Mark Houlahan
Brand Manager, Mustang Monthly
March 13, 2012

Today, some 46 years later, the K-code "Hi-Po" Mustang is still one of the most sought after classic Mustangs. With just three years of production and something less than 13,000 produced, the K-code certainly has higher numbers than, say, a Shelby Mustang. But when you consider the overall production numbers of the '65-'67 Mustang, the K-code represents barely one percent of production. That's what makes the K-code rare.

The K-code option has had many stories written about it over the years, most based on accurate information, but there have been some discrepancies over the years as information is gathered from various places. It doesn't help that, like most Mustang production information, Ford often made assembly line changes during production shifts just to keep the assembly line moving. Thankfully, the K-code engine itself, built at Ford's Cleveland Engine Plant No. 1 in Brooke Park, Ohio (now home to Ford's EcoBoost V-6 engine), had very few production changes.

With the help of Bob Mannel, author of Mustang & Ford Small Block V8-1962-1969, we've delved deep into Bob's book, original Ford literature, archive photos, and even tore apart a K-code engine ourselves locally that was in need of a rebuild. Along the way, we learned plenty of K-code details and have a new-found appreciation for the performance small-block that Ford initially offered in the '63 Fairlane and eventually installed in Mustangs, as well as the Shelby 289 Cobra, G.T. 350, and several other vehicles. We hope this inside look at the Mustang's K-code 289 Hi-Po engine answers your questions, helps you find that K-code Mustang you're looking for, or assists you in building an accurate Hi-Po 289 for your restoration project.

Engine Block

We'll start with the foundation for every 289 Hi-Po: the engine block. Let's get one thing out of the way right at the start--the Hi-Po block was NOT a special high-nickel block or cast just for Hi-Po builds. The Hi-Po block started life at the same foundry with the same casting numbers as regular 289 D-, C-, and A-code blocks. While there are earlier Hi-Po casting numbers, the one to concern yourself with for '65-'67 Mustangs is C5AE-6015-E (and C4OE-6015-F for '64-1/2). What made the Hi-Po block different was how Ford inspected blocks for Hi-Po use. Ford used a special dye to inspect the block for minute cracks and imperfections. Essentially, Ford used "perfect" 289 blocks as the starting point for the Hi-Po and then added the coveted larger heavy-duty main caps for strength.

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Other misnomers through the years include screw-in oil gallery plugs (a few early '63 blocks had them but it was Shelby that added them during his blueprinting for Cobra and Daytona Coupe use that started this rumor). Standard 289 blocks, including the Hi-Po, had standard 1/2-inch press fit plugs. Hi-Pos did not have screw-in core plugs either. As a matter of fact, the block had no discerning outward differences except that most had the vehicle's VIN stamped in the side of the block (regular 289s did not) and the often used "HP" designation in paint inside the bellhousing area, though not every Hi-Po block got this marking.

Reciprocating Assembly

There are some differences in the connecting rods, crankshaft, and pistons used in the Hi-Po 289's assembly. Let's start with the crankshaft. It has been reported time and again that the Hi-Po's crank was nothing more than a cherry-picked stock crank, inspected and deemed "perfect" for use with the higher revving Hi-Po. While this was true for the block casting, the crankshaft was indeed a revised part for Hi-Po use. According to Bob Mannel's excellent book, the Hi-Po cranks were made in batches with higher levels of nickel and magnesium added to stock 289 crankshafts (they did use the stock casting mold as well). This would give the crankshaft increased nodularity, but it was far from an exact science. As such, each Hi-Po crankshaft from these small runs was checked visually. The rear counterweight was polished and then checked with a microscope. If the crankshaft had enough spherical graphite nodules in a specific measured area, it was deemed to have a "high nodular content" and was used for Hi-Po assembly. Ford also added a small "hatchet" shaped counterweight to the front of the crankshaft to move the counter balancing closer to the core of the engine, which necessitated a thinner timing chain.

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The connecting rods and pistons are much simpler. Ford used the standard 289 connecting rod molds to make the Hi-Po rods. As such, you'll see standard C3AE or C3AE-D markings, but where the difference will be found is in the machining, rod cap, and the fasteners. The Hi-Po rod, due to its larger 3/8-inch retaining bolt, had less material removed from the side of the rod during machining. Ford also used a beefier rod cap for the larger rod nut to seat on and to handle the engine's expected 6,000 rpm. The Hi-Po's pistons were specific to the engine, cast from high-strength aluminum for high compression use and with four valve reliefs to accept the high-lift camshaft. While similar to the later A-code 289 piston, which also had four valve reliefs, the Hi-Po's casting number was different. A Hi-Po piston will have a C4OE-A number while the A-code will have a C5OE-H (both with a 6110 base number).

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Valvetrain

There's certainly a lot going on in the Hi-Po's valvetrain area. Starting with a solid-lifter camshaft, Ford also used mechanical lifters (natch), valve springs with fewer coils than standard 289 springs to prevent coil bind at high lift, a flat-wound damper spring inside the valve spring, special hardened retainers, and thicker camshaft thrust plate (before Level 7 engine change) to reduce flexing/distortion of the cam and its higher loading and rpm. Due to the thicker thrust plate, the cam timing gear had to be reduced in thickness to maintain overall dimensions under the timing cover (just like the crank gear mentioned previously). With the thinner crank and cam timing gears, Ford fitted a thinner timing chain to match.

