To calculate the torque of a 304HP diesel engine at 1600rpm, it's important to note that power remains constant at a given speed, meaning net torque is not directly calculable without additional data. The torque at 2500rpm is known to be 638 lb-ft, but this value cannot be accurately translated to 1600rpm without empirical measurement or detailed modeling. Fuel consumption rates and engine efficiency can provide insights, but measuring the engine under load is often the most reliable method. While modeling can offer estimates, the accuracy depends on how well the model reflects the specific engine characteristics. Ultimately, direct measurement is recommended for precise torque values at different RPMs.