I guess many of you are sick of seeing a stream of graphs that don't answer the basic question, "How can I make better espresso?"
Me, too. But here's one last (maybe) graph. It compares shot-pulling with a rotary pump, with and without a gicleur (flow restrictor). Dose and grind were kept constant.
Nowadays most rotary pump machines (GS3, GB5, Synesso, etc) come with a gicleur that restricts the flow of water going to your portafilter so that the initial buildup of pressure takes 5-8 seconds rather than 1-2 seconds.
Interestingly, this initial slowdown makes the shot run far faster once full pressure is reached (see graph). It also means that baristas use a significantly finer grind on gicleur-equipped machines as compared to non-gicleur machines. How that affects the resulting shot is a mystery.
It's worth noting that gicleurs may be unnecessary on vibe-pump machines. Their slow buildup to full pressure mimics the action of a rotary with gicleur. Numerous studies, however, have shown that vibe pumps eventually cause hearing loss and may result in prolonged bouts of insanity.
There's also a shot on the graph that represents a non-gicleur shot that was preinfused with line pressure water (~3 bar) for five seconds before the pump came on. This had a similar effect to the gicleur shots.
Sometime soon when I get most of this preliminary stuff out of the way I'd like to compare the taste and texture of gicleur vs non-gicleur shots.
Just kidding about the insanity part...maybe. :-)