I had a modeler ask me what is the difference I noted in the horsepower designations of the Germans and British in WWI. The following is the best definition I have found in the fewest words.

"The standard German Pferdstärke metric horsepower unit was expressed in the early 20th century as being a unit of almost exactly 735.5 watts, while the British unit for mechanical horsepower was based on the older 33,000 ft-lb/min figure, which translates to 745.7 watts instead.

German and British horsepower ratings differed. Postwar British tests put the rating of the BMW IIIa at 230hp. This corresponds to British ratings of the Mercedes DIIIa engine being rated by the British as 180hp (German rating of 170hp) and the DIIIaü at 200hp (German-180hp). This discrepancy may explain the significant difference in performance of the BMW IIIa equipped Fokker D.VIIF both against Mercedes powered D.VII and their Allied opponents. . ."

When noting WNW engine designations this is a good piece of info to carry with you.