Donald Farr
Former Editor, Mustang Monthly
September 17, 2006
Photos By: The Mustang Monthly Archives

Forty years ago this month, Ford added the 390 big-block to the Mustang's engine lineup. It would be the start of a five-year production span for big-blocks, a relatively short period in the Mustang's 42-year history that produced some of the most powerful and legendary Mustangs of all time. Between '67 and '71, we saw Shelby's dual-quad 428, the 428/429 Cobra Jets and Super Cobra Jets, and the Boss 429. With tons of torque and plenty of image with Shaker hoodscoops and Mach 1s, it was a great time for Mustang performance.

'67-'69 S-Code 390
For once, Lee Iacocca didn't get his way. The father of the Mustang and then-president of Ford Motor Company never intended for the Mustang to grow beyond its original format as a small and sporty car. But when Chevrolet countered for '67 with the Camaro, complete with the big-block 396 on the option list, the Mustang was forced to up the ante as well. To accommodate the FE-family 390, the Mustang grew longer, wider, and heavier.

Adding the four-barrel 390 to the Mustang turned out to be merely a stop-gap measure. Rated at 320 hp, the engine added power and torque to the Mustang (previously, the 271hp 289 High Performance ranked as the Mustang's top performance option) and compared favorably to the Camaro's original 325hp 396 Turbo-Jet, but it was no match for the 375hp L78 396, which was added to the Camaro's engine lineup at mid-year '67. Ford finally countered with the 428 Cobra Jet in '68.

The S-code 390 remained on the Mustang's option list through '69. According to Kevin Marti's Mustang ... By the Numbers book, 29,457 of the 390 Mustangs were sold in 1967. The numbers dropped to 10,650 for 1968 and 10,549 for 1969, a decline that was no doubt caused by the addition of the 428 Cobra Jet.

In '68, Ford also offered an X-code 390, a stump-pulling 390 two-barrel, of which only 733 were built, all with C6 automatic transmissions. The first edition of the '68 Mustang sales brochure also listed a 427 on the powertrain option list, but according to Marti's statistics from Ford, none were ever produced.

'67 Shelby GT500
Like the Mustang, the '67 Shelby grew longer, wider, and heavier. But unlike Ford, Carroll Shelby wasn't content to simply work with a passenger-car engine for his new GT500. Instead of the production 390, Shelby utilized the 428 Police Interceptor and topped it with an aluminum intake manifold with an impressive-looking pair of 650-cfm Holley four-barrel carbs. Shelby rated the uniquely carbureted 428 at a conservative 355 hp. More than likely, it made closer to 400.

'68 1/2 Cobra Jet
Bob Tasca, proprietor of the performance-oriented Tasca Ford in Rhode Island, was disappointed with the Mustang's 390 offering and set out to do something about it. When the chance arose to replace the 390 in a '67 coupe, Tasca's technicians dropped in a 428 Police Interceptor short-block with reworked low-riser 427 heads and a 735-cfm Holley four-barrel. The result was a Mustang built from the Ford parts bin that could turn low 13-second quarter-miles. Tasca knew he was onto something, so for '68, he began offering the package as the KR-8 for "King of the Road, 1968."

The KR-8 drew the attention of Hot Rod magazine, which featured Tasca's creation in the Nov. '67 issue along with an address for Henry Ford II so Ford fans could influence Ford to build a production version. Pressure from Tasca along with letters from Hot Rod readers resulted in the 428 Cobra Jet for the Mustang and Fairlane/Torino.

In December 1967, Ford kicked off the Cobra Jet program by building 50 Wimbledon White Mustang fastbacks for Super Stock drag-racing competition. On April 1, 1968, the 428 Cobra Jet became a factory 335hp option for Mustang GT coupes, fastbacks, and convertibles. The cars were easily recognized by their black hood stripe and functional scoop, which funneled cold air into an air cleaner with a vacuum-operated flapper valve.

'68 GT500KR
At the beginning of the '68 model year, Shelby continued using the 428 Police Interceptor big-block in place of the Mustang's regular production 390. However, the dual-quad induction from '67 was ditched in favor of a single 715-cfm Holley carburetor on an aluminum high-rise intake. But with the introduction of the 428 Cobra Jet in the Mustang, Shelby took advantage of the CJ's regular production status to introduce a new model, the GT500KR, apparently taking homage from Tasca's "King of the Road" name. Basically, the GT500KR offered the upgraded horsepower of the 428 Cobra Jet in the Shelby's sleek and beautiful body style.

'69-'70 Cobra Jet
The 428 Cobra Jet engine settled in as the Mustang's primary performance offering in '69 and continued right through '70. Available in every body style, the CJ found its most comfortable home in the Mach 1, a new fastback performance model with the name and image to complement the CJ's power and performance.

During '69-'70, the 428 Cobra Jet was offered as either a Q-code without ram-air or an R-code with the functioning Shaker hoodscoop. Other than the functional ram air, the engines were the same; both were rated at 335 hp.

'69-'70 Super Cobra Jet
To address durability concerns, Ford began adding an external engine oil cooler, 427 "Le Mans" cap-screw connecting rods, and different balancing to 428 Cobra Jet Mustangs equipped with the optional 3.91 and 4:30 rearend-gear ratios. By February 1969, the digger gears and durability upgrades were promoted as the Drag Pack option. To differentiate the heavier-duty engines from the standard CJs, these 428s became known as Super Cobra Jets. To accommodate the engine-oil cooler in front of the radiator, the passenger-side horn was moved adjacent to the driver-side horn. Contrary to myth, the Super Cobra Jets did not produce more power. They were rated at 335 hp just like standard Cobra Jets. Also, not all SCJs were equipped with the ram-air Shaker hoodscoop; they were available in both Q-code and R-code form.

