I installed a -4 fitting at the top of both the -C drive and also the -A drive that I converted to 3.17 to one ratio.  At idle with oil at about 50 degrees, a gauge connected to that top fitting read about 4 psi inside the gearbox.  With oil at about 180 degrees, the gauge read about 2 psi.  The 2 psi was unchanged when the engine rpm was increased to 5000.  Both gearboxes behaved the same.

I also installed a ball valve in the -6 drain line that I could close immediately after shutting the engine off.  Then I could disconnect the drain line and collect the oil that was inside the gearbox.  The converted -A drive with a 3 pinion planetary contained 22 oz of oil after shutdown while the -C drive with the 6 pinion planetary contained 18 oz of oil.  This was with the top fitting dead ended to the gauge and not venting anything during operation.

The oil flow rate from the prop end of the gearbox was found to be about 1/4 oz per minute while the total oil flow rate through the gearbox was found to be about 90 oz per minute.  This explains why installing separate -6 drain lines to the engine did not change the behavior.  During the flow rate measurement, the oil draining from the gearbox just after starting the engine had a lot of entrained air for a short time.

So the conclusion is that the gearboxes are completely filled with oil during operation and any air initially inside them is entrained in the oil and forced out the drain by the incoming oil.   The gearbox recommendations that I could find do not recommend running with the components completely submerged due to the power consumed by churning the oil and resultant generation of heat.  Those sources also say that if the gearbox is partially filled with oil but the level is too high, the oil will foam and lubrication will be degraded if the system circulates the oil using an external pump.  In our case, it may be better to accept the power loss and extra heat as opposed to returning a lot of aerated oil to the engine.

An impeller was added to the planet carrier of the converted -A drive but that gearbox still operated completely filled with oil.

The gearbox drain and the engine drain connection were increased to 1/2" NPT and fittings drilled out to 21/32" ID were installed.  3/4" ID hose was installed as the drain line.  The drain line was close to level but slightly down hill in operation.  This reduced the gearbox internal pressure to a value not measurable with a normal gauge.  The amount of oil inside the gearbox was measured by clamping off the drain hose immediately after shutting down the engine, disconnecting the hose from the engine, and then recovering the oil remaining inside the gearbox.  The gearbox was found to be completely filled with oil just as before.

When the -4 fitting in the top of the gearbox was connected to the engine crankcase as a vent,  the amount of oil recovered from the gearbox after operation was now found to be about 5 to 6 oz.  The recovered oil did not appear to be foamed and its volume changed very little upon standing for several hours.

Steve Boese