A brief discussion of oil pressure, etc.
03-09-2007, 12:26 AM
#18
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Rich, this is how I think of it. Can you please tell me how it is flawed.
1.Looking from the top, I see a roller in a box. You need oil to completely fill the gap between the roller and the top walls of its container. If you try to provide more oil after you filled the gaps, you will generate pressure, which isn't needed.
2. Now I am looking from the bottom of the roller coming into contact with the bottom of the container. Now I am imaging a 100lb cam sitting touching the bottom of the container. Wouldn't the oil need pressure to pick this cam up so it isn't touching the bottom of the container?
So is anyone using insulator material?
Should I knit an engine cozy?
1.Looking from the top, I see a roller in a box. You need oil to completely fill the gap between the roller and the top walls of its container. If you try to provide more oil after you filled the gaps, you will generate pressure, which isn't needed.
2. Now I am looking from the bottom of the roller coming into contact with the bottom of the container. Now I am imaging a 100lb cam sitting touching the bottom of the container. Wouldn't the oil need pressure to pick this cam up so it isn't touching the bottom of the container?
Should I knit an engine cozy?
Last edited by number77; 03-09-2007 at 12:42 AM.
03-09-2007, 07:24 AM
#19
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It is definitly NOT oil pressure that keeps the shaft and the bearing from making metal to metal contact with each other. Is water on the road under pressure when a car rides on it and hydroplanes? No, and that's the best way to think of the oil wedge - analogous to hydroplaning. Hell, there are sealed bearings that work fine with no oil pressure at all.
One way to prove to yourself that oil pressure at the bearing could not possibly support the shaft: Main bearing diameter in a SBC is 2.45". Let's think of a bearing with a 1" width and conceptualize that the shaft rides on the bottom 1/2 of the bearing (during the power stroke for that particular cylinder). The area of the bottom bearing would be ~3.5sq.in. If oil pressure supported the shaft (pushed it "away" from the bearing) and was 50psi, all it could support would be ~175lb. Obviously, the load is a lot more than that or we could use "spaghetti strap" conecting rods!
The pressure is generated in the oil passages and the oil between the bearing and the shaft is not under pressure, except that created by the force exerted by the shaft. Oil pressure is needed to assure that enough oil is supplied to replace the oil that leaks out from the sides of the bearing. I don't know enough about bearings to know whay a sealed bearing wouldn't work, but it must not or we would see it. I suspect it has to do with heat more than anything else.
As has been stated in this thread, oil serves a cooling function. And in that regard, oil at even 250 degrees is a lot cooler than the temperatures where we worry about metal. Hot oil works best because it slipperier. The trick is that from the oil chemists point of view, it has to maintain shear strength. And as it get too hot, it oxidizes and then you have trouble. I think most OEM's aim for 200-220 degrees. I haven't had a car with an oil temp gauge for years, the last one I did was a VW "Rabbit" GTI that I road raced. I would see 240-250 degrees when racing and never had any oil related issues. I changed the oil before and after each race weekend. I am not a frequent oil changer on my drag race car. Did it once last season (~75 passes). The issue in the race car is getting the oil hot enough to boil off the methanol that finds its' way into the crankcase. Do that, and the oil looks new when drained. I "recycle" it in my tow vehicle, as many old guys do.
I have to chuckle at the idea of 3,000m oil changes with an expensive synthetic oil. If you use synth, unless you are an off-roader or drive such that the oil never gets warm, I'd advise 7,500m change intervals. I use dino in my DD and change about every 5,000 miles.I usually buy my DD's new and put 100-120,000m on them before replacement. Typically, oil pressure does not drop over that period, indicating no significant wear. Around here, the car will rust away long before the motor wears out. I suppose if I bought something very exotic, hard and expensive to fix, that I didn't plan to work on unless necessary, that I would not use when the roads were salty, I would do 3,000m changes. Something like a high-end Ferrari. Otherwise, a complete waste of time, money, and resources IMHO.
Rich
One way to prove to yourself that oil pressure at the bearing could not possibly support the shaft: Main bearing diameter in a SBC is 2.45". Let's think of a bearing with a 1" width and conceptualize that the shaft rides on the bottom 1/2 of the bearing (during the power stroke for that particular cylinder). The area of the bottom bearing would be ~3.5sq.in. If oil pressure supported the shaft (pushed it "away" from the bearing) and was 50psi, all it could support would be ~175lb. Obviously, the load is a lot more than that or we could use "spaghetti strap" conecting rods!
The pressure is generated in the oil passages and the oil between the bearing and the shaft is not under pressure, except that created by the force exerted by the shaft. Oil pressure is needed to assure that enough oil is supplied to replace the oil that leaks out from the sides of the bearing. I don't know enough about bearings to know whay a sealed bearing wouldn't work, but it must not or we would see it. I suspect it has to do with heat more than anything else.
