Per original Ford literature, the 9N is 23.87 belt HP max, rated belt HP = 20.29. The 2N is same. The I&T shop manual lists 9N & 2N at 23.6 HP max belt, 16.3 HP max drawbar and the 8N at 27.3 HP max belt, 23.2 HP max drawbar. Sorry, don't have original Ford specs for 8N and don't know why Ford data is slightly off from I&T.
The 8N is more powerful because the compression ratio for later 8Ns is 6.5 : 1 compared to the original 6.0 : 1.
Hope this helps,
Pete P.
Harborcreek, Penna.