I don't think your getting low enough rpm for a max torque reading. If I remember correctly, the formula is HP= torque X rpm / 5252 . I do remember that HP and torque always cross at 5252 rpm's with torque on the declining line and HP on the increasing line.

This is based on automotive type dyno's that load from low rpm to high. Engine dyno's are, or used to be, the water brake type. I've witnessed actual pulls on my own engines but cannot comment on the workings of the dyno itself. Chassis dyno's use a rolling road type of drum to time drum acceleration by the drive wheels of a car, and calculate torque and HP off of the time to spin of the drum. This is my preferred method for 2 reasons. It tells you what you have, HP and torque, at the ground with all the mechanical losses in the drivetrain removed, and, more importantly, you don't have to take the engine out to do it!

Think about this:I saw an episode of "Celebrity Wings" on Discovery's Wings channel about racer and aviator Bobby Allison. They mentioned how he fashioned an automotive engine dyno out of a propeller. The car engine spins the prop for resistance. There has to be a formula for this somewhere. And since we're already using props, it should be adaptable