Hi Darren,

yes it's controlled by the resistance of the puck itself. The size of the gicleur will effect the ramp up time, but once everything is filled with water (the space above the puck, the whole basket) it's pretty much down to the coffee (any excess water is circulated in the bypass). Bear in mind that the ramp up speed will generally affect the whole shot as it 'sets' the fines in a particular position. Preinfusion for example can fix them somewhere mid puck and shots tend to run faster with preinfusion than without.

Regards,
dsc.

Just as an FYI, Bezzera specs a 100 litre/hour pump (PA104X) for their 1 or 2 group machines and a 200 litre/hour pump (PA204X) for their 3 and 4 group machines.

Pump sizing, I would think, is based on a desire for a short boiler fill cycle, i.e. the pump we may choose solely for espresso making may not cut it when other functions or other groups come into use. But boiler fill could be handled by a completely independent system which is (I believe) the way some machines can be optioned out - edit - or, perhaps, "tinkered out" by the end user