1965 Hi-Po Engine vs. Shelby 289 Short-Block - Mustang Tech
Shelby Rated The GT350'S Cobra 289 At 35 Hp More Than Ford's 289 Hi-Po. Was That Accurate, Or Was It Snake Oil?
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.
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?
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.
The original Hi-Po did receive a special set of cylinder heads, but not one that necessarily provided a power advantage. The Hi-Po 289 heads offered a few desirable features, including screw-in rocker studs (no guide plates), cast-in valvespring bosses, and slotted pushrod guide holes (something shared with nearly all pre-'66 heads) that eliminated the need for either guide plates or rail rockers. These features make the Hi-Po heads both desirable and expensive, but any of the more common pre-'66 heads can be converted to Hi-Po use by drilling and tapping for screw-in studs and upgrading the valvespring package (there is no need to cut spring cups). Since all 289 heads flow the same and given the expense of original Hi-Po heads, we chose to build a replica set using '66 C60Z castings that already featured the slotted pushrod holes. The heads were treated to surfacing and a fresh valve job using the factory 1.78/1.45-inch valve sizes. Having been surfaced at least twice in their long life, our heads ended up with 52cc combustion chambers (right on the minimum service limit).
A few other important components were specific to the Hi-Po 289, including the cast-iron four-barrel intake manifold, cast-iron exhaust manifolds, and Autolite 4100 carburetor (with the larger 1.12 venturis). Good luck finding any of these in your local wrecking yard, but they are available from various sources. We purchased our intake and carburetor in running condition (though the carb came with a rebuild kit), while the cast-iron exhaust manifolds were on loan from Tony Branda Performance. If you need something for your Shelby or Hi-Po Mustang, give them a shout-they probably have it in stock. Tony Branda also supplied some of our Shelby components, including a set of cast-aluminum valve covers and matching aluminum Cobra oil pan.
We finished up the Hi-Po 289 with a 2.5-inch open exhaust, an MSD distributor with the ignition timing locked, and a Meziere electric water pump. Prior to start-up, the new Elgin Hi-Po cam was liberally coated with moly-based assembly lube, treated to a quart of Lucas high-zinc break-in lubricant, and finally prelubed using a drill motor to ensure adequate oiling to all rockers. After a computer-controlled break-in procedure and some ignition and carb tuning, we were rewarded with peak numbers of 275 hp at 5,700 rpm and 288 lb. ft. at 4,300 rpm. These numbers are as close as you'll likely get to Ford's power rating of 271 hp for the Hi-Po.
Now that we had established that Ford was telling the truth about their Hi-Po, the ball was bounced into Shelby's court. This was the moment of truth, so off came the cast-iron exhaust manifolds and on went the tri-Y headers. Like the manifolds, the headers were run with simple collector extensions and no mufflers. Next to go was the Autolite carburetor and cast-iron four-barrel intake manifold. These were replaced by a reproduction Shelby aluminum intake (worth replacing the heavy cast-ion manifold on weight savings alone) and Holley carburetor. Unfortunately, we did not have access to an original 715-cfm Holley, so we utilized a more common 750 Holley instead. Truth be told, both were more than sufficient for the little 289 and both could be tuned to perfection.
With our Shelby mods installed, we yanked the hammer once more and were immediately rewarded with over 300 hp and 310 lb. ft. of torque. The Shelby version checked in at 303 hp at 5,800 rpm and 311 lb. ft. of torque, again just a whisker away from the 306hp rating offered by Shelby. Apparently, those Shelby guys knew what they were doing-not surprising since you don't win FIA championships (against Ferrari no less) without knowing a thing or two about making horsepower.
Mr. Shelby, we never doubted you for a minute!
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|