In what was little more than a necessary step to a larger objective, Ford created the BOSS 429 Mustang in 1969. The company needed to use the newly designed engine in NASCAR’s Grand National competition to regain their competitive position against the Chrysler 426 HEMI engine, which due to its opposing valve, semi-hemispherical combustion chamber design, lent itself to optimal performance at high rpms. NASCAR rules, however, required that the engine, not the car in use on the track, must be available in a minimum of five hundred vehicles available for sale to the public.
Based on the Ford 385 cubic inch big block design, the production BOSS 429 would feature four-bolt mains, a forged steel crankshaft, and newly-designed opposing-valve aluminum cylinder heads that featured slightly hemispherical-shaped combustion chambers. Topped with a 735cfm Holley four-barrel atop an aluminum high-riser dual-plane intake manifold and exhaling through factory header-style exhaust manifolds, the 429 were ranked conservatively at 375hp with 450 lbs.-ft. of twist. In reality, the big mill cranked out closer to 500 horses but Ford fibbed the horsepower output to dissuade potential legal ramifications and rising insurance costs.
Despite the neck-and-neck oneupmanship between the newly-coined Dodge Daytona and Ford’s Torino (rechristened the “Talladega” with a new nose job) in Grand National competition, the decision was made – by short-lived Ford executive Bunkie Knudsen – to use the newly redesigned fastback Mustang for the homologation platform. The effort would not be an easy job. This substantially increased overall size of the BOSS 429 engine made its fitment into the Mustang a significant challenge and would require quite a bit of modification.
Kar Kraft was Ford’s northern performance partner at the time, located in Brighton, MI, about 40 miles northwest of Dearborn. Already involved in converting production Mustangs into Trans Am BOSS 302 competition cars, Kar Kraft’s familiarity with the newly re-skinned Mustang and what would be needed to accommodate the larger plant proved invaluable. That knowledge would serve them well in the process of building the BOSS 429 as work commenced in 1968.
Beginning with Ford’s GT 385 series block (coming in at 385 cubic inches), the BOSS 429 received four-bolt mains, a forged crank, staggered-valve aluminum heads with semi-hemispherical combustion chambers. Capped by an aluminum high-rise intake and a 735cfm Holley four-barrel, the BOSS plumbed the exhaust through header manifolds. While the 429 was ranked at 375hp with 450 lbs.-ft. of twist, it made more like 500.
Images: RM Auctions
To fit the high output motor under the hood, Kar Kraft had to cut out and relocate the shock absorber towers and inner fender sheet metal. All of the suspension mounts were cut and relocated as well, clearing plenty of room for the large header exhaust manifolds. The wider front suspension broadened the front track while the revised wheel spindles and lower control arms lowered the car by an inch all significantly improving the pony car’s front geometry and handling when compared to other big block Mustangs.
With engine compartment real estate being a high commodity, the battery was relocated to the trunk as a stout rear sway bar was installed over the mandatory 3.91:1-geared Traction-Lok rear. Other “mandatories” included an oil cooler, closer-ratio power steering, power front disc brakes, and a four-speed manual transmission. Inside, the BOSS featured plush interior and an 8000rpm tach; air conditioning and an automatic gearbox were non-available equipment. Outside, the car remained remarkably undecorated, with the exception of a “BOSS 429” decal on the side of each front fender, a front chin spoiler, and Magnum 500 wheels wrapped in Polyglas F60x15s.
To feed the big semi-hemi Ford, Kar Kraft cut open the hood and installed a manually-controlled hood scoop, giving the BOSS 429s its iconic aggressive yet austere look. Like the BOSS 302 Mustang of the time, the BOSS 429s were the only other Mustangs ever to be stamped with matching serial numbers – on the back of the block, the inner front fender panels, the transmission housing and the chassis as well. Another way is to locate the special “KK” serial number on the driver’s door. The first car was KK NASCAR 1201. According to most sources, the number of 1969 BOSS 429s built was 858 units, which included two specially-built BOSS 429 Mercury Cougar Eliminators.
