Pump pressure is an extraction variable that is discussed from time to time. Many people make the reasonable assumption that a higher pump pressure will result in a faster flow rate. Espresso is anything but reasonable, however. In Illy's book, Petracco has a graph (p. 160 of first edition, p. 267 of second edition) that shows a faster flow rate at 5 bar than at either 3 bar or 7 bar.
Since we generally don't extract espresso at 3-7 bar, I decided to do a little testing of my own. Fortunately I own two devices that made the project a little easier, the Schectermatic profiling pump
and the Schectermatic Evaluspromatic
(shoutout to Mark P!)
I pulled a series of shots at 201F using Coffee Klatch WBC blend, dosed at 13.5 - 13.6 grams. Grinder was the 3-phase Robur running at 40 hz. Brewing ratios were in the 60-70% range.
The profiling pump was set to preinfuse at ~3 bar for 4 secs, then rise fairly rapidly to either 8.0 bar, 8.6 bar, or 9.1 bar. With the Evaluspromatic I divided the shots into six segments and weighed the amount of espresso produced in each time period (the first segment was uniformly zero).
The data show that an espresso cake is anything but a simple flow restrictor. It appears that fines migration and/or compaction due to pump pressure makes it a very complex restrictor indeed. In fact, the meager amount of data that I collected suggests that in the normal espresso range, higher pump pressures produce slightly lower
flow rates. This is more-or-less consistent with Petracco's data, if one cares to extrapolate from it.
Some people obsess about the effect extraction pressure has on flavor. There may be some significant effects to be found there, especially by manipulating the pressure profile in the crucial early part of the extraction. But I haven't gotten very far into that yet, and I think it will take a blind taste panel to prove that significant changes can be wrought.
Oh yeah, shoutouts to Greg S, John E, and Sean L, who have all built their own profiling pumps. Greg, in particular, has posted a bunch on this topic here on home-barista.com.