Very few SCJs were produced. According to Kevin Marti's production data, as noted on the Web site www.428cobrajet.org, less than 4,500 '69-'70 Mustangs and Shelbys were equipped from the factory with the 428 Super Cobra Jet package. You can find the complete breakdown at the Web site.

'69-'70 Boss 429
Of all the big-block Mustangs, none are more revered in musclecar circles than the Boss 429. Created to homologate the hemi-headed engine for NASCAR competition and with final assembly at a special off-site facility, the Boss 429s boasted a rare combination of super-high performance, mesmerizing underhood looks, low production, and modified inner fenders just to fit the monster under the hood. Even the low-key appearance-no stripes or unneeded doodads-added to the appeal.

The Boss 429 Mustangs came with the most unique of Mustang engines. The aluminum heads were huge, featuring hemi-like combustion chambers and staggered valves to accommodate the large, round ports. Everything had a purpose on the 375hp engine, from the Holley 735-cfm carb to the valve covers, either magnesium or aluminum, with accommodations for the center-mounted spark plugs. The earliest Boss 429s received what is known as the "S" engines with heavy-duty connecting rods attached to a forged-steel crank with 11/42-inch bolts. Most Boss 429s were equipped with the "T" engine with lighter rods and 31/48-inch bolts. In later versions, the hydraulic cam was superseded by a more aggressive solid-lifter cam. A third Boss 429, known as the "A" engine, was essentially the same as the "T" but with revised smog controls.

Most magazine road tests of the Boss 429 were critical of the car's performance, noting that detuning a racing engine for the street compromised the full potential. Perhaps they expected more from a race-bred engine that looked so impressive. Regardless, the magazines reported 13-second quarter-mile e.t.'s.

'71 Cobra Jet
Ford apparently entertained ideas about switching from the old FE-design 428 to the new 385-series 429 in the Mustang for '70, according to factory photos that show "429" stripes on a '70 Mach 1 hood. However, the 429's debut in the Mustang was delayed until '71 when the Mustang itself switched to the larger Torino platform. As it was, the 429 debuted in the Torino/Fairlane in '70 before landing the Mustang gig in '71.

The 429 was Ford's state-of-the-art replacement for the FE-based 428, a design that had been around since 1958. Debuting as a passenger-car engine in '68, the 429 had a skirtless block, like the small-block 289s and 302s, and canted-valve heads, similar to the 351 Cleveland, for improved breathing. For the 429 Cobra Jet, as found in Mustangs, the heads featured 2.19-inch intake and 1.73-inch exhaust valves, along with a 715-cfm Rochester QuadraJet four-barrel carburetor for 370 hp.

Like the 428 CJ, the 429 Cobra Jet was offered in two configurations: C-code for non-ram-air and J-code for functional ram-air (Ford called it the CJ-R), which opened up the NASA-style hood openings to a vacuum-actuated plenum attached under the hood to funnel cooler outside air to the air cleaner, which was open around the outside edge and sealed to the plenum with a rubber gasket. Other than the air cleaner, the engines were identical.

While the 429 CJs are most closely associated with the Mach 1, both versions were available in all body styles, including the Grand.

'71 Super Cobra Jet
The Drag Pack option continued in '71, once again available only with performance gearing, either 3.91 Traction-Lok or 4:11 Detroit Locker. However, unlike '69-'70 when the Drag Pack added only durability components, the '71 version also supplied a high-lift, solid-lifter camshaft and a 780-cfm Holley carburetor, although Ford rated the 429 SCJ at 375hp, just five more than the standard CJ.

Internet Resources
390-www.1967mustang.com and www.scodemustang.com
428 Cobra Jet-www.428cobrajet.org
Boss 429-www.bossperformance.com
429 Cobra Jet-429mustangcougarinfo.50megs.com

Fastest Big-Blocks
 ET/MPHTransGearsMagazine
'68 1/2 Cobra Jet*13.56/106.64-sp.3.89Hot Rod, March '68
'69 Boss 42913.64/104.64-sp.3.91Super Stock, June '69
'69 Mach 1 CJ13.86/102auto3.91Car Life, March '69
'71 Mach 1 SCJ13.97/{{{100}}}.2autoN/ASuper Stock, Feb. '71
'68 GT500KR14.01/102.74-sp.3.50Hot Rod, Nov. '68
'70 Mach 1 CJ14.03/98.84-sp.3.91Popular Hot Rodding, Mar. '70
'68 GT 39014.8/94.6N/A3.25Car & Driver, March '68
*Prototype
Mustang Big-Block Production*
'67 390 29,457
'68 390 10,650
'6811/42 428 Cobra Jet 1,299
'6811/42 Shelby GT500KR 1,571
'69 390 10,549
'69 428 Cobra Jet, Q-code **2,921
'69 428 Cobra Jet, R-code **12,212
'69 Boss 429 849
'70 428 Cobra Jet, Q-code ***800
'70 428 Cobra Jet, R-code ***3,159
'70 Boss 429 500
'71 429 Cobra Jet, C-code ****353
'71 429 Cobra Jet, J-code ****1,512

* Courtesy Marti Auto Works. For more complete breakdowns by transmission type, see the book Mustang ... By the Numbers by Kevin Marti, available from www.martiauto.com or call 623/935-2558.

** Includes 3,373 Super Cobra Jets, including Mustangs and Shelbys.

*** Includes 1,100 Super Cobra Jets, including Mustangs and Shelbys. SCJ numbers provided by www.cobrajet.org. Additional breakdowns by transmission, body style, and assembly plant can be found on the Web site.

**** Includes 615 Super Cobra Jets. Additional breakdown info available at www.429mustangcougarinfo.50megs.com.