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Originally Posted by Irace86.2.0
You mentioned it somewhat, but is it possible the 10w40 oil is also the problem?
I am running 5w30 for the simple reason that Subaru runs 5w30 in all their FI applications, and I am boosted, but I could imagine running 10w40 or 10w50 on the track. With that said, I think I would drive the car for 10-15 minutes at low speeds/rpms to get the oil to operating temperature, with the concern of starving the bearings running the high viscosity oil.
There seems to be a debate these days whether higher viscosity helps to protect the bearings with a thicker layer of oil, or if it is bad. The argument is that pressures will be higher, which is good, but if oil flow rate is slowed then oil temps can creep up; thinner oil will flow faster, so it will deliver cool oil faster. Total system oil temps may not raise because the oil cooling measures post cooling could be adequate, but locally to any one part of the engine there could be a spike. Thick oil could also cause back pressure, creating resistance against restricted flow, which could be why outlet pressure is static in your chart after increasing inlet pressure.
Also, you mentioned thinner oil having less problems with cavitation. Why is this the case? Intuitively, it seems like warmer oil and thinner oil would result in more bubbles.
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I think thinner oil will have less problems with cavitation because it has less resistance to flow which results in more pressure on the inlet (or less vacuum). Interestingly enough, higher viscosity does seem to reduce damage done by cavitation:
http://www.aun.edu.eg/journal_files/80_J_7365.pdf
I think the most important factor here for cavitation is the inlet pressure which is caused by major and minor hydrodynamic losses. Major being mostly friction, minor being mostly changes in direction of the fluid. So a thick fluid running through bendy pipe and small diameters is going to cause more vacuum on the inlet. A thin fluid running through straight large diameters is going to cause less vacuum on the inlet.
This is just examining the inlet side of the pump though with the purpose of reducing or eliminating cavitation and increasing flow rate.
You make good points about whether or not this is actually significantly helping on the other side of the pump. Even if most of the increased flow is going to the head and not the block, there should be some small amount of increased flow even through a bottleneck. Again, whether it's significant or not, or even the correct solution, is another question.
Edit: Removed a possibly incorrect statement about oil viscosity and its relation to cavitation.