In product specifications of bulldozers, the manufacturers give several figures for the engine power, such as flywheel power, net power - Net SAE J1349/ISO 9249, ISO 14396 power, and Gross SAE J1995 power.

What is the difference between them? Which is relevant and which one do you consider characteristic to represent the bulldozer's power (e.g. for comparison between two models)?

For example:

D9 spec

The engine specs of a Caterpillar D9 (the new model): https://www.cat.com/en_US/products/new/equipment/dozers/large-dozers/104260.html


2 Answers 2


I don't have those specifications available to me, but each one is a measure of horsepower with different stuff attached to the engine.

I.e., "Gross" horsepower is probably an engine on a dyno optimized for power: with all of the accessories (i.e. fan, alternator, possibly the water pump) being powered externally, and probably also with no air filter and possibly with the exhaust manifolds off.

"Net horsepower" and "flywheel power" will be with the engine in some semblance of "as installed" condition.

I wouldn't put too much weight on differences of 5% or even 10% between engines -- given a choice between an engine that'll run all day, needs no special maintenance, and never misbehaves, I'll give up a lot of horsepower if the alternative is some finicky fragile thing.

  • $\begingroup$ If I understand correctly, "flywheel" is the power the engine generates to the dozing after some of its power is transferred to auxiliary systems. That is: Gross = Net + Aux? $\endgroup$ Aug 15, 2023 at 20:04
  • $\begingroup$ Yes, but I have no idea if there's any legal definition of that "Aux". If things are being measured to a well-known specification, pay your $$$ and get your specification. $\endgroup$
    – TimWescott
    Aug 15, 2023 at 20:07

Flywheel power is measured at the flywheel - usually against a dynamometer.

The others are measured against a specific set of conditions maybe at the flywheel or output of the geartrain or some point.

All of these will be specified and the losses or differences can be seen by 487, 479 and 452 hp.


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