Finally sorted out my .eng files ! This is prob old hat but I couldn't see this trick anywhere clearly so I thought I'd run a quick new post. Sorry if it is something that is around.
Been playing around with "real" numbers for ages to get some realistic performance and to make driving narrow gauge locos interesting.
After a bit of reading around it was clear that things like the info on the "eng file calculator" was a bit thin on the ground as most NG information is pretty limited. (often weight, cylinder size, and maybe power output). Going back to first principals using info out there in the forums I was able to get some estimate of the required maxboiler output using the loco speed, wheel circumferance etc etc. (a well known formula).
However, and this is the bit where I stuggled, X2 scale locos with the correct weight and power outputs very quickly ran into a steam production deficit. Then I realised that the wheel raduis figures in my eng files (around 22") were actually the diameter. So I halved it and tried again. Magic ! the steam use and boiler output worked, but the con rods and cylinders were going around like mad silly things
Then a lightbulb moment
At a critical speed the recipricating movement of the cylinders used more steam than the boiler can produce i.e at a particular rpm of the wheels. Having all the right numbers in made the mathematical rpm to boiler output ratio correct. But it made the visible rpm twice as fast. If I doubled the cylinder capacity then the mathematical steam use per rpm doubled but the visable wheel rpm remained correct.
And it works !
Ok so this is prob old hat but I am such a happy bunny now. What this means is that now I need to have max regulator and max cutoff to get the train moving. Upto a critical speed steam prod > steam use but at that speed steam use starts to really increase and so boiler pressure is lost. But (and this is the bit that I really like) if I start to reduce the cutoff then I can lower the steam use = steam production. So the loco starts to do correct behavoir going downhill. It uses more steam depending on its speed not the regulator position. Going downhill it is possible to greatly exceed steam production rates even with the regulator set low. the way to preserve boiler pressure being to match the cutoff to the speed.
Oh I am so happy !
I am putting together a little spreadsheet with this on.