Oil Pressure Does Not Equal Lubrication

[bruce381]

this thread is confusing Lubrication with flow not the same.
i think I miss the point but that has never stopped me before.

I can tell you this as a truth using positive displacment Gear pumps running at the SAME speed the following will apply.

Pressure will rise and flow will be lower with a heavy oil compared to a thin oil which would have a lower pressure and a higher flow rate. same temp same day same everything.
bruce

 

[1sttruck]

"Pressure does NOT mean increased flow. Ever turn on a faucet connected to a hose, then kink the hose? The pressure builds, but, if the kink is sufficient, you can have very little, or even zero flow. If you have molasses coming out of the spigot at 60 psi, it won't flow through the hose as fast as water at 60 psi."

But opening the spigot generates more flow as well as more pressure, as water will go farther coming out of a nozzle.

I think that people are trying to use a generalization to cover all specific cases, which won't work:

1. Engine cold - higher pressure, due to cold and/or thicker oil, doesn't always mean more flow.

2. Engine warm - higher pressure with thicker oil doesn't always mean more flow, but it could as lower pressure can indicate lower flow.

As I recall from my motorcycle days a roller crank likes more flow at lower pressure than a plain bearing crank, so in all cases you need adequate flow, which means in all cases a minimum pressure due to the way that our lubrication systems are designed.

 

[XS650]

Bruce, you are absolutly correct; however, for a reasonable range of conditions, the differances in flow are small enough that they don't matter in the big picture.

 

[bruce381]

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bruce, does the system that you are quoting have a lot of long, small bore pipes ?

if so, then maybe the internal friction is increasing pump internal leakage, or reduced suction head is causing incomplete pump filling.

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Something like that must be occurring. Otherwise X gpm was X gpm last time I checked. Only the pressure should vary with visc.

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System uses 2 and 3 inch piping with flooded GEAR pumps in a sump like a car engine SAME syle as in a ford or chevy motor. Just like a BIG IC engine same thing delivery system is static and tank end is like a bearing bye the way tank fill is at top so NO back presure due to filling from bottom aganst a pressure head.

loss is due to pump internal "slipage" or "leakage" as pressure rises due to higher vis just as in a car engine.
same deal

bruce

 

[Bryanccfshr]

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Bryan, it sounds like you need a big oil pan heater. A block heater does a limited amount of good for the oil in the pan.

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We have a policy to let the vehicles continue to idle anytime the temperature is below -30. That does not mean a sudden drop in temp does not sneak up on us and allow our fleet to experience a loud chorus of cold starts with the associated noise. You can tell the difference in the trucks that have 0w30 vs- 15 w40- most definately on those mornings.

 

[Ron AKA]

Camarolt76 has it figured out. The key point here is that positive displacement pumps are essentially constant flow devices. If all other things are equal, the pressure just indicates how much resistance there is in the system. As bearings wear resistance goes down and pressure goes down. Or, the pump can wear and bypass internally and pressure goes down. That is why low oil pressure is felt to be bad - and it is. Flow passages should never plug up with good oil and reasonable OCI's, and a result high oil pressure is not a symptom of a bad engine.

The exception to this is when the maximum pressure control valve setting is reached. At that point then flow is bypassed through the valve and flow is reduced to the engine. However, that is essentially not going to happen unless the oil is too thick (cold), because the wrong viscosity if being used.

Changing oil viscosity (weight) is another matter. Yes you can have a worn out engine and put in a heavy oil (or add STP), then oil pressure will go up. I would agree with the original poster that this is not necessarily a good thing as it really just confirms the engine is worn. However, it could provide a better distribution of oil in the engine than you would have if oil pressure is too low, and may be about all you can do short of rebuilding the engine.

I disagree with the concept that low oil pressure means more flow. It does not with a positive displacement pump. The only way you can get more flow is to change the oil pump. In that case if the same oil and engine is used then the increased flow resistance pushes up pressure.

I would argue however, that is not necessarily good, as the engine needs the right amount of oil - not too much or too little.

