RE: 4 stroke, for speed planes?
Speed
When we went from the 16k rpm Rossis to the 10K rpm Webras in FAI F3A Pattern many of us (myself included) were surprised that the speed of the ships actual rose.
The prop doesn't care about the power source. But you will need much the same power. Maybe a little less as there are less losses (drag and vortex) with the slower prop. But the Reynolds number the blade is operating at will be lower. So going with allow RPM set up I'd look for a wide blade prop with high pitch. The APC prop with respond to heating to facilitate re-pitching (you have a pitch gage?). This is not a recommended practice but I have done it successfully. Do this at your own risk!
Yes, I have re-worked the HP 49VT for more speed running 9X8 to 9x10 props (no poppet valves to limit RPM). But I have to say I liked the Irvine 4cc engine on pipe (heck even on muffler) to the four cycle for the same size plane. The Irvine was much lighter.
With the VT changing the valve timing is just as easy as changing the timing of the inlet rotor on a two cycle engine.
Give it a try and report your findings.
All the best,
Konrad
Sorry, I had no intention of bashing the HP brand or the VT model. Or even the Rossi!