i found the formula in a issue of car magazine last august but like you i didnt know where they got the 5252 from so i had a quick look on google for it and how stuff works turned up this
http://www.howstuffworks.com/question622.htm
its worked out from 1 hp = 550 ftlb per second over the number of radians in a seconds
its easier to read it yourself than me trying to explain it