Returning to a thread from a few months ago....
It's a common belief that an oil cooler inlet somehow steals air from the adjacent rear cylinder, causing it to exhibit a high CHT. I've suggested it does
not do so, based on some previous observation of CHT with changes in oil cooler air supply location. I've also said that measuring static pressure at various points within the upper plenum space would tell a lot.
As it happens, I recently rigged my plenum with pressure taps at each FI nozzle location, a story for another time. In the context of stolen air, I thought everyone might like to see the reality.
Below you can see the pressure taps on the #1 and #3 nozzles. They are simple aquarium bubble rocks, the goal being to record static pressure, not dynamic. The taps on #2 and #4 are similar, but note they are on the forward facing part of the head, where the conversion of dynamic pressure to static is a little higher, rather than on the rear facing part like 1 and 3.
Key point here is the location of the #3 tap, immediately in front of the oil cooler duct inlet:
And the numbers, at three altitudes, with three speeds at each altitude:
Note that #3 pressures are not significantly different, and in fact are just a bit higher than those recorded for the #1 nozzle well forward of the oil cooler intake.