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?
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!