While maintaining a basic resemblance to previous Mustangs, the 1971 Mustangs were the largest and heaviest yet. Every dimension increased, including wheelbase – to 109″. Engine choice still remained high – a total of ten different engines was available on the Mustang.
The 428CJ was replaced by the 429CJ as the top Mustang engine option. The 429 belonged to the 385 Engine Series and as such, no parts were interchangeable with the 428. Wider, larger, and heavier, the 429 would not readily fit into the 1970 Mustang engine compartment, which is one of the reasons that the 1971 Mustang got bigger. The bottom end and cylinder block were a variation of the 429/460 block, of which the Boss 429 was also based on; the cylinder heads were similar to the 351 Cleveland in design. You could describe the 429 as a large Boss 302.
As equipped in the Mustang, the 429CJ came with four-bolt mains, forged rods and pistons, 11.3:1 compression ratio, a hydraulic cam,(similar to the Boss 429’s) very large ports and valves, 2.25″ intake and 1.72″ exhausts. Regular production 429/460 engines came with cylinder heads that had smaller ports and valves. The 429CJ came with a Rochester Quadrajet four-barrel carburetor, and some early units had adjustable valve trains. All 429CJs came with aluminum valve covers.
The 429CJ became a 429SCJ if the Drag Pack option was ordered. It consisted of a 3.91 or 4.30 rear axle ratio with Traction Lock or a 4.11 ratio with a Detroit Locker rear. Both of these engines have the engine code C. Some Mustangs with the Drag Pack also included an external engine oil cooler, though of a different design than previously used. The SCJ engine came with a Holley 780 cfm carburetor rather than the Rochester Quadrajet, a mechanical lifter camshaft and adjustable rocker arms.
Mustangs equipped with the 429CJ Ram Air engine got the letter J for the engine code in the VIN. It was rated at 375hp. The 429CJ-R engine could either be a CJ or SCJ, if it had the Drag Pack option or not.
There were 353 429CJ-4V non-Ram-Air engines with the Engine Code C and 1,512 429CJ-4V engines with Ram-Air and the engine code J. Convertibles with the 429CJ engine were quite rare – 9 with the engine code C and 9 with engine code J, according to Kevin Marti.
The competition suspension, available only on Mustangs with 351-2V engines and larger, consisted of heavier duty shock absorbers, springs and front and rear stabilizer bars (rear bars for 351 four-barrel and larger engines only), and staggered rear shocks (except for the 351-2V engine). Mustangs with this option got power steering with variable ratio. The competition suspension was also mandatory with the optional 15×7 chrome Magnum 500 wheels.
The dual ram induction option consisted of a hood with two functional NASA type hood scoops, which are controlled by engine vacuum. Twist type hood locks, a Tu-Tone paint treatment, black or argent and Ram Air decals round out the option. It was available only on 351 and larger engines.
While the rear deck spoiler remained on the option list, the Sport Slats were deleted, due to the low angle of the SportsRoof’s rear window.
For the first time, power windows and an electric rear window defroster were optional. The electric defroster was not available on convertibles.
The Mach 1 sports interior, available on the Mach 1 and all other SportsRoofs consisted of knitted vinyl seats, the 2-spoke deluxe steering wheel, molded door panels, black dash panel appliqué, wood grain appliqué for the center instrument panel, rear ash tray, electric clock, the instrumentation group, bright pedal pads, and color-keyed rubber floor mats stitched directly on the carpets (front only).
The Mach 1 for 1971 used a different grille/bumper combination. The honey comb grille housed two driving lamps while the front bumper was covered with urethane color-keyed to the Mustang’s paint. Fender moldings were also color keyed. The NASA hood was standard equipment (nonfunctional).The hood along with the lower body were painted either black or argent, depending on body paint. Color keyed dual racing mirrors were standard and all Mach 1s came with front fender Mach 1 decals. There was also a small Mach 1 decal on the rear deck above the pop-open gas cap. Boss 351 side stripes were optional.
Standard wheels on the Boss 351 were 15×7 with trim rings/hubcaps. Optional were the 15×7 chrome Magnum 500s. Tires in both cases were Goodyear F60x15 RWL.
Further watering down the overall performance image of the Mach 1 was the availability of the sports hardtop option late in the model year. Based on the hardtop body it used the Mach 1s honeycomb grille and color keyed bumper, the standard Mach 1 hub cap/trim rings, the nonfunctional NASA hood and the Boss 351’s side stripes.