I have only one question. What was the point of the original post?

 

[Gary Allan]

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I have only one question. What was the point of the original post?

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Not to assume that high pressure automatically means good lubrication. It's to counter heavyhead's notions. More does not always mean better.

 

[Ron AKA]

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Not to assume that high pressure automatically means good lubrication. It's to counter heavyhead's notions. More does not always mean better.

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I would agree then, if the high pressure is caused by high viscosity oil. However, if the oil viscosity has not been changed and the pressure is down then that is not good either.

 

[Bryanccfshr]

I apologize for getting the tread off track with my extreme example, but this extreme is repeated evey time a vehicle is started to varying degrees and durations.

If there is a "clogged artery" in the oil system this can cause false hopes of good oil pressure. I also believe the primary reason for oil pressure loss in older engines is wear on the oil pump itself...not on the other parts in the engine.

I am no medical practioner but this is basic plumbing and fluid dynamics. If we consider the oil pump the heart of the engine what is done in medicine when the heart is not providing enough flow? What is normally prescribed for the blood? If a heart is failing what should be done with the heart? Should the blood be thickened so the heart gets good pressure despite the fact that circulation in the body suffers or do we have leaky valves repaired and thin the blood to ensure circualtion to all the organs?

If an old engine is showing low oil pressure what is the better solution? is it to replace/repair the pump or put in thicker oil?

This thread is very stimulating.

 

[bruce381]

""I disagree with the concept that low oil pressure means more flow. It does not with a positive displacement pump.""

Very system dependant.

In my case wrong.

As I posted above filling a 7,000 blend tank with a thin oil will have low pressure and high flow and a fill time of about 45 minutes.

A thicker oil with a higher pressure and lower flow rate will fill in about 90 minutes.
bruce

 

[bruce381]

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Bruce, you are absolutly correct; however, for a reasonable range of conditions, the differances in flow are small enough that they don't matter in the big picture.

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see above post

 

[427Z06]

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System uses 2 and 3 inch piping with flooded GEAR pumps in a sump like a car engine SAME syle as in a ford or chevy motor. Just like a BIG IC engine same thing delivery system is static and tank end is like a bearing bye the way tank fill is at top so NO back presure due to filling from bottom aganst a pressure head.

loss is due to pump internal "slipage" or "leakage" as pressure rises due to higher vis just as in a car engine.
same deal

bruce

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bruce, how long are the pipes (friction variable)? How far are the pumps raising the fluid(head variable)?

 

[ffxfina3]

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However, when the oil is up to temp and the engine is idling, my pressure is low. My plans are to replace my oil pump with a higher volume pump, and then see if I can go to a lower viscosity oil (currently 15w40 HDEO -- would like to then try a 5w30 HDEO).

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Bryan, that's not a good thing to do. With thinnner oil and higher volume pump with what sounds like worn bearings, you will be flinging a lot of excess oil around inside your engine from the piston rod big ends. That will load up your cylinder walls and your worn piston rings may have problems keeping up.

Also, as long as your oil pressure at operating speeds is OK, don't get too alarmed about idle speed oil pressure.

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OK, perhaps I'll hold off on this plan. I did some more research and it appears that a higher volume (than stock) pump can also suck my oil pan dry. My first course of action is to measure the actual oil pressure at idle (all I have on my dash is an idiot light). Then I'll proceed from there. Thanks for throwing the warning my way.

-Bryan

 

[Shannow]

bruce, does the system that you are quoting have a lot of long, small bore pipes ?

if so, then maybe the internal friction is increasing pump internal leakage, or reduced suction head is causing incomplete pump filling.

 

[bruce381]

Quote:

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System uses 2 and 3 inch piping with flooded GEAR pumps in a sump like a car engine SAME syle as in a ford or chevy motor. Just like a BIG IC engine same thing delivery system is static and tank end is like a bearing bye the way tank fill is at top so NO back presure due to filling from bottom aganst a pressure head.

loss is due to pump internal "slipage" or "leakage" as pressure rises due to higher vis just as in a car engine.
same deal

bruce

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bruce, how long are the pipes (friction variable)? How far are the pumps raising the fluid(head variable)?

