I think the drops 'impacting' from a distance will actually give rise to uneven extractions. If the puck is close to the shower screen, their won't be any drops. Compare with your water tap in the kitchen, if you open it just a little bit, for 2-3mm of flow there aren't any drops, just a uniform flow, but from 1+cm the flow has picked up speed and formed drops.
But i could be totally wrong of course.

Actually, the more head space between the top of the puck and the shower scren, the less channeling and the longer the delay from pump start to first drops (on machines with no explicit preinfusion mechanism).

In fact, the easiest, and perhaps the best, way to get preinfusion is to either dose less or use a deeper basket.

wfallon wrote:Has anyone experimented with a sort of manual preinfusion, along the lines of this:
-Remove portafilter
-Dose, Level, Distribute, etc
-Tamp
-*Place portafilter directly under group
-*Run pump for a few seconds, soaking the puck evenly with the spray from the shower screen
-Pump off
-Lock and Load and Brew!

Another thread got me curious about how quickly the headspace fills up with water, and I tried this. It's hard to do without making a mess. Try it yourself. I predict a soupy liquid of watery grinds will overflow your portafilter before any appreciable preinfusion takes place.

another_jim wrote:Actually, the more head space between the top of the puck and the shower scren, the less channeling and the longer the delay from pump start to first drops (on machines with no explicit preinfusion mechanism).

In fact, the easiest, and perhaps the best, way to get preinfusion is to either dose less or use a deeper basket.

This makes sense. The extra headspace acts as a preinfusion chamber that must fill entirely before the full pressure of the pump is exerted on the coffee bed.
Since this is something I have the means to try, I went and pulled a regular double dose into a triple basket. I normally don't down dose a basket that much because it makes for a slightly messier cleanup in my experience. But the resulting shot was very nice. The pour was even with only a slightly faster start at the edges.

RapidCoffee wrote:Try it yourself. I predict a soupy liquid of watery grinds will overflow your portafilter before any appreciable preinfusion takes place.

Did you actually pull the shot?

I initially ranked this suggestion up there with welding oil, a bag of post holes, a board stretcher, and blinker fluid. Despite my better judgment and with total lack of regard for public ridicule I went ahead and tried it. Not surprising was the accuracy of you prediction about the mess. This was extra messy and only a silly home barista would even consider it. Lucky for me I have been known to be silly on occasion. I was surprised at the result. I prepped the basket then held it under the dispersion screen, just outside of the bayonet ring. I added enough water to fill half the empty space in the basket, maybe .5 ounces or less probably more like .25. I counted to 5, but I did not notice any significant water absorption. I then locked it in, flinched at the resulting overflow, and activated the pump. The resulting pour began very evenly across the surface of the basket with nice color and flow. I did notice the flow seemed a little faster than the previous shot. The actual pump time was only about 23 seconds instead of the normal ~30. I am guessing this is due to the prewetting of the puck, even though I was not able to perceive any absorption. In either case the shot was fine, for lack of better descriptors. It was messy and resulted in a beautiful pour but the resulting puck was the worst I have ever seen.

This was fun and surprising but it is not something I will be doing as a regular preparation step. Maybe I will do a couple more experiments, just to see if there is any consistent result, other than being consistently messy.

If there are no issues with channeling, I doubt there's a lot of benefit to adding head space or dwell time (the delay from pump start to first drops) by using a deeper basket. Changing dose affects both the headspace and other extraction parameters, so is more complicated in its effect.

Gus wrote:Did you actually pull the shot?

I did. It was drinkable... but not something I would recommend.

BTW, I've been using a LaSpaz Vivaldi I for the past few months. This is a classic non-preinfusion setup: rotary pump (with rapid pressure ramp up) and non-preinfusing grouphead. I have the preinfusion cylinder retrofit kit, but removed it after a few weeks of testing. Prolly time to give it another try.

wfallon wrote:................
-*Place portafilter directly under group
-*Run pump for a few seconds, soaking the puck evenly with the spray from the shower screen
-Pump off
-Lock and Load and Brew!

................

With a single boiler dual use, you don't have to go to that mess to get extra pre-infusion time. Just open the steam wand a bit when you start the shot (portafilter locked in). A couple-three seconds will slow the pressure ramp, then close the wand.

Thank you all for the steam wand tip! I'm even more excited for my machine to get here now : )

Hopefully this will keep me away from upgraditis for at least another few months..... lol.

Sincerely,
Bill