Originally Posted by jedijeb

I found one thing that is partially incorrect in the oil 101 section so far. He says

" In fact the relationship between pressure and flow is in opposition. If you change your oil to a thicker formula the pressure will go up. It goes up because the resistance to flow is greater and in fact the flow must go down in order for the pressure to go up. They are inversely related. Conversely if you choose a thinner oil then the pressure will go down. This can only occur if the flow has increased."

If your oil pump is moving let's say 10 gallons per minute of oil, then it should be moving the same amount of oil regardless of its viscosity, but with higher viscosity comes more resistance to move the same amount of oil per minute therefore the pressure increases while the flow stays the same. If you increase viscosity and reduce flow then the pressure will remain the same if you decrease flow proportional to the increased viscosity.

With the design of modern systems it is still better to run thinner oil because of the oil bypass valve which will limit the oil pressure. If you run high viscosity oil you will open the bypass valve at a lower flow than you would with a lower viscosity oil when they both reach the pressure limit of the bypass. He is correct that higher flow provides more lubrication of the bearings so in this case the low viscosity oil is preferred. If though you are not getting any pressure when using the low viscosity oil it means there is too much leakage around the bearings and using a higher viscosity oil will not necessarily fix the problem, only new bearings will.

I'm not an automotive engineer by any means but I am a chemist and do understand fluid dynamics pretty well and this is the only thing I saw that really stuck out as incorrect. It would be correct if the oil was a compressible gas, but not a noncompressible liquid.

Originally Posted by jedijeb

That would depend on the type of pump. A centrifugal pump would do this, but a positive displacement pump would not. With a positive displacement pump a restriction would increase pressure while the gpm remained the same, unless the backpressure causes the pump to turn slower which means that its gpm would become less. Things like the water pump in a vehicle are centrifugal, things like a piston hydrolic pump are positive displacement. What would a gear mesh pump like an engine oil pump be? If there is not enough gap between the gears to allow the oil to back flush then the pump will deliver the same gpm at the same rpm regardless of pressure, if there is enough gap that oil can backflush then it could have reduced flow at the same rpm.

The water hose analogy works that way because there is a limited amount of pressure and flow available to the entrance of the hose, which is like the bypass valve on an oil pump, if you restrict the hose flow it will reach a maximum pressure then no more. The point I was making was that as long as you are pumping water through that hose your analogy will hold, but try pumping the same volume (gpm) of molasses through that water hose and what is the difference in pressure? It will be a higher pressure at the same flow rate because of more restriction caused by higher viscosity.

Originally Posted by bigbadon

An engine oil pump is a positive displacement pump. Flow rate in US gal/min = Fluid Density X Pump Capacity X rpm - internal slippage (leakage). Depending on the amount of clearance, a simple gear pump has limitations. The other factor is the wide viscosity vs thermal range. As oil heats and viscosity decreases, so does restriction decrease also. It is possible to have 10 GPM at either 2 psi or 40 psi....the lubricating protection would remain the same. The job of the oil pump is to DELIVER oil, pressure is irrelevant as far as lubricating goes. Pressure is used to benchmark engine wear. The real oil pumps in an engine are the connecting rods...with them having enough clearance to create an elliptical movement they suck oil from the feed hole in the crank journal and then compress the oil to over a thousand psi.