Who believes you really need 10 PSI oil pressure....

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Re: Who believes you really need 10 PSI oil pressure....

Post by Belgian1979 »

340king wrote:There is actually a thermal balance equation that needs some attention here. The work that goes into compressing that oil more adds heat to the oil. So the oil coming out of the pump at a higher pressure has to be at a higher temperature. So once it reaches equilibrium temp, it may well be higher. The key point to all of this is that the corollary is probably more important. If you are well over that 10 lbs per 1,000 rpm rule of thumb, you are probably wasting energy in the oiling system.
correct. The main advantage of a bypass to the pump is that it doesn't heat up the oil but imo the oil passing throught the tiny bypass hole will experience a pressure drop which could (!) cause cavitation in that area...+ it places the pickup at a disadvantage.

The whole oil circuit and pump is basically a balanced situation.
Last edited by Belgian1979 on Thu Aug 14, 2014 2:23 am, edited 1 time in total.
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Re: Who believes you really need 10 PSI oil pressure....

Post by Belgian1979 »

joespanova wrote:
MadBill wrote:The higher pressure increases the flow rate so that at a given temperature delta within the bearing, more of the heat generated there can be carried off. Wet or dry sump, the temperature will stabilize (hopefully at an acceptable level) at some point, but the bearings will run cooler at the higher flow rate regardless. Visualize the opposite extreme: A small sealed volume of oil surrounding the bearing, with zero flow in or out. The temperature will just go up and up...
Bill.........I get it....theoretically..........I just am not buying that in "reality" there is a measureable difference........sorry.
What's important here to remember is that oil viscosity changes in respect to the oil temps. You need to stay within a specific range (let's say 200-212). However, you have to remember that at the bearing oil temps are way higher...

What's also vitally important is that the wedge forming/load bearing capabilities of the oil strongly correlate to the viscosity of the oil. If the viscosity is too low, the load bearing/wedge forming capacity of the oil will go down and you get into boundry lubrication.

It's almost the same as when you are running 20/50 oil with 60 psi of pressure and you change to 0/20 and your oil pressure drops to 40 psi. This is a strong indication that you oil flows out of your bearings more easily with the tin oil. If however that 0/20 doesn't have enough viscosity it could be that you are in boundry lubrication at that point.

I believe for a sbc the limit is somewhere around 5/30 but I could be wrong.

And even then, assuming you run thin oil and low pressure, you could also cool your oil in the sense that you get a higher viscosity. Maybe that's how the Nascar teams get by with 35 lbs of oil pressure. Don't know.
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Re: Who believes you really need 10 PSI oil pressure....

Post by RednGold86Z »

340king wrote:There is actually a thermal balance equation that needs some attention here. The work that goes into compressing that oil more adds heat to the oil. So the oil coming out of the pump at a higher pressure has to be at a higher temperature. So once it reaches equilibrium temp, it may well be higher. The key point to all of this is that the corollary is probably more important. If you are well over that 10 lbs per 1,000 rpm rule of thumb, you are probably wasting energy in the oiling system.
Oil (without foam or bubbles) is relatively incompressible - it's not going to heat up simply by having it's pressure increased - the work isn't going into the liquid (it's volume isn't changing) - it's just using it to transmit the work elsewhere (holding parts apart from each other, and squirting oil around through those crevices and pushrods). Almost all of the heat comes from the parts it's in contact with that get combustion heat, some from getting sheared around in bearings, and some from valvesprings.

Excess oil pressure is a power robber in one way at least - it takes more power to drive the pump, and if that's coming from the cam, it could be twisting it.
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Re: Who believes you really need 10 PSI oil pressure....

Post by clshore »

Belgian1979 wrote:
clshore wrote:
MadBill wrote:The higher pressure increases the flow rate so that at a given temperature delta within the bearing, more of the heat generated there can be carried off. Wet or dry sump, the temperature will stabilize (hopefully at an acceptable level) at some point, but the bearings will run cooler at the higher flow rate regardless. Visualize the opposite extreme: A small sealed volume of oil surrounding the bearing, with zero flow in or out. The temperature will just go up and up...
Please explain how a fixed displacement pump running at a given RPM can exhibit different flow rates ...
Not at a given rpm...at a given rpm, the pump has X amount of flow. But the flow increases linear with the rpm.
Of course, that's the point of my question, in response to the statement that higher pressure would yield higher flow.
The reality is that in a given motor, a pump with greater fixed swept volume per revolution will result in greater pressure at same RPM (until bypass is reached).
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Re: Who believes you really need 10 PSI oil pressure....

