Floyd,
A very nice engine indeed, maybe a little post by itself would be most appreciated by the membership, rather it being lost on the back end of this one.
But you did raise a very valid point about steam pressures.
The higher you go with the pressure, then the more inefficient an oscillator or piston valve will get. Whereas the higher you go with a slide valve, the more efficient it gets.
Just to explain a little.
If you put too much pressure into an oscillator, the port faces start to separate, and so you get inefficient running.
The same sort of thing happens with a piston valve, but at much higher pressures. Instead of blowing off the port faces, which it doesn't have, it starts to leak steam or air between the pressure and exhaust lines because of the very small shutoff and overlap timing of the piston valve.
With a slide valve, the pressure is actually acting against the valve plate, so the more pressure, the harder it seals.
Bogs