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.

also interested, I know mines about 60 PSI cruising around 2K RPM

Oil actually gets thicker as it gets hot. Its thinner for startup so it can get through the motor and all the parts easier and faster.

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?

you use 5w in colder weather instead of 10w because it is thinner on startup. Once they warmup the viscosity is the same. You answered your own question.

Bearing clearences have a lot to do with oil pressure too, and since metal expands and contracts with temperature changes, your pressure would vary because of those factors as well.

That's a great point!

Thanks for bringing that up.

Wow...

Omg...

I dont know about that one. Pull the drain plug on a pan that has an ice cold motor and then on a hot motor and tell me wich one is empty first.

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?

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.

This is what I have always thought as well.

very true. the hotter the oil the thinner it gets. that's why oil pressure is lower when the engine is hot.

Thank you for your input.

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.

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

see my last post sugar tits :eyes:

lol :lol:

ok wow but yes the second number means the oil it thicker when running but let me give you an example here 5w30 5 is when it starts up and the 30 is when the engine is hot.. BUT engine oil get thinner when its temp goes up so you had straight 5 wieght oil then it would thinn out to 0 when hot but since the hot number is 30 it thinns out to maybe 15 or 20 weight so you still have oil volume not pressure, pressure is created by main and rod bearings and cam bearings, when egine heats up metals expand and tolorences get bigger in which oil pressure get lower but volume stays the same get it