Hi there, Ironman,
That's an interesting demonstration, as said above, 'the proof of the pudding etc.'
But one ingredient of the 'pudding' is the moulding sand - please tell us more about the sand you use.
Could it be that some folks in other parts of the world use sand that is less porous than yours and so needs more feed head and venting? Maybe because of the additives they use to make their sand cohesive? Or constituents that are in the sand because of the geology of its source?
I guess foundry videos would get a bit boring if there's too much ramming of the sand shown but some other makers' videos seem to ram harder and longer than you do - could this also be a result of your sand being naturally more cohesive?
I've never done any casting in my life but I have watched a lot of videos - I'm not challenging your results, I just ask these questions to prompt discussion about this aspect.