Ford: High altitude requires 0W oil. Why?

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Kudos to me for approximating the pressure loss at 7500' at roughly 3 PSI, off the top of my head without reference to a chart! :cool:
After converting units, the chart says it's about 3.2 PSI.
PS: none of this explains why the oil should be thinner at high altitude, due to altitude alone not the related temperature drop.

It's not thinner, not remarkably so anyway, and could be more viscous when up to temp. there's a 20% or so loss in cooling capacity at those elevations.
 
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Could we say an oil pump creates a vacuum and the atmospheric pressure pushes the oil into the vacuum created by the pump in a wet sump engine? Could we say then at high altitude a cold thicker oil would have a longer time to go from the sump to the pump? Planes aren't started cold started at 10,000 feet , A turbo at high altitude will not produce as much boost as at sea level ? Already been posted but it is that.
 
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Cooling of the oil is less at elevation, 0W-30 would contain more PAO and be more thermally stable than a group III 5w-30. There's a higher viscosity index involved aswell, so viscosity drops less above 100°F. And higher loads are a given driving up to thos higher elevations.
The 0w part is for cold starting. and the VI is more but that is a finished oil which doesn't necessarily mean it will keep the parts separated any better.
 
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I understand the theory. But Ford, who has produced the most turbocharged GDI engines of any manufacturer and has a gigantic database of what actually works and what does not, disagrees. And they do think that the "altitude thing" is relevant, or they would not have included the recommendation in 2021 and 2022 Ecoboost user's manuals. The unanswered question is why?
Where is the theory in what I said? Worry over altitude with no actual corroborating facts is less of a theory. One would think that if there was an absolute concern over altitude then every manufacturer would publish such a warning in their manuals, not just this one. Another thing is that depending on the temperature there is no universal guarantee that within the same grade an oil with a 0W rating is thinner than one with a 5W rating.
 
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Could we say an oil pump creates a vacuum and the atmospheric pressure pushes the oil into the vacuum created by the pump in a wet sump engine? Could we say then at high altitude a cold thicker oil would have a longer time to go from the sump to the pump? Planes aren't started cold started at 10,000 feet , A turbo at high altitude will not produce as much boost as at sea level ? Already been posted but it is that.
Oil pumps are positive displacement / constant volume. They should not be sensitive to atmospheric pressure.

Regarding the turbo pressure question, it depends on the turbo. Most turbos I've read about are designed to produce the same absolute boost pressure, which means at high altitude they produce higher pressure relative to ambient air (yet this is the same absolute pressure provided to the engine). That means little or no power loss with altitude. Put differently, at 7500' altitude a NA engine will make roughly (on average) 80% of its rated sea level power. A turbocharged engine will make roughly 100% of its sea level power. The turbo "works harder" to compensate for the lower ambient pressure.
 

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Oil pumps are positive displacement / constant volume. They should not be sensitive to atmospheric pressure.
The inlet side of the pump is. A PD pump can only pump out what flows in.

If there is only 11 PSI of ATM pressure pushing the oil to the pump inlet, then a thinner oil will flow better to the pump when cold than a thicker oil. Example: A 0W at 7500 ft altitude and -5F might flow about the same from the sump to the pump inlet as a 5W at -5F at sea level.
 
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You may be confusing atmospheric pressure with gravity. Even in vacuum, gravity would pull the oil down into the pump inlet.
 
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You may be confusing atmospheric pressure with gravity. Even in vacuum, gravity would pull the oil down into the pump inlet.
Down? It is atmospheric pressure that causes the oil to flow into the pickup, that was the problem back in the 80s with inadequate winter performance measurements where the "suction" from the pump inlet caused a cavity to form and the oil would not pump. The pump relies on creating low pressure for the oil to flow into the pickup, it's pretty much the only instance where flow is critical in the system.
 
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Down? It is atmospheric pressure that causes the oil to flow into the pickup, that was the problem back in the 80s with inadequate winter performance measurements where the "suction" from the pump inlet caused a cavity to form and the oil would not pump. The pump relies on creating low pressure for the oil to flow into the pickup, it's pretty much the only instance where flow is critical in the system.
It's true that as the pump draws oil in, the oil in the pan must fill in to replace the oil drawn into the pump. If the oil is too thick, it won't do that fast enough and the pump will draw in air.

So the question is: what exactly causes the oil in pan to fill in and replace the oil drawn into the pump: ambient air pressure or gravity? It may sound like a meaningless question because it is gravity that creates ambient air pressure. However, it's a good question because at high altitude gravity is (almost) the same while air pressure is a lot less.

If we had 14.7 PSI of ambient air pressure without gravity, the oil wouldn't rest at the bottom of the pan. It would float around inside the engine, sticking to parts through surface tension. The oil pump would not work. So gravity is essential. But is air pressure also essential? If we had vacuum with gravity, the oil would vaporize into mist with nothing to resist its internal vapor pressure. The oil pump would not work. So we need gravity, and we also need enough ambient pressure to keep the oil in liquid form.

