It looks like the Ford power ratings for the Hi-Po 289 were pretty accurate and, surprisingly enough, so were those given to the Shelby 289. We know Shelby is famous for the Cobra, but he wasn't above selling a little snake oil every now and then. The modifications performed on the Hi-Po 289 to transform it into a Shelby version (circa 1965) cooked as good as they looked. The combination of the aluminum intake manifold, Holley carb, and tri-Y headers were worth as much as 28 hp and 26 lb. ft. of torque over Ford's Hi-Po version. There are obviously more powerful modifications available for a small-block Ford, but the Shelby mods were certainly tried and true.