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A Dyno is a load unit that measures HP using different methods by different makes. On the old M&W's the input shaft drives a hydarulic pump, and the load is applied with a restrictor valve at the output. The gauge that reads HP is simply a pressure gauge. That model does not consider RPM when displaying HP, thus the reading is only correct at rated RPM. The only way you can read HP at any RPM other than rated is to use the calculating wheel that is provided
The formula for HP meausred at a rotating shaft is: Torque X RPM divided by 5252.1 = HP. Let's look at some examples and plug some numbers into the formula. 1200 Ft Lb torque X 540 RPM divided by 5252.1 = 113 HP. Lets pull the engine down that was running at 2400 RPM to 1800, the PTO shaft is now turning at 405 RPM. If we still have the same amount of torque the number would be: 1200 Ft Lb Torque X 405 RPM divided by 5252.1 = 92.5 Hp.
Now work the formula backwards. If we still have the same power, 113 HP X 5252.1 divided by 405 RPM and the torque would = 1465 Ft Lb. Thus we would need a increase in torque from 1200 to 1465 for constant HP. In a gas tractor you won't see a lot of rise in torque = rise in HP. You gain some from the fact that everything slows down, like the hydraulic pump, engine fan and generator, less drag in the powertrain & etc.
The modern dyno's with the digital displays will show Ft Lb of torque, RPM and Horsepower. Current dyno's the input shaft drives a brake drum. Brake shoes are applied with a torque arm that can be calculated by the operator for correctness. You can see the increase in torque and Hp on the display. The current engines have progressed to the point that they develope a lot more HP at reduced RPM due to huge increases in torque rise. It took a lot of time and $ to get them to where they are today.
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