All of the fabrication work that went into accommodating the massive 429 benefited the Mustang’s overall handling. As the front springs were widened, so did the front tracking. As suspension perches were relocated and new spindles fabricated, the ride height was lowered. While not designed for cornering, the BOSS 429 offered drastically improved road manners over the March 1s and BOSS 302s.
For 1969, five exterior colors were available; Raven Black, Black Jade, Wimbledon White, Royal Maroon and Candy Apple Red as five entirely different colors were used in 1970 production. Despite the apparent formula for success, the BOSS 429 was far from a sales booster. Reportedly, unsold BOSS 429’s sat on dealer lots even as 1970 model deliveries began. The 429 engine was not particularly streetable and delivered about the same overall performance as the 428 CobraJet, which was also much easier to drive in normal use.
Unfortunately, street-bound BOSS 429s suffered from the same maladies as the Dodge and Plymouth HEMIs for which it was designed to compete with; namely that the semi-hemispherical cylinder heads thrived on high revs. Even worse, the initial batch of BOSS 429s came with the incorrect valve springs which handicapped the copycat HEMI’s reputation even worse. The faulty springs quit at 4,500rpms, stopping way short of their intended 6,000. In the hands of aftermarket tuners, the BOSS saw some success but only after the addition of Hurst shifter linkage, traction bars, a high-performance cam, and re-jetted carb. Even then, quarter-mile times disappointed enthusiasts.
Priced at $4,798, the clean and clandestine BOSS 429 drew attention not by its standing performance – an average 0-to-60 of 6.8 seconds and quarter mile pass of 14-seconds flat at 103mph – but what the specialized Mustang represented, namely a direct volley at the 426 HEMI-powered Chryslers.
As costly as the BOSS 429 package was, the interior strode to provide buyers the best bang for their buck. Although air conditioning and automatic transmissions were completely unavailable, rich wood grain accents and soft bucket seats helped smooth the BOSS’ tough appearance.
The second model year’s production was available in all-new exterior colors, including Grabber Orange, Grabber Green, Grabber Blue, Calypso Coral and Pastel Blue. All 1970 models had the hood scoop painted in a gloss black finish. Having met the homologation requirements for NASCAR and given the relaxed sales performance of the model, it was decided that 1970 would be the last model year for the BOSS 429. Only five hundred cars were built, the last one having serial # KK NASCAR 2258.
The storied background of these cars and their limited production numbers would certainly seem to make them of interest to collectors. Two BOSS 429’s were recently offered at RM Auctions’ Classic Muscle & Modern Performance event in San Diego, without reserve bid and both sold well into the six-figure range. These are pretty good results for a car that originally sold for a little over $4,000. The 1969 model was KK NASCAR 1447, with just 28,169 miles on the odometer and having been through a ground up restoration. The dark red with black interior BOSS went to a new owner for $181,500.
Also in the Top Ten from that auction was a 1970 model, KK NASCAR 2272, in Calypso Coral over black. Similarly restored to pristine, factory original condition, this no-reserve auction ended on a final bid of $156,750. To see how these results compare to past auctions, we scoured the auction houses’ web sites to find more BOSS 429 data. There is less out there than you might imagine, but we did come up with enough to make a few observations.
With record of nineteen 1969 BOSS auctions since 1996, there appears little doubt that – whether you agree with the title or not – these are “investment grade” cars. Selling in the $20K to $40K range in 1996, these cars have appreciated geometrically in the intervening years. Final year models seem less plentiful at auction, befitting their lower production number. We only found seven examples auctioned since 2007, with no clear trend visible beyond the fact that the least expensive example was $154K and the most expensive topped $230K. These are the most expensive of the non-Shelby Mustang models ever produced. Their rarity, no doubt, has much to do with the prices that some people are willing to pay for them. Meanwhile, there are plenty of other Mustangs out there, but few that represent such a significant piece of ponycar history.
(Co-authored with Gordan McDonald)