Ooh, yeah. Those engines are timed for 20k++. Those props are wayyyy too much load, unless as CP suggests the pipe could be tuned for those prop sizes, in which case you might be zipping around pretty good but the engine is impatiently tapping its foot waiting for you to let it rev. You're lugging it in other words. In Europe, for pure RC speed models, they run props as small as 6.5x10 on .45 DF engines.

First kick at the cat I would say trim an 8.8 x 8.0 APC pylon prop down to 8" diameter and get thte thing over 20k. I wouldn't recommend the 8x8 sport prop.

MJD