This timing chain measured 13/32-inch thick versus the regular 289's 1/2-inch wide chain. Using the wrong combination of gears, chain, and thrust plate will cause wear on the “hatchet” counterweight or even timing cover wear/damage. To confuse things further, Ford used two different cam gears in the timing set. Early Hi-Pos (before Level 7 engine change) used an iron gear; while from March 1965 on, the gear was aluminum with nylon teeth. The iron gear used an additional C-shaped spacer to properly locate the gear on the camshaft, while the aluminum gear has the spacer cast in. In a nutshell, use the iron gear, spacer, and thicker thrust plate together, or use the aluminum gear, no spacer, and standard thrust plate together. Do not mix the combinations.

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Cylinder Heads

The cylinder heads used on the Hi-Po 289s are a point of much confusion and misinformation. Let's put some of these fables to rest right here. First off, the K-code heads didn't come with larger valves; they used the same 1.78-inch intake and 1.45-inch exhaust valves as the regular 289 (referring strictly to Mustang based Hi-Po engines here). The heads were also not ported, gasket matched, or anything of the sort. They were cast in the same foundry as regular 289 heads, but did have some obvious features suitable to the Hi-Po's higher rpm ability. One of the most obvious is screw-in rocker studs. The press-in studs found on regular 289 heads would not survive the high rpms that the Hi-Po was engineered for. Guide plates were not required; the heads used the same "close tolerance" pushrod holes as the early 289 heads. Another easily visible difference is the cast-in valve spring seats, which helped stabilize the valve springs at high rpm. The rocker arm is guided by the pushrod on the Hi-Po, thus standard 289 rail rockers are a dead giveaway (especially since the standard and Hi-Po heads have the same combustion chamber size and shape). Lastly, you'll find two dots (versus one) over the 289 designation cast into the head in the valve spring area, and on the end of the head, visible with the head installed on the block, the number 19, 20, or 21. No production head ever had "HP" cast into it, but the C8ZE-B service replacement head does have the "HP" designation, as well as Thermactor Air injection.

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Induction

The Hi-Po's factory induction setup was a standard cast-iron, dual-plane intake manifold with an Autolite 4100 4V carburetor installed with an open element air filter housing. The iron manifold is identical to those used on the A-code 289 and only needs to match your Mustang's model year and build date codes. Ford never installed an aluminum intake manifold on the Hi-Po 289. Where some confusion comes into play is that Shelby did swap out the stock induction for Shelby Mustangs with aluminum manifolds and Holley carburetors, plus Ford did offer some over-the-counter induction setups like dual fours and three 2Vs, and eight-stack Webers, all on aluminum manifolds. Many of these high-dollar induction kits found their way to Hi-Po Mustangs, but they are not factory.

The Autolite 4100 used on the Hi-Po does differ from the A-code version, however. Where the A-code 289 used a 480-cfm 4100 with 1.08-inch primary bores, the Hi-Po was larger (although physically the same size) with a 600-cfm rating and 1.12-inch primary bores. Furthermore, all K-code Hi-Po 4100 carburetors on the Mustang utilized a manual choke mechanism.

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External Hardware

When it comes to the external differences of the Mustang’s Hi-Po 289 versus the D-, C-, and A-code brothers, there are several quick and easy checks you can make. However most of them can be easily be recreated with reproduction parts or restored original parts as these items simply bolt onto the engine block. We’re talking fuel pump, water pump, crankshaft balancer, alternator, distributor, and other such parts. Since these parts are so easily swapped, we’ll go over them briefly as points of information.

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Front and center is the engine’s distributor. The Hi-Po Mustang only used centrifugal advance, and therefore there’s no advance diaphragm and housing on the distributor body. The distributor is also a dual-point unit, which can be easily verified by simply popping off the distributor cap. Also easy to spot is the Hi-Po’s four-blade aluminum fan and thin steel fan shroud with rubber isolator (near the battery tray). Other Hi-Po only features visible at quick inspection are the larger than stock crankshaft balancer, larger diameter alternator pulley (or generator pulley for ’64-1/2), Hi-Po specific cast-iron exhaust manifolds, and chrome valve covers.

Closing Comments

As noted at the beginning, the Hi-Po engine came to life first in the '63 Fairlane. Due to its March 1963 introduction, there was more than a year of Hi-Po production before the engine was ever dropped into a Mustang. As such, there are numerous design changes and part number differences we didn't have the room to discuss here (we kept this conversation Mustang-specific). If you want to learn a lot more about the Hi-Po in all of its iterations, not to mention every bit of minutia on the Ford small-block family from 221 to 302, then you owe it to yourself to check out Bob Mannel's excellent book. Bob graciously offered his assistance in gathering info, supplying photos, and verifying our story and we barely scratched the surface. His book is available via RPM Press at www.fordsmallblock.com or you can reach RPM Press at 423/245-6678 for phone orders.