Here's a topic for consideration: If all of the best commercial espresso makers have settled on 58mm as the ideal portafilter size, why are some prosumer machines produced with 53mm portafilters? And why are most consumer lever machines made with 49mm & 51mm portafilters? Is there a reason for 58mm portafilters, or is this another instance of industrial inertia?
Here's what I've found. If you dial in an espresso grinder for a commercial machine tuned to exactly 9 bar group pressure, you'll have to redial it in for a 53mm machine tuned to 9 bar. The same size coffee grounds that work so well for any of my three 58mm equipped machines won't work on a friends VII 53mm portafilter equipped machine. There's something about the puck mass in a smaller/deeper portafilter that doesn't directly translate to a larger portafilter.
This confuses me; why exactly 14 grams of coffee, ground to the same particle size, takes differing times to produce the same volume of coffee, depending on portafilter diameter. And if the extraction period changes (and it does), how does that affect the taste of the coffee (again it does)? Using exactly the same grind settings, and the same amount of coffee (14 grams), and the same technique (WDT & light tamp), the coffee from the 53mm portafilter tasted over extracted and harsh. We had to adjust and readjust the grinder to hit a sweet spot that produced good coffee with the VII. Whereas with my HX machine, a two or three click change in grind settings is insignificant, for the VII there was only a two click difference between good and bad coffee.
Why should the portafilter diameter affect the shot quality so much?