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Maybe 30 feet but there is NO varible whole oil piping system is NOT changed only vis of oil pumped from tank NOTHING changes other than temp BUT both oils I quoted on were at 80F at the time I mesured this stuff.

Like I said like a BIG oil system in any old car motor just much bigger with "set" clearances. What happens here happens in a IC engine.

bruce

 

[mechtech2]

An electric motor doesn't have the heavily biased side loading of a rod or main bearing, and operates under much less demanding conditions. I really don't see how it's particular lubrication needs correspond to that of an internal combustion engine.
I don't believe that capillary action is responsible for rod and main bearing lubrication, in a system designed with an oil pump. [I am well aware of splash oiling on lawnmower type engines.]
If you tap into the upper, unloaded half of a plain rod or main bearing, you will have system oil pressure, not zero pressure.

 

[den]

Look at some of the old Harleys.Mostly roller and needle bearings.Very little pressure.The book use to tell you even if the oil pump fails you good drive easy for some time.I have seen this myself.

 

[Titan]

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If there is a "clogged artery" in the oil system this can cause false hopes of good oil pressure. I also believe the primary reason for oil pressure loss in older engines is wear on the oil pump itself...not on the other parts in the engine.

I am no medical practioner but this is basic plumbing and fluid dynamics. If we consider the oil pump the heart of the engine what is done in medicine when the heart is not providing enough flow? What is normally prescribed for the blood? If a heart is failing what should be done with the heart? Should the blood be thickened so the heart gets good pressure despite the fact that circulation in the body suffers or do we have leaky valves repaired and thin the blood to ensure circualtion to all the organs?

If an old engine is showing low oil pressure what is the better solution? is it to replace/repair the pump or put in thicker oil?

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First, when people, even doctors, use the term "thin the blood", they are not talking about lowering the viscosity. They are talking about decreasing the tendency to clot. Thinning the blood is one of the WORST terms ever devised to describe a medical condition. Heparin, coumadin, etc., do NOT thin the blood, they just make it less likely to clot.

When a heart is failing, the cure depends upon why it is failing. IF it is failing because of lack of pumping force, that could be because the arteries are too restrictive (High Blood Pressue, or Hypertension)...so, you relax the vessels...make them more open. If it is failing due to inadequate blood flow through the coronary arteries, you open up the blockages in the coronary arteries (Stent, bypass operation). If it is failing due to a primary muscular weakness, you try and stimulate a more forceful contraction...but, in this case, you may be on a very slippery slope and heart failure can soon occur no matter what you do. If pressure is low due to valvular stenosis, you have to open the valve wider. If pressure is low due to overdilation of the heart, you decrease vascular volume or replace a leaking aortic valve. In almost all cases, with the exception of hypervolemic conditions, you aren't messing with the viscosity of the blood, you're fixing the pumping problem.

Unlike an engine, blood viscosity changes are usually a RESULT of a problem, not the cause of the problem. There are exceptions, but, let's not get more boring than I already have! Blood that is too thick requires the heart to work harder...and can cause sludging problems (not the same sludge as in an engine).

 

[Gary Allan]

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bruce, does the system that you are quoting have a lot of long, small bore pipes ?

if so, then maybe the internal friction is increasing pump internal leakage, or reduced suction head is causing incomplete pump filling.

---

Something like that must be occurring. Otherwise X gpm was X gpm last time I checked. Only the pressure should vary with visc.

 

[Ron AKA]

[quoteAs I posted above filling a 7,000 blend tank with a thin oil will have low pressure and high flow and a fill time of about 45 minutes.

A thicker oil with a higher pressure and lower flow rate will fill in about 90 minutes.
bruce

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Are you sure this is a positive displacement pump. What you describe fits the charateristics of a centrifical pump. The exception would be if the pressure is so high the relief valve is opening.