Oil Pressure

Is oil pressure important? In a sense, NO. But that's an exaggeration to get your attention. What IS really 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 separates 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 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 pressurized oil. This oil pressurization 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 balls 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 centers 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. 

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 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 when normal clearances and parts are used. 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.

Our preference is 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. In most of our race cars, we use a semi-synthetic 20W-50 and a HV pump because of the bearing clearances. In our blown Hemi, we use a straight 50 weight non-synthetic oil. Our preferred brand is Brad Penn, which we sell at a competitive price. 

There is a lot more to be said about oil and oil pressure. But the above should get you started.