what HP is my D4 6U

i am a bit confused about the HP of my D4-6U8613. were all of the U the same HP or did Caterpillar increase the HP as the years increased? Thanks

There was an increase in hp about 1955. "Belt" hp went from 51.x to 58.x. There were also other changes to accomplish this. One was increased full load rpm. Later a mechanical balance was added to balance the engine vibrations resulting from the increased rpm. The governor got a different main spring and a different rack setting.

i checked and i have a 6 blade fan. i think my year of mfg is 1953

The Rack Setting Charts are the best source for determining horsepower....