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RednGold86Z wrote:
340king wrote:There is actually a thermal balance equation that needs some attention here. The work that goes into compressing that oil more adds heat to the oil. So the oil coming out of the pump at a higher pressure has to be at a higher temperature. So once it reaches equilibrium temp, it may well be higher. The key point to all of this is that the corollary is probably more important. If you are well over that 10 lbs per 1,000 rpm rule of thumb, you are probably wasting energy in the oiling system.
Oil (without foam or bubbles) is relatively incompressible - it's not going to heat up simply by having it's pressure increased - the work isn't going into the liquid (it's volume isn't changing) - it's just using it to transmit the work elsewhere (holding parts apart from each other, and squirting oil around through those crevices and pushrods). Almost all of the heat comes from the parts it's in contact with that get combustion heat, some from getting sheared around in bearings, and some from valvesprings.

Excess oil pressure is a power robber in one way at least - it takes more power to drive the pump, and if that's coming from the cam, it could be twisting it.
Red,

I think the amount of hp it takes to drive an oil pump is exagerated. I can prime my oil circuit to 60 psi with a simple hand drill which has something like 0,75 hp. This was with a HV pump. I think a SV pump would still need 0,5 hp, so the difference between a HV HP pump and a SV SP pump is maybe 0,25 hp. At higher rpm, higher volumes it would take more hp, but I do not think it will take 20 or so hp to drive the pump. For the same reason I do not think a cam would twist because of it.
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Re: Who believes you really need 10 PSI oil pressure....

Post by Kevin Johnson »

clshore wrote:Guys, please stop and think a minute:
Oil pumps are positive displacement devices.
Each turn of the pump flows the *same* volume (except for internal leakage, which is minor).
So unless you have a variable speed oil pump, the *volume* is always proportional to the RPM.

Figure the cu in (or cc) per revolution, and multiply by RPM/2 (if driven from cam) to get pump flow.

The idea of measuring the amount of bypass oil is a good one, but the pump is still going to flow same.
With respect to rod bearing flow, you are neglecting the centrifugal force imparted against the flow on the approach in the circuit in the rotating crankshaft to the closest proximity to the crankshaft longitudinal axis of rotation. There will be some minimum pressure required to overcome this force that varies with rpm. If the minimum pressure is not met there will be no flow despite the presence of oil pressure from the pump.

There are other ways to provide oil to a crankshaft so any of these discussions should be prefaced with an understanding of the circuit involved and the rpm in question.
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Re: Who believes you really need 10 PSI oil pressure....

Post by bmcdaniel »

clshore wrote:Guys, please stop and think a minute:
Oil pumps are positive displacement devices.
Each turn of the pump flows the *same* volume (except for internal leakage, which is minor).
So unless you have a variable speed oil pump, the *volume* is always proportional to the RPM.

Figure the cu in (or cc) per revolution, and multiply by RPM/2 (if driven from cam) to get pump flow.

The idea of measuring the amount of bypass oil is a good one, but the pump is still going to flow same.
This is probably why my old Ford service manuals from the late 60s & early 70s say to measure oil pressure at 2,000 rpm. There had to be some common standard used to compare different engines/pumps. IIRC, for the 289s and 302s they specified 20 psi @ 2,000 rpm which would be 10 psi for each 1,000 rpm, probably someone extrapolated that for other rpms, but that wasn't Ford's intention.
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Re: Who believes you really need 10 PSI oil pressure....

Post by novadude »

Let's take typcial street /strip SBC in reasonable condition (OK bearing clearances) and 6000 rpm limit. Why would anyone ever use anything other than a standard volume pump with stock 45 psi relief spring and 5W-30 or 10W30 oil?

I honestly can't think of any good reason to use anything else, but maybe I am missing something.
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Re: Who believes you really need 10 PSI oil pressure....

Post by Belgian1979 »

oil lines, oil cooler, accusump, valve spring oilers to name a few.
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Re: Who believes you really need 10 PSI oil pressure....

Post by les327 »

Smoke Y would quote "10 psi per hundred HP." Lots of good info in this thread thanks.
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Re: Who believes you really need 10 PSI oil pressure....

Post by clshore »

Kevin Johnson wrote:
clshore wrote:Guys, please stop and think a minute:
Oil pumps are positive displacement devices.
Each turn of the pump flows the *same* volume (except for internal leakage, which is minor).
So unless you have a variable speed oil pump, the *volume* is always proportional to the RPM.

Figure the cu in (or cc) per revolution, and multiply by RPM/2 (if driven from cam) to get pump flow.

The idea of measuring the amount of bypass oil is a good one, but the pump is still going to flow same.
With respect to rod bearing flow, you are neglecting the centrifugal force imparted against the flow on the approach in the circuit in the rotating crankshaft to the closest proximity to the crankshaft longitudinal axis of rotation. There will be some minimum pressure required to overcome this force that varies with rpm. If the minimum pressure is not met there will be no flow despite the presence of oil pressure from the pump.