As has been stated in this thread, oil serves a cooling function. And in that regard, oil at even 250 degrees is a lot cooler than the temperatures where we worry about metal. Hot oil works best because it slipperier. The trick is that from the oil chemists point of view, it has to maintain shear strength. And as it get too hot, it oxidizes and then you have trouble. I think most OEM's aim for 200-220 degrees. I haven't had a car with an oil temp gauge for years, the last one I did was a VW "Rabbit" GTI that I road raced. I would see 240-250 degrees when racing and never had any oil related issues. I changed the oil before and after each race weekend. I am not a frequent oil changer on my drag race car. Did it once last season (~75 passes). The issue in the race car is getting the oil hot enough to boil off the methanol that finds its' way into the crankcase. Do that, and the oil looks new when drained. I "recycle" it in my tow vehicle, as many old guys do.
I have to chuckle at the idea of 3,000m oil changes with an expensive synthetic oil. If you use synth, unless you are an off-roader or drive such that the oil never gets warm, I'd advise 7,500m change intervals. I use dino in my DD and change about every 5,000 miles.I usually buy my DD's new and put 100-120,000m on them before replacement. Typically, oil pressure does not drop over that period, indicating no significant wear. Around here, the car will rust away long before the motor wears out. I suppose if I bought something very exotic, hard and expensive to fix, that I didn't plan to work on unless necessary, that I would not use when the roads were salty, I would do 3,000m changes. Something like a high-end Ferrari. Otherwise, a complete waste of time, money, and resources IMHO.
Rich
03-09-2007, 09:21 AM
#20
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Sealed bearings would probably be tough to get around a crank and onto the journals, wouldn't they?
03-09-2007, 03:42 PM
#22
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A lot has been said about this subject in this thread and other threads. HP spring is good for higher revving engines as it will keep the oil pressure rising as the revs rise. More oil flow (and hence pressure) is needed at high rpm because more "leaks" and is thrown out of the bearings.
A motor with stock clearances does not need a HV pump. HV pump and a stock 4th gen oil pan can lead to disaster (do a search).
Rich
A motor with stock clearances does not need a HV pump. HV pump and a stock 4th gen oil pan can lead to disaster (do a search).
Rich
03-09-2007, 05:55 PM
#23
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I'm running a built 355 with a high volume pump (slightly more than stock clearances) and a stock pan. The pump is also feeding a T-76 turbo through a -4an line. I'd sure like more info on the possibility of a disaster. Is that from pumping the pan dry before oil has a chance to return to the pan?
03-09-2007, 07:12 PM
#24
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Probably not literaly sucking it dry. But if you combine a stock pan, HV pump, perhaps a little bit low oil level, and high G manuvering I think what happens is an uncovered pickup with brief periods of oil starvatrion.
I am one of the many who lost an LT1 in this situation. The HV pump got on there through a series of mistakes, not intentionally.
Rich
I am one of the many who lost an LT1 in this situation. The HV pump got on there through a series of mistakes, not intentionally.
Rich
03-09-2007, 09:58 PM
#25
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It is definitly NOT oil pressure that keeps the shaft and the bearing from making metal to metal contact with each other. Is water on the road under pressure when a car rides on it and hydroplanes? No, and that's the best way to think of the oil wedge - analogous to hydroplaning. Hell, there are sealed bearings that work fine with no oil pressure at all.
One way to prove to yourself that oil pressure at the bearing could not possibly support the shaft: Main bearing diameter in a SBC is 2.45". Let's think of a bearing with a 1" width and conceptualize that the shaft rides on the bottom 1/2 of the bearing (during the power stroke for that particular cylinder). The area of the bottom bearing would be ~3.5sq.in. If oil pressure supported the shaft (pushed it "away" from the bearing) and was 50psi, all it could support would be ~175lb. Obviously, the load is a lot more than that or we could use "spaghetti strap" conecting rods!
The pressure is generated in the oil passages and the oil between the bearing and the shaft is not under pressure, except that created by the force exerted by the shaft. Oil pressure is needed to assure that enough oil is supplied to replace the oil that leaks out from the sides of the bearing. I don't know enough about bearings to know whay a sealed bearing wouldn't work, but it must not or we would see it. I suspect it has to do with heat more than anything else.
Rich
One way to prove to yourself that oil pressure at the bearing could not possibly support the shaft: Main bearing diameter in a SBC is 2.45". Let's think of a bearing with a 1" width and conceptualize that the shaft rides on the bottom 1/2 of the bearing (during the power stroke for that particular cylinder). The area of the bottom bearing would be ~3.5sq.in. If oil pressure supported the shaft (pushed it "away" from the bearing) and was 50psi, all it could support would be ~175lb. Obviously, the load is a lot more than that or we could use "spaghetti strap" conecting rods!
