As mentioned previously, vacuum secondaries are basically useless inside the pressurized enclosure. Since most vintage-Mustang owners utilize vacuum-secondary carburetors and will need to make the swap to a mechanical secondary carb, Bob recommends upgrading to a QuickFuel four-barrel, which provides removable jets on the top of the carburetor for tuning the high-speed air bleeds, unlike most carburetors that supply a predetermined orifice size. Compared to one large opening in more traditional boosters, the QuickFuel carbs also have annular boosters with 12 small openings for improved fuel atomization.
For tuning the carburetor, Bob recommends investing time on a chassis dyno, preferably one with an O2 sensor to determine air/fuel ratio, as opposed to "running up and down" the highway for seat-of-the-pants evaluations. "I like to run the car on the dyno without the blower belt to make sure the carburetor is providing a good air/fuel ratio without the supercharger. For most street applications, you can then go up four to six numbers for the secondary carburetor jets. The rest of the tuning takes place with the high-speed air bleeds. That's the air leak between the jet and the booster. The more air pressure you have going down the high-speed air bleed, the less fuel is going to come up to the booster. Fine tuning can be done by changing the high-speed air bleeds; most tuners end up between 0.024 and 0.028 sizes."
For timing, Bob recommends backing off from the stock Ford small-block's 34-36 degrees to 28-30 degrees. "You're not always sure of the octane rating from your local gas station, and unlike newer cars, carbureted Mustangs don't have the benefit of electronic spark control to keep everything safe. It's best to sneak up on the timing. It's hard to sneak up on it if the engine is broken."
Supporting Cast
When adding supercharged power, whether on a '66 hardtop such as Steven McCarley's or a late-model 5.0, it pays to make sure the rest of the car can handle the added horsepower. Most importantly, the engine needs to be in good shape; boost puts a lot of pressure on the rods, crank, and bearings. Horsepower also makes heat, so be sure the cooling system is capable of handling the additional temperatures. Steven elected to replace his stock-size radiator with a larger-capacity Griffin aluminum radiator from Mustangs Plus. He also shelved his vintage crankshaft balancer in favor of an SFI-rated ProSport balancer, also from Mustangs Plus. To keep up with the boost, an Auto Meter vacuum/boost gauge was installed. Underhood, a fuel-pressure gauge, also from Auto Meter, was added to the fuel line.
Supercharged power gets you moving fast quickly, so sudden stopping becomes a priority. Steven plans to replace his factory front discs and Stainless Steel Brakes Corporation rear discs with a complete SSBC Force 10 system, including 12.8-inch front rotors, huge four-piston calipers, new brake lines, and a master cylinder.
This view from the passenger side shows how the Paxton throttle linkage connects to the vintage Mustang linkage. | 
The supercharger bracket bolts to the cylinder head with provided 3/8-inch bolts. | 
With Jimmy Grindle, Patrick lowers the NOVI 1500 supercharger into place. |
A braided line is provided for the oil feed. It supplies engine oil to the supercharger from the previously installed oil-pressure sending-unit extension. | 
The oil return line attaches to the fitting at the oil pan. | 
The belt wraps around the crankshaft and supercharger pulleys. Belt tension is applied via an idler pulley that mounts to the bracket. Pressing down on the idler while tightening the bolts sets the tension. |
After mounting the bypass valve and its filter, the discharge tube connects the supercharger outlet to the elbow on the carburetor enclosure. Rubber sleeves with clamps secure the tube. | 
Paxton provides a high-flow air filter for mounting on the inlet side of the supercharger, which faces the rear of the engine on a driver-side installation. | 
Here's the complete installation on Steven's 289. Not only does it look impressive, it also makes impressive horsepower. |