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On 13 Feb 2004 at 21:14, Keith Park wrote: > But... isnt it a problem trying to stuff molasses into a tight bearing > clearance? it just wont get in there and the bearing will starve. Well, not really. Thicker oil will certainly run into the bearing spaces more slowly than thin oil, but it also spills out faster. In the meantime it takes a lot more power to shear it in the bearing so that's just power wasted, all for nothing. This shear also results in a torque on the bearing shells which can cause BIG problems if it gets too big and spins one of them. > I understand what your saying about rotational pressure... BUT, I still > disagree that feed pressure isnt important. To keep plenty of oil in the > bearing when its getting pressed out at high RPM it requires good feed > pressure right? otherwise performance engines wouldnt be so concerned with > such high oil pressure right?? It's not so much that feed pressure isn't important, it's more that it's just the wrong parameter to look at. The important one is FLOW, but that's really hard to make a cheap gauge for, so we measure pressure. It's natural to assume that the 2 go hand in hand, but you quickly realize that this isn't true when you think about what would happen to the pressure if you blocked off all the bearings: The pressure would skyrocket as the flow dropped to zero. The oil is actually fed into the bearing at a location where the hydrodynamic pressure is at a minimum, so that there can even be some suction there at times. Use light oil and you get lots of flow with little shear and torque in the bearings, and little wasted engine power. You'll still get pretty much the same flow, because the oil pump is a positive displacement pump, but the pressure will be less. Now keep in mind that the HP that the oil pump uses = the flow times the pressure. So shooting for max pressure also maximizes the power consumed by the pump (and this does NOT count the power also wasted in shear in the bearings.) In the long run, people spend WAY too much time trying to get high oil pressure, when all they really need is good, continuous, flow. > I do agree that proper lubrication our engines dont need the pressure they > have when new, but without it the engine will wear down to insufficient > pressure long before they reach end of life, and if they dont have it > because a bearing is loose the bearing will hammer out the case and make > things even worse. I keep having to go back to the fact that the important pressure is that which is created IN the bearing. It is caused by the rotation of the bearing as the journal is offset to one side in the bearing space and the oil is dragged into this decreasing volume. In this case, I suspect that the pressure that can be built up is proportional to the RPM, the change in bearing clearance from one side to the other, and the viscosity. One is tempted to point to the viscosity here and think that this make the viscosity important, but you have to note that if the clearance goes to zero on one side, the pressure there goes to infinity, so we don't need thick oil to keep the journal from touching the bearing. Thin oil DOES let the journal get farther off center, but that's not a problem since the amount off center that it can be is still limited by the bearing clearances, which are small. And, cases get hammered out NOT because the journal hit the bearing. Whatever force is applied to the bearing by the journal (thru the oil, by way of the hydrodynamically generated oil pressure) is also passed on to the case. It is perfectly normal to find a damaged case behind a bearing that looks just fine. So why would anyone use anything other than the lightest oil? Well, there are several reasons that I can think of. You want as much oil to still be there when you restart a cold engine. You have kinds of wear other than journal bearings, rings and cylinders, for example. i don't know how viscosity affects their operation, but you also want them to still have some oil film on them when you restart. I'm not saying that you should ignore your oil light. It is important, because we know that a properly assembled engine will almost always keep that light off. If it comes on, we know that something is wrong. If I "fix" the problem by adding super heavy oil, then I'm just covering up the symptom without doing anything about the problem. -- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org