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1983 HO edition.

      182 The lone exception to the J-code 302 Mustang’s single-exhaust-only penance came when it was combined with the GT Equipment Group. Then, fashionable chrome tips emerged from both sides of the specially stamped rear valance panel. Advertised horsepower remained at 230.

      183 The demise of the spirited solid lifter K-code Hi-Po 289 at the end of the 1967 model year forced Shelby Automotive to adopt the new 302 for the 1968 GT350 Mustang. Naturally, the A.O. Smith assembly team started with J-code 4-barrel cars, then added cast-aluminum single 4-barrel intake manifolds, finned (black) Cobra valvecovers and a mild vacuum-secondary 600-cfm Holley 4-barrel under an ovoid cast-aluminum Shelby air cleaner. The standard J-code iron exhaust manifolds and Mustang GT-spec dual-exhaust system remained in place, although Shelby added a new pipe-within-a-pipe dual chrome-tip configuration. The Shelby top-end engine upgrades bumped power from 230 to 250 hp. With 1,657 built (1,253 fastbacks, 404 convertibles) the 1968 GT350 was the only Shelby Mustang, but the 302, and at 250 hp, the least powerful of the breed.

      184 Many of the first 1968 GT350s delivered to customers lacked the Shelby intake and Holley carburetor due to delays in the emissions certification process. Afflicted cars received the special Shelby valvecovers and air cleaner but retained the clunky Ford 4-barrel induction system. Vouchers were issued with these cars so that (after the certification delay was ironed out) their owners could have the Shelby inductions installed at a later date, free of charge, by the retail dealer. Surprisingly (or maybe not, considering the GT350s de-emphasized performance theme), many customers ignored the Shelby alert and never returned for their upgraded parts.

      185 Restoring some of the performance lost during the 1968 302 escapade, the 1969 GT350 took full advantage of the 1969 arrival of the 351 Windsor on Mustang option sheets. Fundamentally similar to the 221, 260, 289, 302 small-block engine family, the 351 shared the 302’s 4.002-inch bore diameter but received its extra displacement via a taller engine block (9.480 deck height) with which to accommodate its 3.50-inch stroke. Sharing virtually identical Windsor-style (inline valves) cylinder heads with the non-Boss 302, the 351 Windsor was offered with a single 2-barrel and 250 hp (engine code H) or a single 4-barrel and 290 hp (engine code M).

      186 For the 1969 GT350, the A.O. Smith conversion workers (who’d been handling the job since Mustang’s 1968 model run) were spared the task of swapping carburetors atop the new 351 Windsor. Its standard 470-cfm Autolite 4-barrel was retained for emissions reasons, but the job of swapping from the stock iron manifold to an aluminum unit remained, as did the installation of finned aluminum Cobra valvecovers. Oddly, Shelby didn’t revise Ford’s 290-hp rating despite the undoubtedly improved intake efficiency provided by the larger ports within the replacement manifold.

      187 Research shows that Ford and Shelby American product planners toyed with launching the big 351 Windsor aboard the 1968 GT350 rather than settling for the comparatively weak-kneed 302. The idea mirrored Chevrolet’s 1967 launch of the 350 small-block exclusively in SS350 Camaros. In the end, Ford waited until 1969 to launch the 351 across nearly all model lines with less fanfare.

      188 Despite magazine advertisements, dealer sales brochures, and a factory-designated engine VIN code (W), Ford never offered the 427 big-block in any 1968 Mustang. Mustang’s sister, the Cougar, shared the W engine code, and 357 low-riser 427 Cougars were built in 1968.

      189 The only factory-blessed 427 Mustangs came in the form of the 1965 Holman & Moody–built 427 SOHC and high-riser wedge A/FX cars (10 built) and in 1967 when Shelby built three GT500s with medium-riser 427 side-oiler engines. The cost of the 427 nearly equaled the 1967 GT500’s $4,195 base price.

