Oil weight and oil pressure

[Threeofnine]

Roughly how much of an effect would going up, or down, one weight of oil have on engine oil pressure at operating temperature? For example going from a 10w30 to a 10w40, or a 5w50 to a 5w40 etc. Is the effect significant or, typically, negligible?

 

[Chris142]

I think it really depends on the bearing clearances. Tight clearances will have more pressure than loose clearances.

 

[eljefino]

It depends on everything.

If your bypass valve would open on the thinner oil you've done nothing but slow down that oil's trip through the engine, making it pick up more heat.

There's also the question of where the sender for your pressure gauge is tapped, and what happens to the oil that bleeds off between the pump and your sender, and after it.

Yuppies comparing oil pressure gauges in the golf club parking lot to brag about engine "health" got us the dummy gauges we have now.

 

[Propflux01]

The oil pump (or block) bypass would keep oil pressure the same, provided it didn't get overloaded.

 

[Lubener]

Roughly how much of an effect would going up, or down, one weight of oil have on engine oil pressure at operating temperature? For example going from a 10w30 to a 10w40, or a 5w50 to a 5w40 etc. Is the effect significant or, typically, negligible?

Little change if any in your oil pressure with the weights you mentioned. I have experimented in the past with various weights and I had very little change.

 

[ZeeOSix]

The oil pump (or block) bypass would keep oil pressure the same, provided it didn't get overloaded.

Doesn't work that way. A thicker oil will produce more oil pressure at the same RPM. Just compare cold oil pressure to hot oil pressure at idle, it's not the same because it takes more pressure to move the same oil volume through the oiling system.

 

[ZeeOSix]

Yuppies comparing oil pressure gauges in the golf club parking lot to brag about engine "health" got us the dummy gauges we have now.

 

[ZeeOSix]

I think it really depends on the bearing clearances. Tight clearances will have more pressure than loose clearances.

True, most of the oil pressure at the sensor is due to the journal bearings. And because of that the HTHS viscosity of the oil tracks oil pressure more linearly than the KV100 does.

 

[Propflux01]

Doesn't work that way. A thicker oil will produce more oil pressure at the same RPM. Just compare cold oil pressure to hot oil pressure at idle, it's not the same because it tales more pressure to move the same oil volume through the oiling system.

Little change if any in your oil pressure with the weights you mentioned. I have experimented in the past with various weights and I had very little change.

This has been my experience as well—the hot VS cold works equally with a heavier oil. As long as the bypass isn't overloaded, the pressures will remain nearly the same.

 

[SHOZ]

On my Gen Coupe 2L at 50 mph and 2k rpm the oil pressure is around 50 psi on hot oil when new. I used some 5w30. 0w40 and now 5w40 over the years. At 141k miles the pressure is down to 45-48 psi now but no matter what the oil grade the pressure was the same at operating temp.

 

[Mechanician]

It depends on everything.

^This^
There are different types of oil pumps, with different displacements. There are even variable displacement pumps on some engines.
Engines have different tolerances depending on the manufacturer.
If a pump produces more volume than is allowed through the tolerances, the pressure relief valve will open, and the pressure will be set to that calibration. Not all engines are the same.
The RPM of the engine is a big factor.
If the tolerances of the pump itself are loose, a thinner oil will not pump as much volume as a thicker oil.
How thick is thick? How thin is thin? 0w16, and 20w50?
Blanket statements can't really be made accurately.

 

[ZeeOSix]

This has been my experience as well—the hot VS cold works equally with a heavier oil. As long as the bypass isn't overloaded, the pressures will remain nearly the same.

What do you mean by "as long as the bypass isn't overloaded" ?

So you're saying you see the same oil pressure at idle (say 800 RPM) when the oil is both cold and at full temperature? Far from what I see on every engine without a variable displacement oil pump. To clarify, when talking about variable displacement oil pumps, it can get complicated because of all the difference control logic.

What engine with what kind of oil pump are you talking about, and what's the location of the sensor and resolution of the oil pressure gauge?

 

[Propflux01]

A relief is a relief, be it in the block, or by variable pump. What I mean by an overloaded bypass or relief is when the amount of oil put out overwhelmes the amount of oil diversion (relief) that goes back to the pan. Example: Old air cooled VW blocks had only one relief and suffered this issue, and in the later years they added a second relief as the single would get overwhelmed at higher rpm’s. Since the relief was no longer overwhelmed, you’d get the same oil pressure on the gauge using 20w50 as you would with 10w-30.

 

[Jetronic]

What do you mean by "as long as the bypass isn't overloaded" ?

So you're saying you see the same oil pressure at idle (say 800 RPM) when the oil is both cold and at full temperature? Far from what I see on every engine without a variable displacement oil pump. To clarify, when talking about variable displacement oil pumps, it can get complicated because of all the difference control logic.

What engine with what kind of oil pump are you talking about, and what's the location of the sensor and resolution of the oil pressure gauge?

