Steve,
Sorry, I've probably missed something. Where's the PCV located on the image above? I can't see it.
Is that 5" or 0.5" under hard acceleration? I think 5" may be a bit on the high side. If it is 5" I would say the valve isn't reaching the "sonic flow" condition because the valve orifice is a bit on the large side. It would be interesting to try a smaller valve rather than a larger one.
I'm proposing to use a manometer instead of a gauge because all of the gauges I've seen here measure 0 - 30" Hg. I don't expect cheap gauges to be accurate enough to measure 1" Hg reliably. Anyway why spend 30 bucks when I can make a manometer out of a bit of plastic tubing!
On another point, I'm not claiming this is rocket science. When you search the web you find MANY technical papers on this subject by University's, Ford, Caterpillar, Toyota, etc. and you start to wonder why so much money has been spent on it. From my reading of the few I have downloaded, drawing a vacuum on the crankcase will affect oil distribution and engine performance. I guess the manufacturer's are trying to avoid warranty claims by funding this research!
Most papers I've seen agree that the depression isn't arbitrary. They're trying to avoid sucking oil/air/dirt through engine seals or cavitation/slip in the pump.
To conclude, I agree, apart from the fact that it interests old retired engineers with little else to do (Me!) most of us are probably trying this to prevent oil leaks. After all it's cheaper than pulling the engine out. As you get older you have to work smarter, that engine is heavy!
AJ
ps I wouldn't dream of leave my Porsche idling with the oil cap off. Why would you do this?