The Big Three's New Diesels put on a Dyno

ShinerBock wrote:
Dynos measure torque and calculate horsepower based on the torque reading at a certain rpm.

Super Chevy wrote:
"What dynos actually measure is torque, not horsepower," explains Roberts. "Since horsepower is a mathematical equation, the dyno measures torque then calculates horsepower."

Learn About Dyno Testing - How Dynos Work

This is a quote from Wikipedia on how an inertia dyno works.

Inertia Sweep: an inertia dyno system provides a fixed inertial mass flywheel and computes the power required to accelerate the flywheel (the load) from the starting to the ending RPM. The actual rotational mass of the engine (or engine and vehicle in the case of a chassis dyno) is not known, and the variability of even the mass of the tires will skew the power results. The inertia value of the flywheel is "fixed", so low-power engines are under load for a much longer time and internal engine temperatures are usually too high by the end of the test, skewing optimal "dyno" tuning settings away from the optimal tuning settings of the outside world. Conversely, high powered engines commonly complete a "4th gear sweep" test in less than 10 seconds, which is not a reliable load condition,citation needed, as compared to operation in the real world. By not providing enough time under load, internal combustion chamber temperatures are unrealistically low and power readings - especially past the power peak - are skewed to the low side.