The pressure is generated in the oil passages and the oil between the bearing and the shaft is not under pressure, except that created by the force exerted by the shaft. Oil pressure is needed to assure that enough oil is supplied to replace the oil that leaks out from the sides of the bearing. I don't know enough about bearings to know whay a sealed bearing wouldn't work, but it must not or we would see it. I suspect it has to do with heat more than anything else.
Rich
I know this, but I can't visualize oil running under the bearing.
I can't use you hydroplaning example because hydroplaning has to do with you spreading out downward force of the car, over a long stretch of liquid in a short period of time and never retouching that surface.
In the engine I am thinking of a top fuel dragster piston firing and pushing down on. and I am thinking of the liquid just sitting there and this huge powerful force pushing it down. logically the liquid has to be pushed out.
My thinking is flawed because the liquid is constantly moving. and it is reverse of hydroplaning. it is as though the water is moving and the car is standing still (the crank or camshaft staying still vertically).
It took me a while to try to figure out what that looked like, until I remembered this thing.
voila
03-10-2007, 12:20 AM
#26
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03-10-2007, 03:48 AM
#27
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The classic definition of a bearing is a device or system which can transmit a load through two elements of the system moving relatively to each other. The other obvious requirement is to allow this relative movement with the minimum amount of power being used, in other words, with the least friction. There are many ways of achieving this, but one of the most efficient and cost effective is to separate the two surfaces of the elements of the bearing with a thin film of pressurised oil. This oil pressurisation could happen in two ways � we could supply oil to the bearing at sufficiently high pressure to force the surfaces apart, or we can use the geometry of the two surfaces and their relative motion to generate the pressure. This latter method, which is known as hydrodynamic action, is that which is found in the bearings of an internal combustion engine. Sometimes the term �plain bearings� is used signifying that there are no ***** or rollers separating the two surfaces.
How does the hydrodynamic action work? The diameter of the part of the connecting rod which connects to the crank is slightly larger than the diameter of the crankpin. This means that when the centres of the crankpin and big-end are slightly offset, a wedge-shaped space is created between the two. Oil, being viscous like syrup, is drawn into this wedge shaped space and is squeezed or compressed, causing a pressure to be generated in the film of oil between the surfaces. If the speed is high enough, the pressure will be sufficient to push the two surfaces apart. Figure 2 shows this hydrodynamic action
03-10-2007, 11:46 AM
#29
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In some of my reading I have seen it proposed that the reason engines with HV pumps sometimes exhibit apparent oil starvation may be due to cavitation as much as momentary sucking air in high G turns.
Maybe I missed it but I think a point that goes along with what you have written is that as you said the oil pump is positive displacement so volume is completely rpm dependant. As such what we accomplish by putting a high pressure spring in is simply using oil the pump will move anyway for actual lubrication rather than bypassing it back to the pan.
I am sure most involved in this post know that but I am sure spelling that out directly will help someone.
I never paid much attention to oiling nowing the SBC has a good system, but in september I lost my 100k+ stock shortblock, rod bearing let loose, I might have caught it sooner had I had a functional oil pressure gauge but all I had was the factory idiot light style gauge that in the Caprice was just an on/off switch for a sender.
When I found a lower milage mint shortblock to put in I put a 0-60psi sender from a truck on and made the stock gauge somewhat functional and added a "white" spring. The cop cars had functional gauges but the 0-80psi sender made for poor resolution which is another point I think good to make. When you buy a gauge get one with sufficient range but not a huge amount more, that way smaller changes from normal will result in more needle sweep and be noticed more quickly.
Maybe I missed it but I think a point that goes along with what you have written is that as you said the oil pump is positive displacement so volume is completely rpm dependant. As such what we accomplish by putting a high pressure spring in is simply using oil the pump will move anyway for actual lubrication rather than bypassing it back to the pan.
I am sure most involved in this post know that but I am sure spelling that out directly will help someone.
I never paid much attention to oiling nowing the SBC has a good system, but in september I lost my 100k+ stock shortblock, rod bearing let loose, I might have caught it sooner had I had a functional oil pressure gauge but all I had was the factory idiot light style gauge that in the Caprice was just an on/off switch for a sender.
When I found a lower milage mint shortblock to put in I put a 0-60psi sender from a truck on and made the stock gauge somewhat functional and added a "white" spring. The cop cars had functional gauges but the 0-80psi sender made for poor resolution which is another point I think good to make. When you buy a gauge get one with sufficient range but not a huge amount more, that way smaller changes from normal will result in more needle sweep and be noticed more quickly.
03-10-2007, 12:16 PM
#30
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Good advice about choosing a gauge. Cavitation may be part of the problem, or the whole problem for all I know. Thing is, it seems that the HV pump is not a problem with a larger pan with better oil control. This leads me to think it is more likely a problem with the pickup sucking air intermittently. Regardless, in case any one still misses the point:
Do not use a HV pump with a stock 4th gen. oil pan!
Rich
Do not use a HV pump with a stock 4th gen. oil pan!
Rich