Al, I'll agree on the brake system "minute" difference, I was only using that as an example of a system where the fluid is no longer moving once pressure is achieved (but it moves a lot until the pads and shoes snug up to metal).
The oiling system is considerably different because it's always moving. The true "end of the passage" is everyplace that the oil squeezes through to get back to atmospheric pressure (zero guage). So at the last little bit of tube before the bearing is where the pressure goes from whatever it was (say 40) to zero on the edge of the bearing exposed to air. The details of when and where it goes to zero often depend on the individual bearing and how fast the crank/rod or camshaft is turning, as well as the clearance. There is a "pressure distribution" across the bearing face, highest at the centerline where the outlet hole is, and lowest (zero) at the edge. But it is not necessarily a straight, predictable amount of change per distance, and it changes with rpm.
Backing up the system into the block, you have to go through small diameter drilled tubes. If they are too small for the amount of oil flowing through, then the velocity of the oil is quite fast. The pressure loss from friction in the fluid stream depends on the square of the velocity. Going back upstream is the large-diameter feed tube that has the guage sender threaded in at the back. The velocity of the oil in this tube is quite a bit less than the small feeders to the crank, especially at the back of the motor. This is due to the fact that about 75% of the engine has already taken it's needs from that galley. So the oil is moving slow here, and won't lose hardly any more pressure in that part of the motor. You don't want to put a guage before the filter, because the important value is the pressure that the bearings are getting, which is close to that indicated in the galley.
Really, the idea of pressure and hydraulic losses can be visualized by thinking about the water system where you live. The users open valves that allow fluid out of the pressurized tubes back to atmospheric. Those valves are no different than the lifters and bearings in an engine, except the holes in the motor just never turn off. They can, however, open up more due to wear. If there are too many open valves (lawn sprinklers), or a huge valve is opened up too much (think fire hydrant), or a device breaks and makes a huge hole (a worn out or spun bearing), then the entire system pressure becomes lower. But the closer you get to the source of the pressure, the higher the pressure still is, because the fluid has not had to go through the long pipe system at high velocity and get past all the little open valves that take fluid out.
Bottom line is: pressure is pressure. It can only decrease as it travels through a system. The faster it moves, the more energy it loses due to friction. In order to have high pressure at the end of the line, you have to limit the fluid going out (keep bearing clearances tight), or limit the velocity (drill passages larger), or put a bigger pump in at the start (high volume). If the fluid is not moving at all, then it has no friction and loses no pressure.
I sincerely hope I haven't caused any confusion with this too-long-winded explanation.
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David
newbie #4153
