Engine cooling test, oil psi curiosity

[jdeas]

Phase 1 testing was done in spring conditions so the upper cooling limits were never really challenged. In preparation for my LA to Atlanta flight last month I did some testing while we were in record heat (SoCal). While I believe I know the reasons for my results I would like to get some feedback from the engine gurus.

Setup:
IO-360-B1B
Stock Vans oil cooler (-8 hoses)
Plenum
Firewall mounted oil cooler

Conditions:
OAT 85f-91f@ 5500ft (@ takeoff 108f) 6-7quarts of oil
CHT peak 370 cruise 340


RPM temp/psi Time Notes
2500 193/80 0min Initial climb
2500 197/75 7min psi changed over 10 seconds -5 psi
2400 211/69 25min psi changed over 40 seconds -6 psi (as low as 65 psi with 5 quarts)
2500 211/69 25min Changing RPM between 2300-2500 made no change in psi
2300 206/75 27min 40 second change +6 psi even with lower RPM as oil cooled


I am quite happy with the overall cooling of this system but the psi regulation is curious. I am assuming these quick psi changes are the results of the vernathrem shifting oil flow from bypass to the oil cooler. So, with the pressure relief valve down stream of the cooler bypass, why would I see a pressure change?


Does the stock supplied cooler have that much restriction?
Could the oil pump volume be inadequate to feed the system with this restriction?
Does the fact that all these pressure values drop another 5 psi at 5 quarts raise any flags?
Does prop governor changes of +/- 40 rpm that accompany each pressure change point to anything?

[David-aviator]

...the slight changes you are seeing go un-noticed by me. The pressure is well within limits.
Why it is occurring is a mystery. Could be the pressure sensor which is a low voltage converted to psi, could be flow variances and certainly rpm will affect it as oil pump is gear driven. Temperature of oil will affect pressure.
I doubt anyone here can answer the question with certainty, just speculation like what I am doing.

[airguy]

Higher oil temps will result in lower viscosity (even with a multi-weight oil, the effect is less but it's still present) and that will result in lower pressure. Your temperature/pressure curve looks pretty standard to me, and you're well within limits.