Horsepower = Torque x RPM/5252
Torque is the force that can be measured with a dynometer. The trouble is that there are several factors that must be applied to achieve a comparable and repeatable results between dynometers. Also the type of dynometer, whether inertia, hydraulic, or electric have different variables.
These extrapolate rear-wheel power output by analyzing the dyno drum's acceleration rate using a sophisticated accelerometer and computer software. Hydraulic Dyno:
Like most engine dynos, the hydraulic or water-brake chassis dyno relies on a constant speed brake or absorber coupled with a rotor with a rotating element and a stationary element. The rotor elements generate braking force using water or hydraulic fluid to absorb or match the powertrain output. Usually a strain gauge measures the torque reaction between the rotor's rotating element and a stationary element.
Electric or Eddy-Current Dyno:
This type of dyno controls the brake/absorber using electric current instead of fluid, measuring torque output and calculating power based on a strain gauge. Electric current provides much more precise control and minimal spool-up lag time, but you need a gonzo electric supply and the dyno itself is more expensive than other types.
The horsepower and torque numbers are altered with a weather correction factor. The raw power figures are multiplied by the correction factor, giving you the ability to adequately compare engines tested in varied conditions of temperature, pressure, and humidity. Because engines make more power at oxygen-rich sea level locations than they do atop the Rocky Mountains, or in dry and cool climates as opposed to hot and humid ones, it's necessary to have a formula that takes into account these conditions to level the playing field between different dynos, or even different test days on the same dyno.
The Society of Automotive Engineers have developed several correction factors for dyno testing, and it's important that if you compare numbers from one dyno with the next that the factor be the same.
The SAE J1349 correction factor, utilizes the following numbers for reference: 77 degrees Fahrenheit, 29.234 in-Hg barometric pressure, and zero percent humidity.
The other correction factor, SAE STD factor, references 60 degrees Fahrenheit, 29.92 in-Hg, and zero percent humidity.
Using the STD factor usually pushes the raw power numbers approximately 4 percent higher than the J1349 correction factor.
I would avoid comparing results from different dynos, and stick with one and just compare results using the same correction factors.