My commercial, non E61, rotary, 90ml HX, 4L boiler, machine (Rancilio S26) seems to have a very narrow espresso flow rate requirement to delivers its flattest brew temperature.
I drew a representative (but not pretty) illustration to communicate this:
Unless the shot flows relatively fast (a bigger normale), brew temp will climb up during the second half of an extraction. It's so bad that for a true ristretto, brew temp graph will look like half a circle (top line in illustration).
I haven't seen many questions on this before so I'm assuming this isn't common HX behavior. Besides sticking to a faster-flowing extraction, my attempts at flattening brew temp included cooling my grouphead (don't ask how!) during an extraction. While cooling the GH works, it isn't predictable nor repeatable nor convenient nor pretty.
I did try different boiler pressure settings (from 0.5 up to 1.5 bar) but couldn't notice a change in behavior. For a long time, I kept my p-stat at around 1.0bar for a flush'n'go routine. I also tried keeping my p-stat at around 0.5bar or so for some time. But what I settled on for a long time now is keeping my p-stat at 1.4bar and 2-3 minutes before I brew I turn off the heating element and quickly bring boiler pressure down to 0.5bar(via manual boiler fill and releasing steam). I settled on this because it keeps the group head hot enough and ready so I don't have to do long warming flushes. I also settled on this because it was the most predictable way for me to manage my machine; of all other things I tried over the years. I did read a lot about managing temp on a HX over the years, but every time I try to apply what I read, my machine doesn't seem to match what's expected. Ideally, if a pro spends enough time on this machine, he'd crack its code, but, alas, I'm not pro and I couldn't crack this machine after years of trying.
Any one sees possible ways for me to have a flat-ish line temp ristretto with this machine?