Gross and Net HP of Tank engines

Horsepower is officially defined as "the amount of energy required to lift 550 pounds, one foot, in one second." This definition dates back to when cars didn't even exist and, despite the worldwide acceptance of the metric system (America not withstanding) in which feet have been replaced by meters, the definition remains in use today.

Generally speaking, horsepower is an indication of how capable a vehicle is at moving itself around and, consequently, the more horsepower a vehicle possesses, the quicker it can move between two points. In addition to determining how quickly a car or truck can move, horsepower has a direct relationship to top speed. As a vehicle's speed rises, additional horsepower must be expended to push it.

So if the definition of horsepower is so straightforward (550 pounds moved one foot in one second), how can the same vehicle have different horsepower ratings? For instance, Leopard-2 MBT have the 47.6-liter  MB-873 Ka-501 engine at 1500 horsepower. Yet  in realiti the tank  has 1252 horsepower. So what gives?

Gross output, is the output of a ‘bare’ engine running on a test stand with no external engine-driven accessories (e.g., alternators or water pumps), free-flowing exhaust headers with no mufflers, and optimal ignition timing. Gross ratings are also mathematically corrected for standard atmospheric conditions. In other words, gross output represents a particular engine’s maximum output under ideal conditions.

Net hp is what the engine is capable of in a car. It is also taken on an engine dyno at the flywheel, but with the engine equiped as it would be in the car. Stock air filter assembly, cooling system, stock exhaust mainfolds running through stock mufflers, various engine-driven accessories, including the engine’s own water pump and generator/alternator and add-ons like the power steering pump and air conditioning compressor. Engines intended for on-road use typically also have restrictive air cleaners and exhaust systems, sound-deadening mufflers, and emissions-control add-ons like catalytic converters and thermal reactors. Engine tuning is further compromised in the interests of reduced noise, better drivability, improved cold-weather performance, and lower exhaust emissions. All of these factors reduce the engine’s maximum output in ways that the gross rating methodology does not reflect. 

Below we can see Gross and net HP of different tank engines