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How To Install A Paxton Supercharger
Adding A Supercharger To A Carbureted 289 Proves That Nothing Beats Boost
Boost is a wonderful thing. That's why Carroll Shelby added the Paxton supercharger to the '66 GT350's option list. If you've never been in a blown Mustang, we recommend you avoid the experience unless you're prepared to install a supercharger on your own. Once you've experienced boost, you'll never want to go back to natural aspiration.
In 2002, Paxton reintroduced its supercharger system for '65-'68 Mustangs with carbureted small-blocks. Utilizing the modern, gear-driven NOVI 1200 supercharger and a carb enclosure similar to the original, Paxton's kit made supercharging accessible to the masses, not just to those who were able to piece together the rare and expensive components from Shelby's original system. This kit provides everything needed to supercharge a carbureted Ford small-block, including the mounting bracket, a boost-referenced mechanical fuel pump, a bypass valve, a crankshaft pulley, and a belt. Recently, Paxton made the NOVI 1500 available with the carburetor kit, so now there's a choice: boost, or even more boost.
Before embarking on a supercharger installation, there are some things you need to know. Boost works best with 8:1 to 9.5:1 compression ratios. If your small-block is set up for 10.5:1, consider replacing or modifying the pistons to drop it to 9.1:1, which Paxton considers ideal. If your engine is equipped with two-barrel induction, you'll need to swap to a four-barrel intake and carb. If you already have a street four-barrel with vacuum secondaries, you'll need to switch to a carb with mechanical secondaries because boost throws off the vacuum signal to the secondary diaphragm. In that case, consider upgrading to a carburetor from QuickFuel that offers more tuning possibilities than a standard Holley or Autolite. With a stock or Edelbrock Performer intake, the carburetor enclosure will fit under a factory hood; taller intakes may dictate the use of a hood with a cowl rise or scoop.
The NOVI centrifugal superchargers use engine oil for lubrication-oil needs a way to flow from the engine, through the supercharger, and back into the oil pan. Paxton provides fittings and hoses to obtain oil pressure from the oil-pressure sending-unit port in the engine block. Some cringe at the thought of punching and tapping a hole in the oil pan for installation of the oil return-line fitting, but the Paxton instructions detail how to do it in the car without the risk of metal shavings or paint chips falling in.
You may have to make some sacrifices for the pleasures of boost. Paxton offers options for installing the supercharger on either side of the engine. The passenger side requires the relocation of the battery; with a driver-side installation, you have to give up air conditioning and power steering unless you're willing to improvise.
Paxton supplies a comprehensive kit designed to bolt on to a stock '65-'68 small-block, but it can't anticipate the various modifications made to older Mustangs. Wider radiators can interfere with the crankshaft pulley, aluminum Cobra-style oil pans require removal for drilling and tapping for the oil return fitting, and carburetors may require linkage modification to fit inside the enclosure. Be aware of possible interference problems with larger, late-model-style distributor caps and some valve covers. On '65-'66s with the factory 5/16-inch fuel line, plan to upgrade to at least a 3/8-inch line. In other words, be prepared to replace components and tweak others. Don't worry-it will be worth it.
At last year's Mustang Club of America 30th Anniversary Celebration in Birmingham, Alabama, Paxton's Gil Cormaci was impressed with Steven McCarley's '65 Mustang hardtop ("Homebuilt Hot-Rod," Mar. '07, p. 36). The only problem, Gil said, was that it wasn't equipped with a Paxton supercharger. We figured we could remedy that situation. Since Steven and his Mustang are located near Atlanta, we called our old friend Chuck Gutke at Cobra Restorers for installation assistance. With more than 25 years of experience with Cobras, Shelbys, and high-performance Fords, Cobra Restorers is well versed in adding superchargers to carbureted Ford engines. Tech Patrick Kelley handled the installation, with frequent comments and an occasional helping hand from Jimmy Grindle.
After some initial fuel delivery challenges which turned out to be little more than a new-but-defective fuel filter, Scott Milner at Coupe Performance made minor adjustments to the QuickFuel carburetor to take Steven's 289 from 240 naturally aspirated horsepower to 355 hp with the Paxton supercharger. That's an impressive 115hp gain for a small-block Ford.
Steven put his own spin on how it feels to drive a 355hp supercharged Mustang: "First of all, it's amazing the respect a blower gets on the street. Nobody expects a '65 or '66 Mustang to have a supercharger underhood. It just blows-excuse the pun-people away. Of course, the car is a handful. If you just dump it, First, Second, and Third gears are basically useless, sending the BFGs up in smoke. With some pedaling, you can keep the tail out all the way up an interstate entrance ramp-ask me how I know."
Steven is ready for more. His high-mileage 289 will be replaced by a roller-cammed 306 small-block with a steel crank and prepped rods. The goal is 450 rwhp.
On the Dyno
Bolting on the supercharger was a simple task at Cobra Restorers; Patrick had the engine running and the supercharger whining within a day and a half. Immediately afterward, Patrick and Jimmy trailered Steven's Mustang to Jeff Harris and his Dynojet at ProSpeed Performance, where it became apparent from the first aborted pulls that the engine was leaning out at higher rpms. Instead of risking damage, the dyno testing was postponed until the problem could be investigated. After haggling with carburetor tuning and adding larger fuel lines, the culprit turned out to be a defective fuel filter with an internal restriction, purchased new from a local auto parts store. Sometimes you need to look for the small things.
Back on the chassis dyno, this time at Coupe Performance and with additional tuning by Scott Milner, the 289 responded with 355 rwhp at 6,000 rpms, a 115hp increase over the 240 hp without the supercharger.
Tuning for Top Performance
Boost crams more air into the engine, usually meaning more fuel is needed than a factory-style fuel system can handle. Because tuning for boost is different than tuning a naturally aspirated engine, we turned to Paxton's Bob Endress for advice.
"You need a fuel line that's large enough to provide an additional 150 to 200 hp worth of fuel," Bob says. "In some cases, people begin chasing around the carburetor looking for a problem that doesn't really exist. They get futher away from their base tune until they discover the fuel pump isn't getting enough fuel in the first place. It has to be there."
Bob notes the fuel pump needs to add a pound of fuel pressure for every added pound of boost. "If you have a static fuel pressure reading of 6 or 7 pounds with a total of 5 pounds of boost, you're going to need 12 or 13 pounds of fuel pressure at wide-open throttle. That's critical because you must have more fuel pressure coming into the float bowls than the pressure in the enclosure. Otherwise, fuel will stop flowing into the carburetor."
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."
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.