I just got to it today and I got 105C water out of the group at 1.25 bar boiler pressure. That's quite high I think?
I dropped the pressure to 1 bar and got 98C but lower than that and steam production would suffer.

Does it make sense to try and put some sort of a serpentine on the upper thermosiphon loop to try and cool down the water temp while still maintaining higher pressure in the boiler? Would something like that cause the thermosiphon to stall?

Would increasing the water level have any effect on the group temp? Is it possible the level is low and hot steam is getting in the thermosiphon loop?

What are you using to measure the brew water temp? Levers are especially difficult to measure because of the dynamic water flow.

Here's a thread where DrGary measured his Prestina. Apparent Overheating of Lever Espresso Machines Measured with Scace Thermofilter

The water is superheated (coming out of 1+bar boiler) but the coffee still tastes great. That's because when you have resistance (coffee), the water has more time to cool down.

That's right. The group is designed to act as the heatsink; but it has to have a resistance like the puck, and fill with water, in order to do its job.

Ah, I see.
I have a homemade device attached to a PF with an adjustable valve at the end. I put an empty basket and a T type thermocouple sits right under the basket so the thermocouple is not directly exposed to the water but I guess almost.
I was measuring the temperature by opening the valve a bit to allow for air/steam to be pushed out, pulling the lever down and closing the valve completely after a 2-3 seconds. I guess that would have put more water in (since there is no coffee and the water fills up the whole PF and attached pipes) and would not allow the group to cool it down fast enough.
However, wouldn't the scorching hot water still hit the top of the coffee puck immediately?

It does; but in practice, it seems to make no difference. Overheated coffee has an instant coffee flavor. You can get an unmistakeable hit of this from Pavoni home levers when the group overheats; but I've never gotten it from a commercial spring lever. I guess the exposure is too brief.

My Londinium machine does the same thing, yet I can still get good tasting shots from it.

pinky-and-me wrote:I just got to it today and I got 105C water out of the group at 1.25 bar boiler pressure. That's quite high I think?
I dropped the pressure to 1 bar and got 98C but lower than that and steam production would suffer.

Does it make sense to try and put some sort of a serpentine on the upper thermosiphon loop to try and cool down the water temp while still maintaining higher pressure in the boiler? Would something like that cause the thermosiphon to stall?

Would increasing the water level have any effect on the group temp? Is it possible the level is low and hot steam is getting in the thermosiphon loop?

I have the SCL and have thermocouples on the inlet and outlet tubes feeding the group. At idle, I get ~112C on the hot (top) leg of the thermosyphon and ~92C on the cold (bottom) leg of the thermosyphon. I also have a thermocouple on the bottom of the group where it is mounted to the frame. Here the temperature is ~84C. The group temperature itself is below this. So when hot water from the thermosyphon loop enters the group it is cooled below boiling and to a brew temperature that produced neither sour or acid extraction.

In the hot leg, I have installed a needle valve to control thermosyphon flow and thus cool or heat the group as needed. Without the needle valve, I was getting ~95C at the bottom of the group and the extractions were a bit acidic. Closing the valve, so that the bottom of the group was ~75C, I got slightly sour extractions. I think there is a relatively wide range of intermediate temperatures that produce good tasting extractions. Different coffees might benefit from dialing in the temperature, however, I've not done much with temperature tuning.

Here is a photo of the needle valve, before I insulated it:

Here are temperature profiles for the hot and cold legs of the thermosypon loop. This group temp is the surface temperature just above the position of the portafilter. This is ~8C cooler that the bottom of the group where it is mounted to the frame.

Have you ever put a temperature Scace on it?

I don't have a Scace. I've modified a bottomless portafilter with a valve and thermocouple attached to a blind basket. When I've tried to measure brew temperature, I have to make several attempts because the mass of the portafilter and valve really cool the brew water. Once everything is hot, I see temperatures in the mid-90C range. I never get boiler water (>= 100C) temperatures in the basket. I think the mass of the group really moderates temperature - as it is designed.