Originally Posted By: KrisZ
Originally Posted By: OVERK1LL
Originally Posted By: GMorg
To correct the typo above,
I disagree that pressure is higher with higher viscosity due to lower flow.
But that's exactly what the higher pressure means. Oil pressure is resistance to flow, higher pressure relative to another lubricant at the same temperature and RPM means less flow.
Something doesn't add up here. I think all it means is that the oil is thicker, and that there COULD be lees flow if the oil is significantly thicker.
If my fluid dynamics memory serves me right, which is a long shot,
, back pressure is directly related to flow and fluid viscosity, all else being equal.
So if you have a pipe with a constant diameter, it will always provide a certain back pressure that will be related to flow and fluid viscosity, just like oil system.
Let's say you're pumping water through that pipe. At certain flow the pipe will provide a certain back pressure and the only way to change that back pressure if to change flow. Higher flow will provide higher back pressure and vice versa. Now, if you want to pump oil through that same pipe and at the same flow, the back pressure will be higher than water because the oil is more viscous, but if you drop the flow, the back pressure will also drop, it has to because that's what the pipe sees. You cannot have lower flow with higher back pressure because that would mean that something else changed in the system, like the pump having not enough power to maintain flow.
I think you're confusing back pressure with pressure. Back pressure is like wind resistance, the faster you go (higher flow) the more drag (back pressure) the car will see. So if a compact car and an SUV go at the same speed, one will see less drag than the other, but simply looking at the drag (back pressure) will not tell you that the SUV is going slower. The SUV could see higher drag at a lower speed than the small compact, but that cannot be assumed by just looking at the drag value.
Yes GMorg is confusing the oil pump pressure and the back pressure that the an OP gauge measures.
Using your pipe analogy, the oil pump pressurizes the system or the pipe. At a given rpm the oil pump pressure is fixed regardless of the oil's viscosity. The fact that thicker oil results in more back pressure is measurement of the reduced oil flow. The thinking that the higher back pressure associated with pumping thicker oil will compensate fully and therefore flow at the same rate as lighter oil is simply not the case.
The oil grade comparison confirms this.
All testing was done below the oil pump by-pass point.
If we had the exact HTHSVs and VIs of the oils tested and not just the oil grades the differences between the light oils and heaviers oil would be more linear.