He's saying, if the bypass is sized too small the pressure could rise beyond the opening pressure. I suppose with a very viscous oil that could happen, but not something I expect to see in normal use.

 

[oil pan 4]

Roughly how much of an effect would going up, or down, one weight of oil have on engine oil pressure at operating temperature? For example going from a 10w30 to a 10w40, or a 5w50 to a 5w40 etc. Is the effect significant or, typically, negligible?

It depends on a lot. Say if you were comparing a typical 10w-40 to say amsoil 10w-30 there wouldn't be a big difference. Because amsoil 30 weight oils are usually hitting about 12 cSt or higher at 100c and typical big box store oil say some something like mobil1 40 weights are usually hitting like 14.
But if you were comparing a typical off the shelf big box store 10w-30 to a typical off the shelf 10-40 weight the 30 weight could be around 10.5 cSt and the 40 weight could be around 14 cSt at 100c so there would much more of a difference.
In theory say you had 10.5psi of oil pressure hot at idle with big box store 30 weight and you switch to big box store 40 weight you'd have 14psi hot at idle. That's probably not exactly what would happen but gives you an idea.

 

[twX]

He's saying, if the bypass is sized too small the pressure could rise beyond the opening pressure. I suppose with a very viscous oil that could happen, but not something I expect to see in normal use.

Oil pressure will start to rise above the opening pressure of the relief valve even when there's only a small amount of oil bypassing. Oil pressure can generally get to at least double the PRV opening pressure if the oil is thick enough.

Thicker oil will increase oil pressure in any operating condition, but it will have a much larger effect when the relief valve is not open.

Here's a chart showing oil pressure vs oil temperature (viscosity). Compare the curve at 210°F to the one at 150°F. At low rpm, when the PRV is not open, oil pressure is 140% higher with the thicker oil. At high rpm, when the PRV is open, oil pressure is only 35% higher, since the relief valve is regulating the pressure (but not perfectly).

 

[wemay]

I've used 20 and 30 without any noticeable difference in oil pressure in the Chevy Malibu. It's the only vehicle i've had in years with an oil pressure gauge.

 

[ZeeOSix]

A relief is a relief, be it in the block, or by variable pump. What I mean by an overloaded bypass or relief is when the amount of oil put out overwhelmes the amount of oil diversion (relief) that goes back to the pan. Example: Old air cooled VW blocks had only one relief and suffered this issue, and in the later years they added a second relief as the single would get overwhelmed at higher rpm’s. Since the relief was no longer overwhelmed, you’d get the same oil pressure on the gauge using 20w50 as you would with 10w-30.

He's saying, if the bypass is sized too small the pressure could rise beyond the opening pressure. I suppose with a very viscous oil that could happen, but not something I expect to see in normal use.

No pump pressure relief valve is perfect, especially just the oil fashioned spring loaded type on a fixed displacement PD oil pump. That's the configuration I'm talking about. When the discussion goes into variable displacement PD pumps with all kinds of different control methods and logic, it becomes much more convoluted.

Anyway, the original question was how does a different oil viscosity effect the oil pressure. The only way to answer that is to see only the effect of the viscosity change with all other factors held constant. Here's some data I took on my Z06 (old fashioned PD pump with a spring loaded pressure relief) which shows how the oil pressure at a constant 2500 RPM changed with the oil temperature, which is a essentially a corresponding change in the oil viscosity. The oil was 5W-30 Mobil 1 full synthetic. What's the viscosity change of that oil in 25 deg F increments ... not much.

It's not a huge change, but as said before, if the viscosity increases so will the oil pressure with all other factors being constant. If someone changed from a 5W-30 to a 5W-40, they may only see a one or two PSI change in OP. And you would need an oil pressure sensor and gauge sensitive and accurate enough to see a small change like that.

 

[Jetronic]

Yes, if you push the oil pressure relief open further, pressure will go up, that's how springs work. The system is a pressure limiter, not a control device.

 

[ZeeOSix]

Yes, if you push the oil pressure relief open further, pressure will go up, that's how springs work. The system is a pressure limiter, not a control device.

If the oil pressure relief opens further to shunt more oil as RPM increases, then the oil volume going to the engine will be reduced (rolls over) as RPM increases, and that will cause the oil pressure to roll over more as RPM increases. The oil pressure is still rising slowly as it rolls over because the spring loaded pressure relief valve can't perfectly control the pump output pressure to a constant value. In a variable displacement oil pump system with some good computer feed-back controlled logic, it might be possible to actually control the oil volume and pressure to a constant value as RPM increases.

Oil pumps in the GM LS series of engines (controlled with just a spring loaded relief valve) show this operational phenomena. Here's a graph showing how the oil pressure still rises, but is rolling off (departure from the exponential part of the curve in the 2000-3000 RPM range) after the pressure relief valve starts opening.