I enjoy a good espresso imponderable.

First we begin with a chart showing the capacity of vibratory (left) and rotary (right) pumps. As you can see, a rotary pump is exceedingly overcapacity when used in an espresso machine (900 ml/min or approximately 375 milliliters in the time it takes to pull an espresso). So if the pump is pushing out 375 milliliters while we're pulling a shot but only 60 milliliters escapes, where is the rest of that water going?



Answer: Round and round in circles. The "excess" water is going from the output side of the pump to the input side, controlled by the bypass valve. Brew pressure is adjusted by turning the screw on its bypass valve. The pressure adjusting screw for a Procon rotary pump is shown below:


From Procon exploded view; also see Adjusting espresso machine's rotary pump brew pressure?

Back to your multigroup espresso machine. When the second group solenoid opens, more water exits the pump and doesn't return through the input side of the pump because it's exiting instead from the grouphead. Double the amount of water is exiting the system, but the pressure remains the same because the pump is well below its overall capacity. By extension, all 375 milliters per minute are doing the aquatic merry-go-round when the barista engages the pump with a blank basket in the portafilter.

miKe mcKoffee wrote:There may be no pressure drop on some multigroup machines if starting a shot while another shot is in progress. Such is not the case with my La Marzocco Linea 3AV. If starting a shot (or flushing a group) while another shot in progress there is a brief drop in pressure observed then returns to full pressure. Therefore I wait until a shot finished before starting the next. And wait until no shots pulling before flushing a group or groups.

How much does it drop if measured at the grouphead? From the last thread on the subject (thanks Matthias):

HB wrote:We tried the flush test on Counter Culture's La Marzocco FB-80, the same model used at last year's WBC. According to the panel gauge, the brew pressure dropped less than 1/2 bar when a second group was flushed while another was pulling a shot. I believe the dropoff could be further reduced by adding an accumulator, but we did not have time to test. Next time I will try to remember to bring a Scace II to confirm more accurately.

The 1/2 bar drop was measured before the grouphead's gicleur. I still have not remembered to bring a Scace II, but the momentary post-gicleur grouphead pressure drop should be smaller than the pre-gicleur pressure drop. The La Marzocco at Counter Culture Coffee has more restrictive gicleurs than stock so the flow from the group is really slow.