[WDShaffer]

Absolutely the duty cycle is important. GMC took an onroad design and applied it to a stationary source. Running at a steady rpm with steady load removes the acceleration & deceleration stress of street driving. Having a fixed load (water pump) removes the variable of accelerating different loads, too. Finally, the operators were trained to maintain the pump engines; as opposed to junior trying to get dad's ole farm truck to beat his buddy at the stop lights (provably not a huge issue for the GMCs).