It's interesting that the bore and stroke for the .65AX is almost square so i would expect this engine to be less of a revver and more for torque.

OS list the practical rpm as 12,000 for the .65AX.

The old .61FX is over square with a larger bore than stroke and OS list the practical rpm as 17,000rpm for that engine.

The claimed power figures for both engines favours the .61FX by about 10%

I'd love to see some comparisons between the two with the larger prop sizes.