Just found this (from a dyno company in ND called RevSearch)...
<snip>
Horsepower is a calculated value. Torque is a real, physical, rotational
force. (Think of a torque wrench.) It's important to remember the dyno
measures torque and rpm and then from these calculates horsepower. On the
dyno it takes more water flow to the water brake to increase the load on
the engine being tested. As the test engine's torque rises more water flow
is needed. As the test engine's torque drops less water flow is needed. The
dyno's water brake does not respond to horsepower. Major adjustments to
water flow are needed as an engine crosses its torque peak but none are
needed as it crosses its horsepower peak. In other words the water flow to
the brake during a dyno test follows the engines torque curve and not its
horsepower curve. Torque is what twists the tire, prop, or pump. Horsepower
helps us understand an amount or quantity of torque. (Torque + time)
Here's an interesting bit of motor head trivia; below 5252 rpm any engine's
torque will be higher than its horsepower, above 5252 rpm any engine's
horsepower will be higher than its torque.
Horsepower = torque x rpm / 5252
<end of snip>
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