[ItBites]

All the supposition from the initial poster is true.

The reasoning is correct when an engine is accelerating. A Brake dynamometer will give more accurate numbers than an Inertial dynamometer... Always!

Inertial dynamometers should be used for comparison purposes for back-to-back tests to determine results of changes, not for determining SAE-certified horsepower. To get true SAE horsepower, you need a Brake Dynamometer.