The reason I don't think altitude makes a difference here is because how quickly the oil in the pan replaces the oil drawn into the pump, depends purely on the oil's viscosity (and thus temperature), and how strong gravity pulls on it. Gravity doesn't change (much) with altitude, and this process shouldn't be sensitive to ambient pressure, so long as there's enough to resist the oil's vapor pressure and keep the oil in liquid form.
 
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The inlet side of the pump is. A PD pump can only pump out what flows in.

If there is only 11 PSI of ATM pressure pushing the oil to the pump inlet, then a thinner oil will flow better to the pump when cold than a thicker oil. Example: A 0W at 7500 ft altitude and -5F might flow about the same from the sump to the pump inlet as a 5W at -5F at sea level.
Remember that any push from air pressure comes from a pressure differential.: a pressure difference in two places. When ambient air pressure drops, it drops everywhere: inside the oil pan, inside the engine, inside the empty oil passages, everywhere. So what pressure differential does higher altitude change?
 

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Imagine a pan of mercury with a vertical tube containing a vacuum inserted into it. At sea level, the air pressure on the mercury in the pan would force mercury to rise in the tube up to the 30 inch level, before gravity would take over and stop any further rise. At 7500 ft, the same thing would happen at around 22.5 inches. A suction pump, inserted into the tube at 22.5 inches, would benefit from 8.5 inches of mercury pressure at sea level, but zero beneficial pressure at 7500 feet. Now, replace the mercury with engine oil and the tube with an oil pump pickup tube. Sucking oil up a tube at 7500 feet is clearly more difficult than at sea level because of less beneficial pressure from the weight of the atmosphere on the oil in the pan.
 
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That's because the end the vertical tube has vacuum on the other side. So when ambient pressure changes, it changes the pressure differential.

In the engine, ambient pressure is on both sides. The other side of the oil pump are the engine oil passages, which are also at ambient pressure. So the drop in ambient pressure doesn't create a differential.
 

Alien

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That's because the end the vertical tube has vacuum on the other side. So when ambient pressure changes, it changes the pressure differential.

In the engine, ambient pressure is on both sides. The other side of the oil pump are the engine oil passages, which are also at ambient pressure. So the drop in ambient pressure doesn't create a differential.
So let me raise the pump in my column of mercury by 1 inch. Now, 7500 feet, it will not pump anything at all, while it will still work at sea level. What the output side of the pump is hooked to makes no difference here or in the engine. The atmospheric pressure on the fluid in the pan is what is making the difference.
 

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That's because the end the vertical tube has vacuum on the other side. So when ambient pressure changes, it changes the pressure differential.

In the engine, ambient pressure is on both sides. The other side of the oil pump are the engine oil passages, which are also at ambient pressure. So the drop in ambient pressure doesn't create a differential.
I do understand what you are saying: what is beneficial on the suction side of the pump is counteracted on the pressure side, and vice versa. But the situation may not be completely symmetrical: if the pump is unable to pick up fluid on the suction side, the fact that it would be easier to move it around on the pressure side is inconsequential. It is possible that in very cold temperatures with certain types of fluids, pumps and geometries, there is a danger of such a situation occurring. No?
 
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I do understand what you are saying: what is beneficial on the suction side of the pump is counteracted on the pressure side, and vice versa. ...
Exactly. Both sides of the pump are at atmospheric pressure, so there is no pressure differential. Thus atmospheric pressure doesn't affect the pump.
But the situation may not be completely symmetrical: ...
Maybe... but I don't see how. Any ideas?
 

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I do understand what you are saying: what is beneficial on the suction side of the pump is counteracted on the pressure side, and vice versa. But the situation may not be completely symmetrical: if the pump is unable to pick up fluid on the suction side, the fact that it would be easier to move it around on the pressure side is inconsequential. It is possible that in very cold temperatures with certain types of fluids, pumps and geometries, there is a danger of such a situation occurring. No?
The scenario described by @kschachn above may be one of those situations:

"It is atmospheric pressure that causes the oil to flow into the pickup, that was the problem back in the 80s with inadequate winter performance measurements where the "suction" from the pump inlet caused a cavity to form and the oil would not pump. The pump relies on creating low pressure for the oil to flow into the pickup, it's pretty much the only instance where flow is critical in the system."
 
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In the 2022 F-150 and Expedition owner's manuals Ford states: "If you use your vehicle regularly above the altitude of 7500 ft (2,286 m) and under the temperature of -4.0°F (-20°C), it is recommended to use the alternative engine oil", where the "alternative engine oil" is 0W-30 instead of the normal 5W-30. Why do you think altitude makes a difference?

