For the record...
As much as I'd like to see 250 ATW in the HX systems I build, I have my doubts about achieving it with the Nemesis and air- cooled engines.
I could be wrong here. But I won't know for a fact until the next one is built. There are a couple more "tweaks" I've designed into it, that should produce more heat, but only time will tell.
Currently, a 25/45 is delivering 203 ATW, but it's not constant. Constant is more like 190 ATW. (That's at 600 PSI through a #6 jet equivalent.) My preference is higher, so I'll continue to work toward that result. A 27 / 47 Nemesis would probably deliver more heat, but running that through a Nemesis exhaust system will take a major accommodation that I haven't spec'd in yet and don't plan to for the time being.
What I do expect to gain reliably, is another 20 degrees, putting the constant output into the realm of 220 ATW (240 to 250 ATM), which is pretty decent, I think . (This is because the current temp of the exchanger-tank is 120, 20 degrees shy of the target 140, and I know how to correct that.)
When the next one is complete, with the tweaks in place, I'll test it thoroughly and report the results.
One thing I will do, is report ATM temperatures, both in and out, and the jet size and pressure at the jet. That way, it's more in terms of figures that actually mean something to most people. I see ATW as ultimately what matters. But, since no one else states their performance ATW, for me to do so, is handicapping my own results by at least 20 degrees- more like 30.
Anyway, that's the current state of things, regarding the pinnacle of my HX designs.
Very soon after the move (we're moving Thursday), I'm planning to design the "Elite", now that the Nemesis is for all intents and purposes, "done". The Elite will be water- cooled, have 2 blowers and deliver enough heat for both to run at 240 or better, each, and simultaneously.