APC 17 x 8 at 9300 RPMs measured 18.5 lbs thrust static.
Thrust HP : 21.67 lbs thrust static.

One could also change the air pressure, which will change the thrust a little bit. I typed 25.54 and got exactly 18.5 lbs thrust. I am just wondering if we could just change this factor to get more accurate measurements.

Another one:
APC 18 x 8 at 8400 RPMs measured 15.5 lbs thrust static.
Thrust HP : 22.22 lbs thrust static.

It is even worse here, this with the same engine plug etc.