What Determines Oil Pressure?
What Determines Oil Pressure? Hello.
I was wondering if someone could explain to me the factors that determine oil pressure. The only thing I'm currently aware of is oil temperature and RPM, where the hotter and thinner it gets, the lower the pressure, and vice versa. But I've noticed that my oil pressure gauge shows mildly higher readings at lower RPM when up to tempterature than with a higher RPM but at lower (about 155°) temperature. Does the amount of load on the engine influence oil pressure? I've been wondering because this is the only changing factor that I can think of, since a colder oil is supposed to give a higher pressure.
For example, oil pressure @ 1700 RPM in 3rd (M6) with about 155° is about 40 psi. But @ same RPM in 5th with about 195° is about 50 psi.
So it's this backward inconsistency that caught my curiosity. It wouldn't be the case that the gauge needle is sticking and not reading accurate until the heat expands the plastic and giving it more tolerance for movement, would it?
Any input is appreciated.
I was wondering if someone could explain to me the factors that determine oil pressure. The only thing I'm currently aware of is oil temperature and RPM, where the hotter and thinner it gets, the lower the pressure, and vice versa. But I've noticed that my oil pressure gauge shows mildly higher readings at lower RPM when up to tempterature than with a higher RPM but at lower (about 155°) temperature. Does the amount of load on the engine influence oil pressure? I've been wondering because this is the only changing factor that I can think of, since a colder oil is supposed to give a higher pressure.
For example, oil pressure @ 1700 RPM in 3rd (M6) with about 155° is about 40 psi. But @ same RPM in 5th with about 195° is about 50 psi.
So it's this backward inconsistency that caught my curiosity. It wouldn't be the case that the gauge needle is sticking and not reading accurate until the heat expands the plastic and giving it more tolerance for movement, would it?
Any input is appreciated.
Thicker when hotter? I thought it was the opposite. If this is the case, then it explains my question entirely.
I was always under the impression that the oil is thickest when it's cold, hence the environmental factor when choosing an oil--10W 30 more suitable for warm weather cold starts and 5W 30 more suitable for cold weather cold starts because the 10-30 could be too thick to circulate for proper lubrication.
Would it happen to be the case that oil thickens when it initially gets hot, but then once it gets really hot, it thins?
I was always under the impression that the oil is thickest when it's cold, hence the environmental factor when choosing an oil--10W 30 more suitable for warm weather cold starts and 5W 30 more suitable for cold weather cold starts because the 10-30 could be too thick to circulate for proper lubrication.
Would it happen to be the case that oil thickens when it initially gets hot, but then once it gets really hot, it thins?
Thicker when hotter? I thought it was the opposite. If this is the case, then it explains my question entirely.
I was always under the impression that the oil is thickest when it's cold, hence the environmental factor when choosing an oil--10W 30 more suitable for warm weather cold starts and 5W 30 more suitable for cold weather cold starts because the 10-30 could be too thick to circulate for proper lubrication.
Would it happen to be the case that oil thickens when it initially gets hot, but then once it gets really hot, it thins?
I was always under the impression that the oil is thickest when it's cold, hence the environmental factor when choosing an oil--10W 30 more suitable for warm weather cold starts and 5W 30 more suitable for cold weather cold starts because the 10-30 could be too thick to circulate for proper lubrication.
Would it happen to be the case that oil thickens when it initially gets hot, but then once it gets really hot, it thins?
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The oil pump is fixed displacement and pressure limited by the bypass valve. If you have higher pressure when under higher load it sounds like something is further limiting oil flow, i.e., more system restriction. How many miles? Any unusual noises?
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Joined: Nov 2008
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From: ohio
the colder the oil is the thicker it is. being in the agricultural industry, its quite a problem, a conventional crank case in a tractor during the winter will actually cause it to roll when started. the oil gets so thick its works like the old school fluid drive transmissions do.
The only thing unusual I can detect by sound is the occasional vibration from the steering rack & pinion (positive without doubt). Thinking again, the oil pressure difference (if present) corresponds mainly to temperature (again, seemingly backward as described above) rather than load. I've noticed that even a moderate load produces a relatively high pressure for its RPM when up to temperature (45 psi with 1600 RPM in 3rd @ 195-200°) Would a bad PCV valve affect pressure? Or does bearing clearance with metal heat expansion produce such a difference?
I've suspected that the needle dial assembly isn't as loose as people would assume, meaning that it'll stay still until the forepart moves substantially. For example, if you have a wire hanging off the edge of a table, it moves freely unless it moves over a notched area, then requiring more momentum for continuation. If this is true, then it negates this whole thread.
Last edited by Crimsonnaire; Apr 19, 2009 at 04:32 PM.
If you are going of the gauge, it would probably be best to put a mechanical gauge on and get true readings before getting too far down the road of suspecting issues with the motor.
"Oil Viscosity
The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil."
http://www.nordicgroup.us/oil.htm
Here's another link for you:
http://www.aa1car.com/library/oil_viscosity.htm
The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil."
http://www.nordicgroup.us/oil.htm
Here's another link for you:
http://www.aa1car.com/library/oil_viscosity.htm
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From: Tampa/Valrico, FL/ Chicago, IL
"Oil Viscosity
The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil."
http://www.nordicgroup.us/oil.htm
Here's another link for you:
http://www.aa1car.com/library/oil_viscosity.htm
The viscosity of multi-weight motor oil is specified using two numbers. The first number is the viscosity when the oil is cold. This is followed by the letter W (which stands for winter, not weight), which is followed by the number that indicates the viscosity when the oil is at operating temperature. The higher the number the thicker the oil."
http://www.nordicgroup.us/oil.htm
Here's another link for you:
http://www.aa1car.com/library/oil_viscosity.htm