On a couple of long flights, my passenger spotted something I never noticed: the oil pressure is noticeably lower (5-10% in absolute terms) at 2575rpm (the TB20 max) than at 2400rpm (the typical cruise).
I wonder what the physical explanation for this might be.
Perhaps one is that the oil pressure controller is at one end of the oil gallery, and the oil pressure sensor is at the other end of it, and I would expect a bearing rotating at a greater speed to be passing more oil.
Maybe your pump is cavitating, thus less, not more oil is being pumped ?
I think LeSavng might have a point in that the oil is foaming more at high RPM resulting in more air in the oil.
This is pure educate speculation and not fact, what I will do is ask a guy know who is in the automotive engine test business if he has any input that would relate to this happening.
The oil entering the pump is coming from the sump and won’t be aware of the engine rpm. So I reckon this is a real pressure drop caused by a higher oil consumption of bearings rotating faster.
Is this in conjunction with higher oil temp ? It happens like that “normaly”.
I don’t think so because the effect has a time constant of seconds.
However I need to do a flight on which I test this properly and perhaps get a video.
The oil temp never moved at all :-)
The oil entering the pump is coming from the sump and won’t be aware of the engine rpm.
But the pump is, and so is the sump. The pump is a constant volumetric pump: the volumetric flow is only dependent on the RPM of the engine.
This has potentially two adverse effects.
Both these effects are easily solved with more oil volume in the sump. It is therefore very easy to check, just fill up to max and check. I would believe the first one to be the most likely, bit you never know.
With a constant volumetric pump I don’t see how pressure could go down with increased rpm, unless maybe the pressure regulator is broken somehow?
With a constant volumetric pump I don’t see how pressure could go down with increased rpm
That is true but after the pump there is a spring loaded pressure control valve, which feeds supposedly a constant pressure (not constant volume per second) oil to the oil galleries.
Then, if the galleries consume more oil, the pressure will fall IF measured somewhere far down the road from the pressure control valve.
Obviously if the pressure was measured right after the pressure control valve it would (should) be totally constant, but then arguably the pressure gauge would be somewhat useless because it would show a perfect pressure until a second after the oil all disappears 
Well, that’s what the oil temperature gauge gives you 
There was plenty of oil – about 9 quarts. The engine is burning very little nowadays.
Peter wrote:
Then, if the galleries consume more oil,
What would make them do that? (not saying they aren’t, just wondering how)
There is also the prop governor. I don’t know the details of that, but presumably oil could in theory leak from high pressure side to low pressure side. How much depends on the exact position?
