Thank you, Sarosh.

The so-called 'specs', aren't... They are just claims made by most engine manufacturers, to make what they are selling appear better then what the other guys are selling...

As they come; i.e. with the stock muffler and on the recommended fuel, virtually none of them are capable of standing up to those claims.
One example with the OS.46AX, even with the muffler baffle removed (which should give it some power advantage), this engine can spin an APC 'toothpick' 8.75 x 8.25 NN pylon racing prop, at just over 16,000 RPM, on 15% nitro fuel.

This engine is rated by its manufacturer at 1.65 HP at 16,000 RPM, but this toothpick spun at this RPM equated to only 0.89 HP.

Adding nitro to the maximum recommended 20% nitro will not make up for the 48.5% power deficit...
In fact, if this engine spins a larger prop, like a 10x6 APC @ ~14,300 RPM, it is making MUCH more power; 1.1 HP to be specific.

It shows the engine, as it comes, is not designed to even be spun at the manufacturer's claimed RPM range...

Much like using the 'bozo-trap' IR thermometer, trying to prop your engine to see the 'manufacturer's claimed performance' is a waste of your precious time...