How hard can a human blow?

I cant help you with that. im not really an engine guy, but i am trying to be since I just bought a junk boat and had to basically take the whole powerhead off the other day. maybe you could go by a shop and ask how they test them... maybe search on the internet for a forum.

either way the hardest I could blow is 2.4 on my calibrated electronic gauge. that doesnt seem like much to test leakage.