We probably won't make the VT.61 because the molds don't fit our die casting machine. The castings for the 25 and the 61 were made by a camera company in Austria.
And chiefss, yes HP does exist, we just happen to own it.
The main problem with the rotary valve is sealing. The larger the valve the more it is prone to heat distortion. If everything isn't perfect you will loose the seal. This is one reason we require 20% oil in the fuel for the VT's. The oil film makes it all possible.
HP actually had designed a VT .91 but never produced it. You probably wonder where the design came from. Ing. Leopold Habsburg designed it for HP.
Like rotary valves? Here you go guys...
http://www.dself.dsl.pipex.com/MUSEUM/P ... alveIC.htm