. I'd imagine a short OCI Bitog nut like myself doesn't worry much about that. I'm sure that goes for you as well. ;)
 
That is awesome Overkill. You mentioned, I believe, that you thought you were at the nominal relief pressure of 65 and the digital gauge dropped below 65 and so you thought the relief valve was closing once the pressure was dropping. Is that correct? Knowing that the mechanism is simply a metal spring ( inside the right hand side of the pump photograph), I think it was in the relief mode the entire time of the video and that those slight pressure fluctuations might be “ in the noise”. You have an extremely sensitive measuring device monitoring a very simple mechanical device ( a spring sliding inside a cylinder with a friction fit piston all at -18 C). However, you have definitely proven the function of the relief valve in a oil pump, and the positive displacement pump theory in cold starts is subject to the relief feature. Thanks for posting.

I guess Ontario is done with the cold weather for now. Maybe next year you can try the proper oil for that car. Pennzoil Platinum Ultra 0w40 :D.
If we assume the relief opened right at 65psi, then yes, I believe I was on the relief briefly, the subsequent start a few minutes later didn't hit 65, and was at 59 as you can see. Driving around had it hovering around 60, a solid 5psi from the relief.

Not a chance I'm putting the SRT oil in this, lol.

We were at -14C again yesterday, a bit of a cold snap, -13C here this AM, so no warm weather yet.
 
I wanted to circle back to this and almost forgot. Thanks to @Snagglefoot for replying to this thread, I got a notification and remembered.

I believe the Hellcat oil pump has wider gears within the same housing to move a bit more volume. They did this by milling the inside of the pump to create extra space. These YouTube videos explain it pretty well:




Would make sense it has a different pump, it has a lot more HP and also doesn't have MDS.
 
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