The Thrill of Overkill
Story Bob McClurg
Craig Conley of Paradise Wheels, Inc. is one of those guys who likes to think outside the box; and this Twin Paxton-supercharged blow-through, carbureted small-block Mustang is certainly proof of that. Talk about the “mad scientist approach!” “They always say if one is good, then two is better,” says Craig.
Inspired by the twin Paxton-supercharged 427 Cobras created by Carroll Shelby, the idea behind both Conley’s Two-Stage and straight Blow Thru twin-supercharger kits for the 260-302 Ford small-block was to create that all important “wow factor,” while delivering between 10 to14 psi of “safe” boost. First up is Conley’s Two-Stage Paxton reciprocating ball drive supercharger system, which consists of a pair of “SN60”outer cases updated with “SN-2000” fluorocarbon seals, a special CNC-cut impeller and “M50” ball races bolted to a sturdy 5/8-inch thick, 6061 T6 aluminum plate. And, like the single supercharged Paxton setups which were an option on the 1966-68 Shelby GT350s, a Paxton air box – albeit slightly modified – works in conjunction with a boost referenced 650 cfm single four-barrel Holley carburetor. Retail price for Paradise Two-Stage Paxton kit is $3,995 FOB San Marcos, California.
Then there’s Paradise’s SN60 cased, 260-302W Straight Blow Thru Twin Supercharger kit, which features one blower per carburetor. Like the Two-Stage kit, the Straight Blow Thru kit – which shares 95 percent of the same components as the Two-Stage kit – also retails for $3,995 FOB, San Marcos, California. Of course, you will have to go out and purchase a 260-302W dual quad intake manifold like a Blue Thunder, an Edelbrock, or Holley-Weiand, or Paradise can supply you with one for approximately $2,500. “We recommend the 600-cfm vacuum secondary Holleys for street applications, or a set of 600- to 650-cfm Holley center squirters for race setups,” says Conley. It should also be noted that either Two-Stage or Straight Blow Thru kits can be ordered with either the “Paxton Blue” or optional polished SN-2000-style supercharger cases. Vintage Shelby GT350 owners already in possession of an OE Single-Stage SN-60 Paxton supercharger system can purchase the Paradise Paxton upgrade kit for $2,000. Of course, it is highly recommended that the original Paxton SN60 supercharger case be brought up to current spec (at an additional charge).
Conley concludes with “any engine that’s slated for supercharging should have a good set of forged-aluminum pistons – no more than 8.5:1 to 9.0:1 compression – a blower cam with a lobe separation of between 110 to 114 degrees to provide for optimum exhaust scavenging, and a good set of cylinder heads equipped with competition-style head gaskets and head studs.”
Recently, we sat in on the installation of a Blow Thru Twin Supercharger kit onto a 1966 Mustang notchback coupe running a “standard” 8.5:1 compression 289 with a .500 lift cam. Dyno technician Bruce Tucker from JBA Racing Dyno Center in San Diego conducted a series of three pulls with the ’66 on Bittle’s Dyno Dynamics chassis dyno. With Chevron Supreme Unleaded in the tank, and 36 degrees advance in the distributor, the following results were achieved. Optimal horsepower increases were congruent with the increase in jet size, as was the increase in air/fuel ratio. Optimal engine torque registered in at 322 to 329 lbs-ft at 3,550rpm. Not bad either. Now, if you’re looking for a “sweet spot,” (219.9 to 222.5hp at 3,550rpm) from an internally stock 289 would be an ideal operating level.
1 To use the twin supercharger system, if you don’t already happen to own either a Shelby in-line dual quad intake, or a pair of Holley 600s, you’re going to need to obtain both prior to bolting on Paradise Straight Blow Thru dual quad twin supercharger system. Automotive aftermarketers like Blue Thunder, Edelbrock or Holley-Weiand have exactly what you need.
2 The first order of business is the removal of the OE external mechanical fuel pump, which will be replaced by one of Paradise’s high volume mechanical fuel pumps. Next, the alternator is removed along with the fan belt and fan in order to allow the installation of the 8-rib serpentine drive belt crank pulley onto the existing V-belt drive pulley setup.
3 Pulley mounting is accomplished by substituting the series of three OE 3/8-inch factory bolts with three 3/8-inch Allen head long bolts provided in the kit.
4 We see the coil being temporarily unbolted and set aside. It will be re-installed to the new supercharger mounting plate with the coil bracket reversed.
5 The installation of a total of five 11/16-inch hex head studs comes next. Three are screwed on threaded mounting bosses on the right side of the head and two more are screwed into the threaded mounting bosses on the left side of the head.
6 Prior to going any further, it may be necessary to trim approximately 1 inch of material off the heater hose nipple on the water pump to not only clear the inner stud, but also provide enough clearance for the rubber coolant hose. Don’t worry, there will still be enough material left.
7 On goes the bracket which is held in place by a series of six 3/8-16 Grade 8 hex head bolts and one 3/8-16 countersunk Allen bolt.
8 At this juncture, our alternator is re-mounted in position at the underside of the supercharger mounting bracket.
9 The next order of business is the mounting of the supercharger idler pulley to the supercharger mounting plate using a ½-inch bolt.
10 We see the coil being re-positioned and mounted with the bracket reversed to the backside of the supercharger mounting plate.
11 The driver’s side Paxton SN-60 supercharger gets installed, followed with the installation of the air intake duct and K&N conical filter assembly.
12 The driver’s side forward blow thru bonnet and hose assembly comes next.
13 This is followed with the installation of the passenger side Paxton SN-60 supercharger. Note that this unit is “clocked” correctly to provide proper engine clearance and to prevent any engine installation mistakes. It is followed by the rear air bonnet and air intake hose installation.
14 Next comes the re-installation of the fan and accessory drive belt.
15 Installation of the 8-rib serpentine blower drive belt comes next.
16 The passenger side K&N conical air filter and air filter bracket are installed next, right where the battery used to reside.
17 Installing the boost lines to both superchargers is the final step. Then it’s off to San Diego’s JBA Racing Dyno Center.
18 They’re off and running! Results from the first pull yielded 392.5hp at 5,645rpm at 8.9 psi, and 430 lbs-ft at 4,132 to 4,277rpm at 7.4 to 7.0 psi. The air/fuel ratio remained fairly constant at 10.5 to 10.7.
19 Conley goes underneath the hood, and pulls the carburetors for re-jetting. The second run produced 402.8hp at 5,799rpm at 9.0 psi of boost, and 402 lbs-ft at 5,147rpm at 7.8 psi with an air/fuel ratio of 10.8 to 10.5. After a little more tuning, They were able to coax 414.8hp at 5,799rpm at 9.2 psi, and 391 lbs-ft at 5,075rpm at 7.5 psi. Air/fuel ratios fluctuated between 11.1 and 12.1. More importantly, this little black and orange striped ’66 produced 219.9 to 225hp at 3,550rpm, which is some pretty serious horsepower at street operational levels; especially for a reasonably stock 289!