RE: Nelson 40 info needed
If the RE engine has high timing, it's best to run it with 7x7.5 carbon props made with continuous strand. It launches at 26,000 just on the front edge of resonance. The preferred fuel is 75/25 for breakin and 80/20 for prime time. Dave suggests lifting the glow plug element about .030". Get a stack of head shims and sneak up on the lowest setting that allows the engine to run right. .040-.050" with either the FAI or nitro head is safe. Over compression will lead to the engine getting grounded quickly. The liner needs to be set around 180-190 degrees exhaust duration for the pipe to work. Without a liner shim the timing is low enough to run open exhaust, but the engine is a weak sister unless it's on the pipe, all the ports are too big to work under the design rpm. You will need to finger choke that soup bowl to coax fuel until the engine is close to launch rpm.
If that engine has a Q-40 liner, guys are using the chopped carbon APCs in the 7.4x8 range. These might be a slight overload, just like compression you want to sneak up on the max load prop or else cook those innards.