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.
As seen in this shot of a...
As seen in this shot of a 289 Hi-Po head, the cast-in spring seats, screw-in rocker studs, and the 289 with two dots are plainly visible and easy to verify with the simple removal of the valve cover.
The combustion chamber shape...
The combustion chamber shape of the Mustang Hi-Po head is identical to the regular D-, C-, and A-code 289 Mustang engines, as are the valve sizes. So do not believe the hype about smaller chambers and/or larger valves when looking for Hi-Po cylinder heads.
Only the service replacement...
Only the service replacement head, C8ZE-B, has
This is our original Hi-Po...
This is our original Hi-Po carburetor on the engine we're tearing down. Note the 1.12 venturi size cast into the main body, plus the 6Z-C stamped into the left front carb foot as well.
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.
According to the carburetor...
According to the carburetor info from Pony Carburetor’s ID book, the '66 Mustang Hi-Po carb with manual trans will have a carb tag number of C6ZF-C, which ours does, and that also matches the 6Z-C stamping number. It looks like everything on our Hi-Po is correct except for the cylinder heads.
The Hi-Po used Ford's standard...
The Hi-Po used Ford's standard iron dual-plane intake, just like the ones used on the A-code Mustang and available over the counter from Ford in their 4V induction kits (seen installed here).
Often found on restored cars...
Often found on restored cars today, as it was available in over-the-counter dress up kits, the chrome open-element air cleaner was a big part of the Hi-Po's look and performance.
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.
The Hi-Po dual-point distributor...
The Hi-Po dual-point distributor has a casting number of C3OF-12127-D for '64-1/2 and C5OF-12127-E for '65-'67 (non-Thermactor engines). As you can see, it has dual points under the cap and no vacuum advance.
The Hi-Po's larger 1-13/16-inch...
The Hi-Po's larger 1-13/16-inch balancer (stock is about 3/4-inch thick) was designed to help dampen increased harmonic vibrations encountered as the Hi-Po reach its rpm potential. Crankshaft pulleys are identical to other 289 V-8s and Hi-Pos only have a single sheave, or groove, as Hi-Pos were not available with power steering or air conditioning.
Seen here are the famous Hi-Po...
Seen here are the famous Hi-Po cast-iron exhaust manifolds that Ford used on the Mustang. The Mustang manifolds differ from the earlier Fairlane manifolds due to the fact that the Mustang had a manual choke and didn't use a heat-riser tube off of the manifold. While cast-iron, they did flow much better than the stock 289 manifolds. Also note the chrome valve covers Ford used on the Hi-Po engine.