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Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 21,897
Not Ranked
Quote:
Originally Posted by blykins
What I like to see for my personal stuff and what the customers like to see are generally two different things.
So, measuring at the top of the oil filter adapter, and this is for your own personal stuff only, say, maybe 15 PSI looks good to you at idle when it's nice and hot, and idling below 1000?
The point of a bearing is to keep the rotating shaft from touching a housing that could be stationary (mains) or moving at a different speed (rods). The oil pressure is not what keeps the two pieces from touching.
If you think of two flat plates with oil between them, when one plate moves the oil molecule touching the moving plate moves with that plate and the oil molecule touching the stationary plate stays put with it. All the oil molecules between are moving at different speed (some speed between zero and the speed of the moving plate). Essentially there are different layers of oil moving at different speeds. This is causing the oil to shear. The shearing action acts like a wedge that lifts the plate. The higher the speed differential the more the wedge can lift. So the pressure is not what keeps the two plates separated. It is very similar to the hydroplane affect, like spinning a tire on water.
Oils have polymerized chains of molecules that are entangled. The shearing action straightens the chains and causes shear thinning (lower viscosity). The shearing action is a form of friction and causes heat, which further lowers the viscosity. The lower the viscosity the lower the wedging affect (the less lifting force to keep parts separated). If it wasn't for the heat, a small amount of oil could remain in the bearing indefinitely. However in an engine at high shear rates, the heat build up is very rapid. So, oil must be flushed through the bearing, at a rate that is fast enough to keep the oil from over heating and thinning out to the point of metal contact.
Therefore the pressure is not what is important. The flow is what is important. The flow can be measured by the pressure drop across an orifice (restriction). Flow is proportional to the square root of the pressure drop. If you consider the entire engine as the restriction, and consider the oil pan to be at a constant pressure, then the oil pressure reading is a pressure drop, and it is proportional to the flow. So the oil pressure is a very inaccurate poor mans measure of the oil flow.
Now the problem is that the pressure drop, flow correlation is different for different viscosity oils. The colder the oil the higher the viscosity and the higher the pressure drop for a given flow. The point to all this is that the pressure is not what needs to be measured. At a given temp on a given engine at a set of conditions a certain amount of oil pressure may be adequate. At a different set of conditions, twice that amount of oil pressure may result in an engine failure.
Also from one engine to another with different paths lengths, and clearances, the required oil pressure to meet the needed flow is different. The exact pressure needed for any one engine is a different number at different conditions. What is known for certain is that zero oil pressure is zero flow and a disaster in a short time. How much pressure is the minimum is a crap shoot.
If you are pumping any more oil volume than you need you are wasting Hp. One drop too little is disaster. With so many variables everyone wants to error on the side of pumping too much.
I'm surprised with all the EPA and other government requirements the manufactures haven't went with variable speed electric motors on both the oil pump and water pump to get better MPG.
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 21,897
Not Ranked
Quote:
Originally Posted by olddog
The point of a bearing is to keep the rotating shaft from touching a housing that could be stationary (mains) or moving at a different speed (rods). The oil pressure is not what keeps the two pieces from touching.
If you think of two flat plates with a oil between them, when one plate moves the oil molecule touching the moving plate moves with that plate and the oil molecule touching the stationary plate stays put with it. All the oil molecules between are moving at different speed (some speed between zero and the speed of the moving plate). Essentially there are different layers or oil moving at different speeds. This is causing the oil to shear. The shearing action acts like a wedge that lifts the plate. The higher the speed differential the more the wedge can lift. So the pressure is not what keeps the two plates separated.
Oils have polymerized chains of molecules that are entangled. The shearing action straightens the chains and causes shear thinning (lower viscosity). The shearing action is a form a friction and causes heat, which further lowers the viscosity. The lower the viscosity the lower the wedging affect (the less lifting force to keep parts separated). If it wasn't for the heat, a small amount of oil could remain in the bearing indefinitely. However in an engine at high shear rates the heat build up is very rapid. So oil must be flushed through the bearing at a rate that is fast enough to keep the oil from over heating and thinning out to the point of metal contact.
Therefore the pressure is not what is important. The flow is what is important. The flow can be measured by the pressure drop across an orifice (restriction). Flow is proportional to the square root of the pressure drop. If you consider the entire engine as the restriction, and consider the oil pan to be at a constant pressure, then the oil pressure reading is a pressure drop and it is proportional to the flow. So the oil pressure is a very inaccurate poor mans measure of the oil flow.
Now the problem is that the pressure drop, flow correlation is different for different viscosity oils. The colder the oil the higher the viscosity and the higher the pressure drop for a given flow. The point to all this is that the pressure is not what needs to be measured. At a given temp on a given engine at set of conditions a certain amount of oil pressure may be adequate. At a different set of conditions, twice that amount of oil pressure may result in an engine failure.
Also from one engine to another with different paths lengths, and clearances, the require oil pressure to meet the needed flow is different.
If you are pumping any more oil volume than you need you are wasting Hp. One drop too little is disaster. With so many variables everyone want to error on the side of pumping too much.
I'm surprised with all the EPA and other government requirements the manufactures haven't went with variable speed electric motors on both the oil pump and water pump to get better MPG.
Well that sounds to me like "if the little red light ain't on, you're good to go."
Well you're engine can fail at 4 times the pressure required to turn off the little red light, when at high rpm high loads. This is why racers like oil pressure gauges, so they can see what the pressure is at those conditions.
The light winking at you at idle is not close to a failure at idle, but what is the pressure when you go WOT. You have no idea. You just know that the light went out.
On a new engine with a clean screen on the oil pump, the manufacturer can count on the fact that a gear type oil pump is relatively a positive displacement pump, thus flow is proportional to rpm. However after many miles, the warranty is gone. A plugged oil pump screen restricting oil flow will limit the flow to a maximum. Now the flow is not proportional to the rpm. The idiot light will not show that, only a pressure gauge will show you that. Many an engine has failed to too little oil flow and the little light never came on.
The manufacturer is not overly concerned about this unfortunate event, because one they do not have to pay and two you need a new car.
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 21,897
Not Ranked
Quote:
Originally Posted by olddog
...but what is the pressure when you go WOT. You have no idea. You just know that the light went out.
Well I can honestly tell you that I have never looked at my gauges during WOT, not even for an instant. But I did spring for the Melling 57HV pump, as my builder had the same view of oil volume flow as yours.
Cobra Make, Engine: CSX #4xxx with CSX 482; David Kee Toploader
Posts: 3,574
Not Ranked
Well, that makes it pretty simple! I remember years ago guys telling me that the trick was "blueprinting" your oil oil pump (clearances), which always seemed to be BS to me!
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All that's stopping you now Son, is blind-raging fear.......
Cobra Make, Engine: ERA #732, 428FE (447 CID), TKO600, Solid Flat Tappet Cam, Tons of Aluminum
Posts: 21,897
Not Ranked
Quote:
Originally Posted by Bernica
Well, that makes it pretty simple! I remember years ago guys telling me that the trick was "blueprinting" your oil oil pump (clearances), which always seemed to be BS to me!
No, a 9 PSI red light on the dash is simpler. And if it's flickering, or even blinking, just keep driving. It's when it stays on for good that you need to start paying attention.
Well I can honestly tell you that I have never looked at my gauges during WOT, not even for an instant. But I did spring for the Melling 57HV pump, as my builder had the same view of oil volume flow as yours.
That's the advantage of a small block. I glance at the gauges frequently, as long as the tires seem to be hooked to the ground.
You macho FE boys are too scared to take your eyes off the road.
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