> Getting back to the original question;

Which does not specify if this is for modeling, field identification or electro-mechanical specification.

> A GP38 has a 16 cylinder normally aspirated (non
> turbocharged) EMD 645 engine and produces 2000hp.

EMD engines are not normally aspirated because they are two-stroke. The GP38 engine has a pair of roots blowers.

> A GP39 has a 12 cylinder turbocharged 645 engine
> and produces 2300 hp.

No mention is made of the difference in electrical systems - after all these are diesel-electrics.

> The main deal about the GP39 is that is was meant
> for slow speed service with more tractive effort,
> but with 4 fewer cylinders, and thus use less
> fuel.

Slow speed service? A GP38, GP39 and GP40 are all rated at 2000 traction hp at 10 mph. At "slow speed", even a GP39 will derate to maintain adhesion. This is often referred to as power matching and was a very common option to allow mixed consists of various horsepower locomotives to run together at slow speeds without slipping.

The GP38 and GP39 are commonly equipped with 62:15 gearing so are quite capable of running at speeds up to 65 mph. Train speed is a function of the horsepower per ton assigned, so it is usually an advantage to use fewer engines to obtain the required horsepower as long as enough powered axles are available to start the train and ascend any ruling grades.

According to fuel consumption charts, a GP39 burns more fuel than a GP38 in run 2 even after correcting for the 15% horsepower differential.

Since this is a discussion forum - I will state this is my opinion and only has a 98% chance of being correct.