Kind of a science question: I've been hacking the electronics of a cheap espresso machine a bit to see if I can get better results, and was wondering - espresso is recommended to be brewed at around 130 psi of pressure. That's a lot. My machine is a single-boiler steam pressure machine, and from what I can tell it barely manages 5 psi when pushing water through the puck.
When I use compressed air to step up the pressure to 60psi, the water shoots through in about 1 second - a tiny bit less than the recommended 25 sec :). So how is this supposed to work in a "real" high-end machine? Seems to me the tamped coffee is never going to be able to offer up that much resistance, to where 130 psi of back pressure could be sustained.