[Bisimoto]

That is a great question. Roller style dynos use the ability of the test vehicle to overcome a large interial roller to calculate torque, and eventually horsepower. Inertial dynos are horrible as a tool to tune vehicles at partial throttle and steady state parameters due to its key purpose: relying on sweep runs to yield data.

The tyre interface introduces many variables as well: tyre growth, tyre adhesion (or lack therof on high hp applications), tyre sizes, pressure of strapping down to dyno, wheel alignment...all of these can skew/marr results.

In regard to peak power, my dyno unit reads 7hp less than a local dynojet (6 miles away from my current facility). As I have spoken to local tuners, cost of investment in this type of dyno is the main reason why shops shy away from the Dynapack unit that I have.