289 Hi-Po Engine ID Guide
Easy guide to build or buy a 289 Hi-po
Cylinder HeadsThe 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.
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.
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.