I played around the past few days with oil and psru. I added a psi gauge on top of psru,and vented the housing back to the oil dipstick tube. I have an oil breather tube from the fill port. We should have a vent on the oil crankcase. Without it, I did a pull and turned off the engine and 5psi stayed within the system until I cracked open the dipstick to relieve the pressure.

On Neil's PSRU, the oil inlet is on the front half. The front drain is on the side, which connects to the back drain that is an10, then to the oil pan.

Anyways, the stock psru inlet restrictor was 9/64. Once warmed up and running at 2200rpm static on the ground, engine oil is about 100psi for me. PSRU pressure was about 2.5psi and lots of oil was venting back into the oil dipstick tube. I kept on sizing down the restrictor until 1/16th. That mostly kept the vent clean and clear of any pressure build up inside the psru.

I'm not sure why Tracy recommends having the psru fuel of oil, maybe uses different bearings then Neil's? From a truck gear case perspective, plantey gears only need a coating of oil, not full and pressurized. I'm probably wrong, but throwing it out there. ?.?

- Matt Boiteau

Others have far more experience than me, but here are my thoughts.

All internal combustion engines will pressurize the crankcase, due to 'blowby' past the rings (apex & side seals, for us). Also, if the test is begun with a cold engine, simple temperature rise will increase pressure in the crankcase.

I'm not familiar with the internal construction of Neil's drive. Does it use pressure fed sleeve bearings, ball bearings, or a combination? If there are pressure fed sleeve bearings (like typical crankshaft main & connecting rod bearings, or the E-shaft bearings in a rotary), limiting flow/pressure more than recommended could compromise the bearings. Even with an all-ball bearing system, limiting flow too much *could* have the potential to overheat the system, since the oil is expected to remove heat in some designs. (Design dependent) IIRC, Tracy has said that his drive (pressure fed sleeve bearing at the input end) should *not* have a flow limiting orifice.

If Neil's drainback port is on the side, that would seem to imply that he expects the drive to operate with the housing remaining filled to the level of the drain port. Tracy's drain ports are on the bottom, at least implying that the drive is ok with only pressure fed lube to the sleeve bearings and directed or random spray to the ball bearings, and no residual oil pooled in the housing. Though I haven't seen Tracy weigh in on the subject, the fact that the drains are on the bottom implies that he didn't intend for the housing to run full of oil. What Steve Boese & others found is that the AN4 drain lines seem to be too small for gravity flow to keep up with the pressure feed, and, combined with the lack of a vent back to the crankcase, causes the drive to carry air back to the crankcase in the return line and eventually fill with oil. The mods that have been made were to enlarge the return lines and add a top vent to the housing, so crankcase air can replace the air carried out with the returning oil.

Charlie