These things really are fascinating. Be sure to click on the “SSME Incredible Facts” link down at the bottom. You know you’re talking about a big engine when its fuel pump produces over 70 thousand HP. They make the comparison that this is the equivalent power of 28 locomotives. Each engine produces the thrust equivalent (in vacuum) of just over 12 million HP, and the shuttle has three of them. If I ran the calculations correctly, each solid rocket booster produces the equivalent of over 75 million HP.
Geof, I just looked up the thrust numbers for the F-1. It looks like though the total thrust produced by all 5 of them used on Saturn V stage I is greater than the the total thrust of the two SRBs, the SRB appears to win hands-down on per-engine thrust. Is the SRB perhaps the most powerful single engine ever constructed?