I will often hit peak flow of 2.5-3g/sec during the main extraction after fully saturating the puck and using a 10-20 second hold stage.

Bear in mind that with a flow control valve in place of the fixed gicleur, your C_v is variable and the aforementioned calculations are useless. With so-called flow control, you can dial in whatever pressure drop you want regardless of the flow rate.

The caveat is that if you generate the full brew pressure at the puck and then close off the flow, you'll have to wait for that pressure to dissipate through the puck before the effect of the flow control will be seen.

Generally, when I have my grind dialed in correctly, I'll run 7.5ml/sec for 4 seconds and then close down to 4ml/sec for an addition 3-4 seconds as the pressure begins to rise and hits ~3bar PI peak. At this point I've added ~46g (~2.5x 18g dose) the weight of the puck in water, allowing it to reach full saturation under pressure.

This is when (~8 seconds from pump on) I'll shut off the flow for 12-17 seconds (depending on the coffee), during which time the pressure will slowly decline to ~1.5-2bar and I'll have ~2g in the cup (~4g with the MC4 for reasons I'm still trying to figure out).

Not sure how my practical experience and numbers fit in with the above discussion. Math was never a strong suit for me.

Jake & Michael, Thanks for the math lessons. I love topics like this while I savor a 'spro...

... and think about how to add in all the group tortured route compensations.

Woweee! What a thread! I love how both are right, just depends which part of the shot you're talking about

As with all things espresso I'll bring it back to "yes but how does it taste?"

The original question was "is this a thing?"

And on my Reservoir fed E61 with group manometer fitted (but no flow control), my group manometer gets to 2bar with no pump. I'm not sure I believe it's 2 bar but for sure there is static pressure from the boiler, which takes 5-10s to build.

Anyway. Shots that I "pre-infuse" that way, which isn't that different to what's proposed with a pump "boost" at the start, taste bitter and slightly over extracted vs. not pre-infusing, all else being equal. And so "is this a thing" for me would be a thing if you wanted to jam even more coffee into a same sized portafilter while keeping everything else the same - and hence reduce the over extraction believe I'm seeing (and reducing the risk of channeling).

So is this "the" thing? For me I'm not sure.