Power Numbers
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.
| Hi-Po 289 | Shelby 289 | Gains |
| RPM | HP | TQ | HP | TQ | HP | TQ |
| 2600 | 121 | 245 | 132 | 266 | 11 | 21 |
| 2900 | 136 | 246 | 147 | 267 | 9 | 21 |
| 3200 | 159 | 261 | 174 | 285 | 15 | 24 |
| 3500 | 185 | 277 | 198 | 297 | 13 | 20 |
| 3800 | 207 | 286 | 221 | 306 | 14 | 20 |
| 4100 | 224 | 287 | 240 | 307 | 16 | 20 |
| 4400 | 240 | 286 | 259 | 309 | 19 | 23 |
| 4700 | 253 | 283 | 277 | 309 | 24 | 26 |
| 5000 | 262 | 275 | 287 | 301 | 25 | 26 |
| 5300 | 269 | 267 | 296 | 293 | 27 | 26 |
| 5600 | 272 | 255 | 300 | 281 | 28 | 26 |
| 5900 | 273 | 243 | 301 | 268 | 28 | 25 |
Unfortunately, we didn't have...
Unfortunately, we didn't have access to an original Shelby version of the 715-cfm Holley center pivot carburetor, so we substituted a more common Holley 750. Both were more than sufficient for the little 289.
One modification not installed...
One modification not installed on the Hi-Po 289 for our dyno session was this reproduction Cobra aluminum oil pan from Tony Branda Performance. We doubt the oil pan would show any power gains, but there is no denying its visual appeal.
Though of limited benefit...
Though of limited benefit on the dyno, in the real world these trap doors keep the sump full of oil to eliminate oil starvation under acceleration, braking, and cornering.