      190 Beyond the scope of showroom available retail offerings, Shelby American did whip up an exciting 427 Mustang test car in 1967. Based on an unsold 1965 GT350 R-Model (5R537), the body was severed at the firewall where a Fairlane-based 1967 front clip was installed. With the big-block-capable, pre-production Fairlane nose clip stampings replacing the narrow Falcon-based structure, the car was used to help develop the 1967 GT500 package, Shelby’s first retail-available big-block Mustang offering. The production 1967 GT500 employed a 428 Police Interceptor-based engine, but this test mule was powered by a full race 427 side-oiler and was completely gutted. With a 1964 Thunderbolt “teardrop” hood bubble and wearing gray primer paint, the cobbled 427 mule car looked more like an NHRA FX dragstrip entry than a road racer but lapped the Willow Springs test track nearly as quickly as Dan Gurney’s GT40 MKII LeMans racer.

      191 It would be easy to dismiss the 11 SelectAire (A/C) 428 4-speed Mach 1s as the hasty work of an over-zealous plant manager looking to dispose of obsolete parts inventory (see Fact No. 142). With the totally redesigned 1971 Mustang’s pending elimination of the 428 engine option, the leftover A/C parts would be useless, so packing them into a few lucky 428, 4-speed Mach 1s might seem a good solution. However, this view is complicated by news that the rule-breaking Mach 1s were assembled at all three Mustang plants. Eight came from Dearborn, two from Metuchen, and one from San Jose. Records show that although all cars were built with the gentle 3.25:1 rear axle ratio, only one car came through with the $54 tachometer/trip odometer gauge. All the better to help the driver avoid over-speeding the compressor.

      192 Kevin Marti’s records on the 11 A/C-equipped 1970 4-speed 428 Mach 1 rule benders offers an interesting insight into Ford’s notoriously conservative stance on limited-slip differentials on its muscle models of the 1960s and early 1970s. While most competing auto manufacturers standardized a limited-slip “posi” differential in more instances than not, unusually large numbers of high-performance Mustangs (and other models) were delivered with “open” differentials, which was an instant handicap in any drag race. To wit, of the 11 4-speed 428 Mach 1s built with A/C, only 2 carried the optional ($43) Traction-Lok differential inside their 9-inch rear axle housings. Years later, Ford made glorious amends by including clutch-type limited-slip differentials in all 1983-up V-8 Mustangs.

      193 It’s only a guess why Ford didn’t include more limited-slip differentials as standard equipment during the super-car 1960s. One theory holds that it was an intentional move to prevent accidental throttle oversteer and subsequent loss of driver control, acting as a precursor to the electronic vehicle stability systems found on modern Mustangs. Instead of using wheel speed (and other) digital sensors to detect tire spin and then manipulate braking force, ignition timing, or throttle position to restore traction and stability, the open differential simply sent the right rear tire up in smoke. With the remaining three tires in contact with the road, directional control wasn’t compromised that much.

      194 Criticism of Ford’s preference for open differentials must be tempered by the fact that Ford offered the industry’s best and strongest differential in the form of the optional No-Spin Detroit Locker. Standard in the 1965 Shelby GT350 (see Fact No. 82), the legendary Detroit Locker was offered sporadically throughout the ensuing years for those who cared enough to step up to its $207 price tag (four times the cost of the standard Traction-Lok). Ford was also very proactive by offering over-the-counter rear axle (and other vehicle system) upgrades through its Muscle Stuff dealer parts retail program. No competing clutch or cone-type limited-slip differential assembly was as strong and effective as the famed Detroit Locker.

      195 Often overshadowed by the headline-grabbing 428 Cobra Jet and Boss 429, the 390 was Mustang’s most common big-block offering with 50,769 built (28,800 in 1967, 11,475 in 1968, and 10,494 in 1969). Without Ford’s decision to switch Mustang’s architecture from the compact Falcon to the midsize Fairlane, fitment of the wide FE series big-blocks would have been impossible. Look for an “S” in the fifth spot of the VIN tag to verify factory-installed 390 GT power.

      196 The most potent of the Mustang’s 390 offerings were the 1967 and 1968 models. Even though major ingredients remained unchanged, their use of 600-cfm Holley 4-barrel carburetors brought them close to their 320 (325 in 1968) horsepower rating. A switch to the smaller 470-cfm Autolite 4-barrel in 1969 undoubtedly dulled the edge, but Ford didn’t adjust the engine’s 320-hp rating.

      197 Many are shocked to learn that the 1967–1969 390 GT engine and 428 Cobra Jet shared the same camshaft. With .475/.486-inch

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