The paddle axle is a mechanical 3-way valve.

And here is the video slideshow that Marshall originally posted:
The preinfusion part is around the 3.00 minute mark.

Marshall wrote:This is simply wrong. I suggest you call La Marzocco service directly at (206) 706-9104.

I just got off the phone with them and they clarified.

The volumetric version preinfuses at line pressure only - for a set amount of time programmed by the user.

The paddle version preinfusion pressure begins at line pressure and can be adjusted up to pump pressure the farther you move the paddle over. Thus preinfusion pressure and time are determined manually. On the back end of shot, the reverse can occur by dialing down pressure again from pump to line pressure.

Italyhound wrote:The paddle version preinfusion pressure begins at line pressure and can be adjusted up to pump pressure the farther you move the paddle over.

Unfortunately, that's not correct, either. Anyone who owns a GS/3 MP will tell you that when you begin the shot, you go from line pressure to full-on pump. There is nothing in between. The new Shot Puller and Strada EP's have more control in that respect.

But, as you end the shot, you can taper the pressure down to a very "soft landing." To me, this is one of the best features of the manual paddle. Where the flow starts blonding and breaking down in conventional machines, you can extend the shot with the MP, maintaining a coherent, brown coffee flow for a few more seconds.

To me, the major (actually only) advantage to the volumetric over a plumbed-in MP is the automatic backflush cycle on the volumetric. If that appeals to you, or you don't want to plumb in, I would go with the volumetric.

"The volumetric version preinfuses at line pressure only..." What happens if the volumetric machine is not plumbed in?

Italyhound wrote:The paddle version preinfusion pressure begins at line pressure and can be adjusted up to pump pressure the farther you move the paddle over. Thus preinfusion pressure and time are determined manually.

I wish...but not in the real world (we're speaking about the GS3 here).

Just goes to show you, that calling the "experts" can give you incorrect information too...

vberch wrote:"The volumetric version preinfuses at line pressure only..." What happens if the volumetric machine is not plumbed in?

Another incorrect statement...this should be the description for the MP model...of course with no line pressure, no preinfusion.

The Volumetric version is summed up by Dick Green on the previous page...it does not make a difference when plumbed in or from the tank, it is performed by the pump.

Peppersass wrote:Presumably, it's done this way (with the solenoid closed) so that pre-infusion works the same whether the machine is plumbed in or not. The alternative method Bill describes would allow water to continue to flow into the basket at line pressure if the machine is plumbed in.

Thank you folks for summing up what seems to confuse the well meaning folks who actually make and sell these machines.

You have made it very clear there is little to be confused about.

Now, the question is how pre-infusion in volumetric GS3 compares to MP in a cup. I tried both volumetric and MP machines, but on different occasions, using different grinders and different coffee, so in my mind it is still difficult to quantify the differences.

It would be very interesting to hear from someone who tried them both side by side.

The difference would most likely be indistinguishable using blind tasting with keeping shot parameters equal.
The difference lies in what can be done with the manual paddle toward the end of extraction. Or when coffees/grind/dose is not dialed in, you have a huge amount of versatility with the MP with running preinfusion, shot timing, trailing pressure on the fly.