There are other ways to provide oil to a crankshaft so any of these discussions should be prefaced with an understanding of the circuit involved and the rpm in question.
Kevin, I understand what you are saying, but my point is that the pump swept volume is fixed per revolution.
That amount of oil WILL flow, unless the HP needed to drive the pump exceeds the motor output, or the oil pump drive breaks, or the pump body explodes.
The oil itself will not compress in any meaningful way.
The amount of pressure developed is precisely determined by the motors' internal resistance to that flow, ie sum of bearing clearances, oil squirters, oil passage hydraulic resistance, etc.
So if internal centrifugal force must be overcome for the oil to flow, it will show up as increased system pressure at the pump outlet.
The oil will flow no matter what.
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Re: Who believes you really need 10 PSI oil pressure....

Post by Kevin Johnson »

Kevin Johnson previously wrote: With respect to rod bearing flow, you are neglecting the centrifugal force imparted against the flow on the approach in the circuit in the rotating crankshaft to the closest proximity to the crankshaft longitudinal axis of rotation. There will be some minimum pressure required to overcome this force that varies with rpm. If the minimum pressure is not met there will be no flow despite the presence of oil pressure from the pump.

There are other ways to provide oil to a crankshaft so any of these discussions should be prefaced with an understanding of the circuit involved and the rpm in question.
clshore wrote: Kevin, I understand what you are saying, but my point is that the pump swept volume is fixed per revolution.
That amount of oil WILL flow, unless the HP needed to drive the pump exceeds the motor output, or the oil pump drive breaks, or the pump body explodes.
The oil itself will not compress in any meaningful way.
The amount of pressure developed is precisely determined by the motors' internal resistance to that flow, ie sum of bearing clearances, oil squirters, oil passage hydraulic resistance, etc.
So if internal centrifugal force must be overcome for the oil to flow, it will show up as increased system pressure at the pump outlet.
The oil will flow no matter what.
There are other places for the oil to flow -- that is not in question. The mains will certainly benefit right up to when the rod punches through the block.
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Re: Who believes you really need 10 PSI oil pressure....

Post by MadBill »

clshore wrote:..The oil will flow no matter what.
You are still overlooking the bypass valve, which can be active to some extent much of the time. In a standard Chev pump, yes, the oil will emerge from the pockets formed by the gear teeth and the housing at a rate proportional to the RPM, BUT thereafter anything from zero percent up can be diverted back to the inlet side by the action of the bypass valve, without entering the rest of the engine's oiling system. If the valve is modulating the system pressure (i.e. more than 0% bypass), then the reduced system backpressure due to lowered restriction at the bearings will result in a slight closing of the valve. The volume will increase and the pressure delivered to the system will remain essentially unchanged.
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Re: Who believes you really need 10 PSI oil pressure....

Post by clshore »

MadBill wrote:
clshore wrote:..The oil will flow no matter what.
You are still overlooking the bypass valve, which can be active to some extent much of the time. In a standard Chev pump, yes, the oil will emerge from the pockets formed by the gear teeth and the housing at a rate proportional to the RPM, BUT thereafter anything from zero percent up can be diverted back to the inlet side by the action of the bypass valve, without entering the rest of the engine's oiling system. If the valve is modulating the system pressure (i.e. more than 0% bypass), then the reduced system backpressure due to lowered restriction at the bearings will result in a slight closing of the valve. The volume will increase and the pressure delivered to the system will remain essentially unchanged.
Bill, if you actually measure bypass flow vs pressure, you will find that there is virtually no bypass flow until you reach the cracking pressure.
Examine a real regulator (as used in hydraulic applications) and you will see a profound difference in how they are made, and how they operate.
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Re: Who believes you really need 10 PSI oil pressure....

Post by Kevin Johnson »

clshore wrote: Bill, if you actually measure bypass flow vs pressure, you will find that there is virtually no bypass flow until you reach the cracking pressure.
Examine a real regulator (as used in hydraulic applications) and you will see a profound difference in how they are made, and how they operate.

The original assertion by Bill was that increased pressure yielded increased flow. "The higher pressure increases the flow rate ..."

In a positive displacement pump running at a given rpm into a given circuit it is not possible to raise the pressure by adjusting the bypass pressure if the bypass pressure has not been already been equaled or exceeded. Davidson's principle of charity in interpreting an argument demands that a specious interpretation of, say, decreasing bearing clearances to increase pressure at a given rpm not be used, nor by lowering the system temperature and thereby increasing the viscosity of the fluid. Therefore, implicit in Bill's argument is that the pump must increase rpms in order to reach the new higher bypass pressure. For there not to be increased flow at the new higher bypass pressure the rpm must not increase. I do not think this is empirically observed with typical engine oil pumps. Indeed, you point out that there must be increased flow at a higher rpm.

Then the counterargument must be altered such that at some chosen asymptotic level of pressure there is no detectable flow change for an incremental increase in pressure via incrementally increased rpm. I am quite sure that such a level exists but as you point out the physical demands placed on the pump and circuit structure would be enormous.
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