hutorihunzu wrote:my last question is why 9 bar and not 11?

Difference in opinion.

The Italian manufacturers say pumps should be set to the pressure for which they designed the machine. They specify adjusting the blind pressure of vibe pumps at 11 bar for two reasons. 1) the pressure on vibes drops with flow much faster than on rotary pumps. so a higher blind pressure is required to keep the pressure equal at normal flow rates. 2) The initial ramp up to full pressure on a rotary pump takes about 2 to 6 seconds, and 4 to 10 on a vibe, depending on the spaces the need to be filled in the group. Therefore, the 11 bar design pressure for vibes is the rough equivalent to the 9 bar design pressure for rotaries.

Third wave cafes and espresso hobbyists say pumps should be set by taste. Based on this, many cafes tend to set their rotaries down to around 8 bar to get a less bitter taste. Hobbyists going by taste usually set vibes to around 9 to 10 bar. If you want to go this route, start at 12 bar, and spend a few weeks trying that setting; drop to 8 bar and spend a few weeks again; then zero in until you find what you like. By starting at the extremes, you'll have a much better idea of what the actual difference is.

Generally speaking, changes of less than 1 bar are very hard to taste. You will also find the best pressure will depend very much on how you like to dose, what roast level you enjoy, etc etc -- so if your practice in these changes, the pump pressure settings should be revisited