OK, 18x6 sounded a little ambitious.

I just wonder if I'm "over-revving" with the 15x7 @ 9400 rpm.

Horizon Hobby specs list the benchmark as 15x6 @ 9400 rpm, which I'm getting with the 15x7, and almost 9900 with the 15x6. They list the "prop size range" as 14x6 - 16x6, and then list the HP as "1.7 hp at 8500 rpm."

These numbers are all over the place - hence my confusion.[&o]

I know for a fact it produces over 14 lbs of thrust and doesn't overheat with the 15x7, so I guess I'll be happy with that for a while. The performance of this engine certainly exceeds my expectations, except for the ignition power consumption.