Here is an excerpt from the manual:

Materials
NameSpecification
Engine Oil - SAE 5W-30 - Synthetic BlendWSS-M2C961-A1
Alternative Engine Oil for Extremely Cold Climates
To improve engine cold start performance, use the following engine oil in climates where the ambient temperature reaches -22.0°F (-30°C) or below.
Materials
NameSpecification
Engine Oil - SAE 0W-30 - Synthetic BlendWSS-M2C963-A1
Cold Climate Oil Viscosity Chart 5W30  0W30

Note: If you use your vehicle regularly above the altitude of 7500 ft (2,286 m) and under the temperature of -4.0°F (-20°C), it is recommended to use the alternative engine oil.
Engine horsepower will be reduced because of lower oxygen content; so, they recommend thinner oil (less oil-viscosity drag) to make up for some of the lost horsepower at high altitudes.

Everyone should use thinner oil regardless or temperature or altitude unless thicker oil is necessary because of extreme high-speed driving or towing. That said, I would usually prefer 5W-30 over 0W-30 because of lower VII content and better availability. I would prefer 0W-20 to 5W-20 though.
 
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Engine horsepower will be reduced because of lower oxygen content; so, they recommend thinner oil (less oil-viscosity drag) to make up for some of the lost horsepower at high altitudes.

Everyone should use thinner oil regardless or temperature or altitude unless thicker oil is necessary because of extreme high-speed driving or towing. That said, I would usually prefer 5W-30 over 0W-30 because of lower VII content and better availability. I would prefer 0W-20 to 5W-20 though.
Oh I see. The manufacturer publishes that statement about the winter rating in the oil section of the owner's manual because they are concerned the operator won't obtain the published horsepower output.

Right.
 

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You may be confusing atmospheric pressure with gravity. Even in vacuum, gravity would pull the oil down into the pump inlet.
It's true that as the pump draws oil in, the oil in the pan must fill in to replace the oil drawn into the pump. If the oil is too thick, it won't do that fast enough and the pump will draw in air.

So the question is: what exactly causes the oil in pan to fill in and replace the oil drawn into the pump: ambient air pressure or gravity? It may sound like a meaningless question because it is gravity that creates ambient air pressure. However, it's a good question because at high altitude gravity is (almost) the same while air pressure is a lot less.
Both gravity and the atmosphere pressure on the oil makes it move towards the pump pickup and inlet side ("vacuum side) of the pump. When a pump starts rotating, it evacuates the volume inside the pump and creates a low pressure (ie, vacuum) at the inlet - meaning the inlet is at a lower pressure than the outlet, and any forces acting on the fluid/oil will move it towards the pump inlet.

Look at the driving forces that are trying to push the oil into the pump inlet - it's actually both gravity and atmospheric pressure. If you removed all of the atmospheric pressure pushing down on the oil in the sump, and was only left with the gravity factor, then the total force on the oil will be less without any atmospheric pressure on it. If you could seal and pressurize the sump to 100 PSI above ATM for example, the driving force trying to push the oil into the pump inlet would be much higher.

If we had 14.7 PSI of ambient air pressure without gravity, the oil wouldn't rest at the bottom of the pan. It would float around inside the engine, sticking to parts through surface tension. The oil pump would not work. So gravity is essential. But is air pressure also essential? If we had vacuum with gravity, the oil would vaporize into mist with nothing to resist its internal vapor pressure. The oil pump would not work. So we need gravity, and we also need enough ambient pressure to keep the oil in liquid form. The reason I don't think altitude makes a difference here is because how quickly the oil in the pan replaces the oil drawn into the pump, depends purely on the oil's viscosity (and thus temperature), and how strong gravity pulls on it. Gravity doesn't change (much) with altitude, and this process shouldn't be sensitive to ambient pressure, so long as there's enough to resist the oil's vapor pressure and keep the oil in liquid form.
Look at an example where there was just enough gravity to keep the oil at the bottom of the sump and enough air pressure on it to keep it from vaporizing. Then vary the air pressure above the oil from say 5 PSIA to 100 PSIA. If there was essentially no gravity and at 5 PSIA there wouldn't be much of a driving force to push oil into the pump inlet. If the air pressure was increased to 100 PSIA the driving force would be much higher. Do the same senario by holding the pressure constant and increasing and decreasing gravity. Both have an effect on moving oil to the pump inlet and filling the voided area (the vacuum side) of the pump with oil.

Based on what Ford says in the OM about using 0W instead of 5W at elevations of 7500+ ft and at -4F and below, it seems they may think it does make a difference. Seems their statement is focused on cold start-up, and oil flow to the pump inlet. What if the elevation was at 20,000 feet like on Mt. McKinley in Alaska? At -5F, would 0W flow better to the pump inlet than 5W at sea-level (14.7 PSIA) vs at 7500 ft (11 PSIA) vs at 20000 ft (6.75 PSIA)? I think there would be a difference as the ATM pressure varied, and at some point there could be a point were pumpability becomes an issue depending on the oil temperature and ATM pressure combination.
 
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