Pounds per square inch varies according to the size of the "pipe" with the same flow rate. An oil filter has an inlet pipe and an outlet pipe.
No - there's no "inlet pipe" or "outlet pipe" in a filter. Pipes have flow loss and pressure loss (when flow is occurring; not applicable in static condition) when the walls of the pipes create drag related to frictional effects and resultant boundary layers inducing fluid shear, etc. Rough walls create more drag and dP; smooth walls less so. With short pipe lengths, these losses are so small that they are reasonably ignored. In really long pipe runs, it's very important to consider. However, there ain't no pipes in a filter, so your assertion means zilch.
Rather, filters have "holes" (or ports, if you prefer) which are the inlet and outlet routes. Those holes present a potential for dP, but ONLY if the holes are significantly smaller in area than the cross-sectional area of the supply line or exit line. Generally, the inlet/outlet holes in a filter are greater in total cross-sectional area than the supply/exit oil circuits themselves, so the "holes" are a moot topic here.
There is no other way in or out of an oil filter. I don't need a test setup to know this, it is not theory, it's very basic physics, units of measurement, math. Wrong?
No - you're not wrong regarding the entry/exit points. One way in and one way out. Did someone say otherwise?
Looking at the pictures, this is the general way I see how the set up controls lube flow ...
- oil flows via the inlet supply line as it goes into the filter mount
- oil touches the inlet pressure transducer which is mounted AFTER that supply line inlet, just ahead of the filter media
- oil flows through the media (or through the BP, if the dP across the media is too great and invokes the BP action)
- oil touches the outlet pressure transducer which is mounted BEFORE the outlet line (mirror to the inlet in concept)
- oil flows via the outlet line as it heads to the engine
There's no "pipe" causing any significant restriction here. The transducers are a close as can be practically placed, and exist mere millimeters ahead and behind of the oil's path through the media. Even if the filter goes into BP mode, that does not affect the dP reading across the two transducers because, as you state, there's only one way in and one way out.
If and only if the "holes" of the filter were significantly restrictive relative to the overall capability of the system, then you could claim that the "holes" contribute to the dP. But that's not how oil filters are designed/made. The several inlet holes in the baseplate represent enough cross-sectional area in total, as does the outlet as well, that the total flow through the filter is not inhibited by the "holes" relative to the engine's other restrictive points.
If you want to acknowledge pipes in the system, I'll mention these ...
- there's the suction pipe coming from the oil-pickup and connecting to the oil pump is typically the smallest "pipe" in terms of cross-sectional area in just about any typical engine lube system. The flow via that pipe is pretty large, because the PD pumps always end up bleeding off pressure via their PRV to protect the entire system from over-pressurization. So the flow capability of the tube has to supply both what is used in the engine and what is dumped back to the sump
- We could consider the engine's oil galleys as "pipes", because they are the flow routes through the engine that have cross-sectional area and also have walls.
- Some engines have small oil-jet cooling pipes under the pistons, but obviously not all engines
- Some engines have oil carried externally (or internally) via pipes to things like turbo chargers for bearings, etc; not all engines, obviously
None of that matters to this experiment that RSMike ran, though. He measured the dP as close as practical to the media as he can, and the "holes" in the filter are not the restriction you imply.
Why? I don't mind being wrong, and don't need to be right.
People leave a discussion as it devolves into personal attack bickering, which is bullying. Because of someone needing to "win", when it's no longer about the science, and they are the wrong ones? No wonder some very good resources in people have chosen to leave this site. The bullying to "win" needs to stop. People should feel free to ask and ponder without being personally judged, that's good discussion. I guarantee you there are people here who think twice about asking something or challenging something because they know what is coming at them if they do. That's wrong.
Board racer
People leave for all manner of reasons; I accept that for what it is. People often confuse legit challenges to their opinions and theory with personal attacks. (Yet, no one here has called anyone else derogatory names, insulted family members, called into question one's genetic lineage, etc.) The quantity of people here whom are intimidated from asking questions are no less in count than those who are unafraid to spout off poorly reasoned garbage. It's an open forum, and as long as the rules are not broken, there's free exchange available to those who willingly participate.