OK here's a summary of the two observations.
1) In both cases the crankcase breather system was blocked on purpose
2) In both cases the engines were just idling
3) In both cases the engine was reported to slow down before a final pressure reading of about 3 PSI was made
4) In both cases the engines either stalled or very nearly stalled at about 3 PSI
What have we learned so far?
Firstly =>
it is not sensible to block the crankcase ventilation <= You could blow out a seal
Secondly both reports say that the engine slowed at idle before the pressure of 3 PSI was reached.
If I have understood this correctly - people are saying that this is because the crankcase pressure acts on the IP side of the shut off valve.
If this is the case I guess one could assume that the shut off valve operates at a very low vacuum level - much lower than the vacuum level needed for a "vacuum required for consistent shutoff"...
...but I don't think so!
13 inch hg ~ 33 cm hg ~ 440mbar ~ 6.3 PSI
Compare those numbers with "the pressure at stall"
of 3.1 PSI ~ 220 mbar
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So I'm thinking the observations could have been influenced by something else.
What about pumping losses?
Could an increase in pumping losses - due to an increase in crankcase pressure - stall the engine?
I think so!
Take a look at this article talking about petrol engines and the effect of negative pressure in a combustion chamber on pumping losses
Part Load Pumping Losses In A Spark Ignited IC Engine
If negative pressure on that side of the piston isn't something that's ideal - I'm sure positive pressure on the underside of the piston is equally as bad.
I reckon 'cos the engine is only idling - at a stage when it has little power and most of its "effort" is used in just keeping going - an increase in crank case pressure is going to make it struggle. And then I think it will make it stall - so long as a seal isn't blown out.
If crank case pressure is increased when the engine is running at higher speeds I think this effect will not be noticed and you are then much more likely to blow out a seal.
So there's my thinking on the subject. What do you guys think?