The Paxton Supercharger Kit was a rare option on Shelby Mustangs, beginning in 1966. These kits are still available today, and now consist of Paxton’s updated Novi G.S.S. compressor housing, custom air filter element, supercharger mounting bracket, pulley assembly, larger capacity fuel pump, supercharger drive belt, and all necessary nuts, bolts, and hoses, making installation a snap.
The Wheeler Speed Shop-built 289 small-block nestled between the frame rails of Rich Fabbri’s ’66 Mustang fastback generated 318 horsepower on Jones Electronic Technologies’ chassis dyno in normally aspired form. How much power can it generate when its cylinders are pressurized with 6 to 7 pounds of boost from the Paxton Novi G.S.S. centrifugal supercharger? Keep reading to find out.
Once the battery has been disconnected, the radiator overflow and windshield wiper reservoir can be removed along with the Mustang’s 700-cfm Holley carburetor.
Though not necessary to install the supercharger, Glenn chose to remove the factory distributor, providing him with more working room under the hood.
If the ignition coil is bolted to the intake manifold from the factory, it must be relocated to clear the Paxton’s intake ducting.
Next, Glenn unbolted the Mustang’s factory crankshaft pulley, which will be combined with the Paxton-supplied crankshaft pulley to drive the supercharger.
Fuel enrichment would be a major concern when under boost, so the Mustang’s Carter mechanical fuel pump was replaced with a new version. The now Carter pump, on the left, which is supplied in all of Paxton’s kits, utilizes a vacuum fitting located on the top of the pump body to sense supercharger boost, then spikes the fuel pressure to stave off detonation.
The high-volume fuel pump is bolted in place using the 9/16-inch Grade-8 bolts supplied in the kit.
The supercharger mounting bracket is next. Remove the two cap screws from the water pump….
…then bolt the bracket to the engine with the bolts provided in the kit. Note: For installation on early 1964 engines with staggered height bosses, 5/16-inch washers will be needed to compensate for the short boss.
Next, Glenn bolted the Paxton Novi G.S.S. supercharger housing into place.
From there, it was on to mounting the Paxton’s special air filter element, which is encased in a custom aluminum housing. But first, the factory voltage regulator must be relocated 4.5-inches above its original position on the front support.
A trial fitting of the air cleaner screen revealed some slight interference with the freshly relocated voltage regulator. The problem was easily remedied by rotating the voltage regulator 90 degrees.
The air filter element was then installed. The rear of the element should slant upward slightly, then two 3/8-inch mounting holes were drilled into the left front fender to secure the air filter base.
With the inlet system in place, Glen moved back to the Mustang’s Edelbrock intake manifold, where he installed the lower half of the Paxton carburetor enclosure with the extra long studs supplied in the kit.
Meanwhile, it was back over to JET, where technician Sean Murphy began the fuel delivery system modifications by thoroughly degreasing the carburetor.
Next, it was over to the workbench, where the Holley’s float bowls were removed.
…Sean replaced them with Holley’s solid float bowls, par number 1163000.
Though not necessary on Fabbri’s Mustang, some blown steeds will need some additional modifications. Sean drilled a 7/16-inch hole in each float bowl, which was then tapped with a ¼-18 NPT fitting. Four addition 1/16-inch holes are drilled in the carburetor’s base plate, then they are connected with the supercharger kit’s hardware.
Next, Sean replaced the Holley’s primary jets with 0.067 versions, while the stock secondaries gave way to 0.089 jets. Paxton recommends starting with these jet settings, then fine tuning the combination to each application. Remember, changes in altitude, camshaft profile, free breathing cylinder heads, and other intake and exhaust modifications can require different jet sizing to optimize the combination.
Sean preset the float level even with the float level bolt, then reinstalled the fuel bowls.
Before completing the carburetor modification, Murphy installed this block off piece into the cam plate’s hot air tube. Carbureted applications that see positive manifold pressure (i.e. supercharging and/or turbocharging) require the fitting to be blocked off.
From there, it was back over to Wheeler’s, where Gruettke placed the carburetor inside its new home.
Another minor snag was encountered when Glen attempted to connect the carburetor’s throttle stud to the bonnet’s linkage. The inner lip of the bonnet was interfering with the stud when the throttle was opened.
A cutoff wheel was used to sever the end of the throttle stud, allowing it to clear the inner edge of the carburetor bonnet without further hang-ups.
Next, Gruettke connected a new piece of 5/16-inch fuel line from the carburetor and out through the hole in the side of the bonnet.
Wheeler technician John Garcia custom bent the throttle linkage to clear the Paxton’s carburetor enclosure.
It was now time to bolt on the top of the carburetor bonnet and connect the Paxton’s air inlet hoses.
The final steps before fire up were to reinstall the distributor and attach the vacuum/boost line from the supercharger housing to the boost-sensitive Carter fuel pump. Garcia also bent a custom fuel line from the pump to the carburetor to clear the supercharger housing’s large inlet.
With the large amounts of air and fuel that would be force-fed into the 289’s combustion chambers, Glenn decided to add a high output ignition system. The ACCEL coil, which now resides on the passenger side inner fender well, fits the bill nicely.
It’s now time to add the lubricating oil to the supercharger housing. Glenn added the recommended 8 to 10 ounces of Paxta-Trac synthetic fluid to the blower, insuring proper lubrication. Paxton recommends that fluid levels be checked at regular intervals, as well.
Finally, the supercharger belt is installed, Garcia then set the timing at 8 degrees initial advance while Gruettke checked for any vacuum or fuel leaks.
After a quick (mighty quick, from our vantage point in the copilot’s seat!) test drive around the block, the Mustang was put on the dyno rollers at JET once again. When the Paxton generation 6-ponds of manifold pressure, the Mustang registered 447 horsepower at the rear wheels. That’s a whopping 129 horsepower increase at the rear wheels! Not bad at all for an afternoon’s worth of labor.