Recreating a Hi-Po or Shelby...
Recreating a Hi-Po or Shelby 289 short-block was not terribly difficult. The Demon Engines' 289 was built from a late-model 5.0L block stuffed with a 289 (2.87-inch) crank, rods, and forged flat-top pistons from Probe Racing. The forged slugs featured four valve reliefs to duplicate the factory Hi-Po pistons and static compression ratio.
Looking back at Ford's history during the mid 1960s, you would be hard pressed not to read about Carroll Shelby. Although he is perhaps best known for gracing the world with the Cobra, one of the most powerful and legendary automobiles in history, he also had a hand in many other success stories, like beating Ferrari with the Cobra Daytona Coupe. He was also partly responsible for the tremendous success of Ford's own GT40 project, winning the LeMans 24-hour endurance race two years in a row.
Carroll Shelby also brought us the fabulous Shelby Mustangs from 1965 to 1970. Impressive as these feats were, Shelby wasn't above a little slight of hand. Case in point: After building the first small-block Cobra, he managed to convince journalists (if not their readers as well) that he had numerous examples built and ready for sale. All it took was repainting the one available Cobra and passing it along to the next magazine.
The success of the Cobra didn't go unnoticed by Ford executives. They quickly realized that racing success (or the illusion thereof) paid big dividends to their then-new pony car, the Mustang. Association with the Cobra immediately elevated the Mustang's status. Even if a buyer didn't opt for the Shelby version (most didn't), he or she could live vicariously through those who did.
The original Hi-Po 289 and...
The original Hi-Po 289 and Shelby version utilized the same solid flat-tappet cam profile. Our reproduction Hi-Po 289 cam came from the Elgin catalog and is still available.
Truth be told, the '65 Shelby GT350 wasn't an ideal daily driver. The Shelby modifications transformed it into a serious road racer, which meant certain concessions had to be made. One was ride quality, as the improvements in handling firmed up the ride considerably. Shelby didn't stop with the handling and braking; Ford's 289 Hi-Po was also Shelbized, most notably with the addition of tubular Tri-Y headers and a Holley 715-cfm carburetor with aluminum high-rise intake manifold. These performance mods were over and above the already potent Hi-Po 289 version offered by the factory, taking the power rating from 271 hp for the Hi-Po to 306 hp for the Shelby Cobra version. The question is-did these modifications really amount to some serious power or was Shelby simply repainting the Cobra?
The '65 Hi-Po engine did receive...
The '65 Hi-Po engine did receive dedicated cylinder heads, but the only real difference between a Hi-Po casting and any other 4V 289 head was the use of screw-in rocker studs (no guide plates) and cast-in spring pockets. From a flow (and power) standpoint, the ports on '65 Hi-Po heads were no different than any other '65-'67 289 casting (221 and 260 heads were smaller).
To illustrate the difference between an original Hi-Po 289 and a Shelby version, we built a replica Hi-Po 289 and ran it on the engine dyno, then added the necessary Shelby components. The back-to-back test is the best way to illustrate the power gains offered by the Shelby modifications, as historical records and racer recollections are often somewhat less than accurate. Given the expense of a numbers-matching buildup combined with our minimal (defined here as "none at all") budget, we did the next best thing by building a replica of the Hi-Po 289. It worked on two levels by providing an accurate assessment of the power offered by the original 271hp 289 without resorting to an actual restoration.
Built by the engine pros at Demon Engines, our 289 consisted of a late-model 5.0L block stuffed with a 2.87-inch 289 crank, 5.155-inch factory rods (with ARP rod bolts), and Probe Racing forged pistons. The flat-top pistons replicated the four valve reliefs employed on the '65 Hi-Po 289 engine. On the original Hi-Po, the forged pistons were combined with 52-55cc combustion chambers to produce a static compression ratio of 10.5:1. The final element in the short-block was the solid flat-tappet cam. Our reproduction Hi-Po 289 cam came from Elgin and featured the factory specs of .477-inch lift and 310 degrees of advertised duration (roughly 228 degrees measured at .050). The lift value is figured assuming a 1.6 ratio rocker and a recommended valve lash of .018.
 We modified our C6OZ heads...  We modified our C6OZ heads for screw-in rocker studs and installed a set of valvesprings that allowed our engine to rev cleanly to 6,500 rpm. |  As was period correct, our...  As was period correct, our 289 heads received standard (non-rail) rockers to work with the slotted pushrod guide holes. The heavy cast rockers are not ideal for high-rpm use, but they were original on the Hi-Po. |  We located a suitable cast-iron,...  We located a suitable cast-iron, four-barrel intake manifold on eBay. The Hi-Po 289 relied on the heavy cast-iron manifold while the Shelby stepped up to a much lighter aluminum version. |