SD-39's are powered by a V-12 645E3 prime mover rated at 2300 hp, the SD-40 by a V-16 prime mover rated at 3000 hp. Due to the reduced horsepower they couldn't make the same top speed as the SD-40, but starting tractive effort was similar. The SP SD-39's were heavily ballasted and for a time had a lower gearing that I'm told was later revised to a more standard 62-15 ratio that most road units had so as to prevent traction motor damage when in consists with other units. SD & GP 39's were found to be more fuel efficient than SD/GP 38's with 300 more horses, and thanks to the turbo they performed much better at high elevations. (Several roads ordered GP-39-2 instead of GP-38-2's with this in mind.) I've always thought the SD/GP 39's sound like an SW1500 on steroids when they switch wide open.