A brief discussion of oil pressure, etc.
03-05-2007, 10:23 PM
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A brief discussion of oil pressure, etc.
There have been a bunch of threads lately about oil pressure. In a (probably vain) attempt to clarify some issues in that regard, I offer the following.
Oil pressure is NOT IMPORTANT! Well, that's an exaggeration to get your attention. But what IS important is oil flow. The plain bearings in an engine work as follows. There is no metal to metal contact except at startup and shutdown. What separate the shaft and bearings is a film (actually a wedge) of oil. Oil is constantly flowing out from the edges of the bearings, so a constant flow of oil is needed to replenish this.
The oil is supplied by a pump, which is of the positive displacement type. The volume the pump flows is directly proportional to the rpm of the pump gears. The oil pump does not produce any pressure!!!!! It produces flow. The oil coming out is at the same pressure as when it went in to the pump - zero relative to the oil in the sump.
So why can we measure oil pressure above zero? Well, the pump is flowing a fixed volume of oil into the oil passages of the block and out through the bearings. This causes resistance to flow and the result is pressure. Oil pressure is thus a surrogate way of measuring oil flow - the more oil flow (from higher pump output), the more pressure will be produced. If we increase the resistance, by using a more viscous oil, the pressure will also go up because of the increased resistance to flow. When the oil is cold, it is more viscous, and there will be more oil pressure.
How much oil pressure is enough? Keep in mind that what counts is flow. A time proven rule of thumb for a SBC is 10psi/1,000rpm. This assumes the normal range of engine bearing clearances and it is well proven that 10psi/1,000rpm will supply enough oil flow. Since we have no convenient way to measure flow, we use pressure as a surrogate. If the bearing clearances are larger, we need more flow to keep an adequate film of oil between the bearings and the shaft. Do we need more oil pressure? No, but it will take more volume to produce the same pressure because the resistance is lower. So, we need a high volume pump to maintain oil pressure. This costs hp - more power is needed to drive a HV pump. The reasons to choose large clearances are beyond what I want to go into now.
What if the bearing clearances are tight? Well, we will see more pressure with the same output but we don't need it. In that case, we can use a lower viscosity oil with less friction and less resistance to pumping. This will lower the oil pressure and gain hp because of the lower pumping losses and less friction. This is what the OEM's are doing to improve mileage and performance. Some new vehicles come with a recommendation for 5W-20 oil. There are real hp gains from using a low viscosity oil. Some hard core racers will use 0W-10 weight. The problem is that there is a general relationship between viscosity and shear strength. Low viscosity oil may not provide enough resistance to shear to protect bearings (avoid metal to metal contact) under very high loads (high boost blower cars, heavy nitrous use, etc.). Many racers are using 5W-20 for the same reasons but with a little more protection.
I am one of those dinosaurs who wants a little more bearing clearance and a heavier oil for a hi-po motor. The idea is that with high loads, more clearance and greater shear strength is needed to avoid metal to metal contact when there is parts deflection. I use dino oil. Actually, a semi-synthetic 20W-50. And I need a HV pump because of the bearing clearances. But I realize I am giving up hp and am slowly coming around to considering using a thinner oil and tighter clearances.
There is a lot more to be said. But maybe now you believe me that you don't need a HV pump unless you KNOW you need an HV pump? The issue of a high pressure spring has been discussed a lot. I think it's not a bad idea if you run over 6,000rpm. The stock pressure relief spring limits oil pressure to 55-60psi, so if using it you will not see the desired 10psi/1,000rpm at high rpm. It does not cost power like a HV pump does.
Rich
Oil pressure is NOT IMPORTANT! Well, that's an exaggeration to get your attention. But what IS important is oil flow. The plain bearings in an engine work as follows. There is no metal to metal contact except at startup and shutdown. What separate the shaft and bearings is a film (actually a wedge) of oil. Oil is constantly flowing out from the edges of the bearings, so a constant flow of oil is needed to replenish this.
The oil is supplied by a pump, which is of the positive displacement type. The volume the pump flows is directly proportional to the rpm of the pump gears. The oil pump does not produce any pressure!!!!! It produces flow. The oil coming out is at the same pressure as when it went in to the pump - zero relative to the oil in the sump.
So why can we measure oil pressure above zero? Well, the pump is flowing a fixed volume of oil into the oil passages of the block and out through the bearings. This causes resistance to flow and the result is pressure. Oil pressure is thus a surrogate way of measuring oil flow - the more oil flow (from higher pump output), the more pressure will be produced. If we increase the resistance, by using a more viscous oil, the pressure will also go up because of the increased resistance to flow. When the oil is cold, it is more viscous, and there will be more oil pressure.
How much oil pressure is enough? Keep in mind that what counts is flow. A time proven rule of thumb for a SBC is 10psi/1,000rpm. This assumes the normal range of engine bearing clearances and it is well proven that 10psi/1,000rpm will supply enough oil flow. Since we have no convenient way to measure flow, we use pressure as a surrogate. If the bearing clearances are larger, we need more flow to keep an adequate film of oil between the bearings and the shaft. Do we need more oil pressure? No, but it will take more volume to produce the same pressure because the resistance is lower. So, we need a high volume pump to maintain oil pressure. This costs hp - more power is needed to drive a HV pump. The reasons to choose large clearances are beyond what I want to go into now.
What if the bearing clearances are tight? Well, we will see more pressure with the same output but we don't need it. In that case, we can use a lower viscosity oil with less friction and less resistance to pumping. This will lower the oil pressure and gain hp because of the lower pumping losses and less friction. This is what the OEM's are doing to improve mileage and performance. Some new vehicles come with a recommendation for 5W-20 oil. There are real hp gains from using a low viscosity oil. Some hard core racers will use 0W-10 weight. The problem is that there is a general relationship between viscosity and shear strength. Low viscosity oil may not provide enough resistance to shear to protect bearings (avoid metal to metal contact) under very high loads (high boost blower cars, heavy nitrous use, etc.). Many racers are using 5W-20 for the same reasons but with a little more protection.
I am one of those dinosaurs who wants a little more bearing clearance and a heavier oil for a hi-po motor. The idea is that with high loads, more clearance and greater shear strength is needed to avoid metal to metal contact when there is parts deflection. I use dino oil. Actually, a semi-synthetic 20W-50. And I need a HV pump because of the bearing clearances. But I realize I am giving up hp and am slowly coming around to considering using a thinner oil and tighter clearances.
There is a lot more to be said. But maybe now you believe me that you don't need a HV pump unless you KNOW you need an HV pump? The issue of a high pressure spring has been discussed a lot. I think it's not a bad idea if you run over 6,000rpm. The stock pressure relief spring limits oil pressure to 55-60psi, so if using it you will not see the desired 10psi/1,000rpm at high rpm. It does not cost power like a HV pump does.
Rich
Last edited by rskrause; 03-06-2007 at 06:19 AM.
03-06-2007, 12:11 AM
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What about once you get into the larger bearings that are found in BBC, BBF, and the 351w motors? Couldn't having tight clearances be bad since you have more surface you will create more heat in the main bearing relative to a smaller main bearing? I would think there would be a point at which tighter clearances on bigger journals would have a negative effect in the aspect of higher bearing tempatures.
Other than that, great write up.
Other than that, great write up.
03-06-2007, 06:22 AM
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I don't know where the point of diminshing return for the OEM's is in terms of tight clearances. I would guess they do know and it has to do with manufacturing tolerances, among other things. I do know that there is a minimum clearance below which the oil film can't form. I believe some motors now call for as low as the low 20 thousanths clearance on the mains.
Rich
Rich
03-06-2007, 12:24 PM
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How about the relationship to oil and cavitation? If the pump introduces air into the oil path then there is a need for higher viscosity oil. As Rich said the bearing clearance is a thing that has as many variables as the oil pressure, but connection to flow, pressure and clearance is inherently linked.
Bret
Bret
03-06-2007, 01:14 PM
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So the traditional rule of thumb about needing 10psi/1000rpms is all dependent upon bearing clearances right?. I hear alot of people quote this like it was set in stone,am I right to assume that this is not correct?.
03-06-2007, 01:51 PM
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During the last Engine Masters competation that was on SPEED channel for some reason I got hung up on watching oil pressure of the competitors. Most did not go over 40 psi and many had the oil pressure go down more at higher rpm's. Now mind you I'm aware these guys may not be looking at 250K engines, but think the old rule of 10psi is a bit dated.
03-06-2007, 04:38 PM
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During the last Engine Masters competation that was on SPEED channel for some reason I got hung up on watching oil pressure of the competitors. Most did not go over 40 psi and many had the oil pressure go down more at higher rpm's. Now mind you I'm aware these guys may not be looking at 250K engines, but think the old rule of 10psi is a bit dated.
10psi/1,000rpm is still a valid rule of thumb for street performance and sportsman level racing with typical oils and bearing clearances.
Rich
03-07-2007, 11:50 AM
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One other thing, that should be mensioned is that the oil is the "other" coolant. People often don't think about how much cooling the oil does. In an air cooled engine it's the only coolant. A large capacity system and HV pump can help keep temps. down.
03-07-2007, 07:32 PM
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I would guess there would be a point at which too hot is not a good thing. I have heard that oil over 200*F no longer is cooling anything and only heats up stuff.
03-08-2007, 11:19 AM
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I've heard Bob Norwood say many times: "Heat is HP". Problem is the LTx engine seems to be real sensative to heat soak. Guess if you can just keep it in the bottom end your OK. May be harder than it sounds.
