When Hot Rod Tech Editor John Thawley wrote up his preview of this 429-powered Mustang for the Feb. 1969 issue he made no mention of the word Boss. In late 1968, designer Larry Shinoda’s nickname had not yet been applied to this or the 302-powered version being readied for Trans-Am racing. So for Thawley, this was simply a Mustang 429. Nothing Boss about it.
Except for its potential, that is. “Y’all gonna see some getting’ on with some gettin’ on when Hubert Platt (‘the mouth of the South’) and Ed Terry start campaigning the 429 Mustang early this spring,” Thawley wrote to start his story. “Seems that Ford’s new NASCAR engine (mentioned as the ‘twisted hemi’ in June ’68 HRM) will shortly become a production engine and thus will be eligible to run the NASCAR circuit along with the stock car classes of NHRA and AHRA.”
Well, “production engine” in name only. To homologate the 429 for NASCAR and, secondarily, drag racing, Ford contracted with Kar Kraft to build a limited run of Mustangs with the wide motor shoehorned in. Just 857 Boss 429 Mustangs were built in that first year, with another 499 made for 1970.
Much of Thawley’s preview dissected the new engine, especially the aluminum heads with their twisted hemi chamber configuration. “Port and valve size is huge. Notice the radical valve angle which allows for the desirable plug location, moves the intake valve guide to one side of the port, and straightens out the runner for better breathing characteristics.”
As you can see from the photo, the wet and snowy conditions in Detroit weren’t optimal for putting the 429 through any sort of acceleration tests. Thawley’s piece contains no driving impression of the car at all; it’s possible he wrote the story from Ford’s press information and wasn’t around when Bob Swaim photographed the car.
Thawley guesstimated that the Mustang would run “low tens” on the strip. Some drag racers did find success with the 429, but not before making their own modifications to the engine. (Hot Rod’s June 1969 cover story was about how Connie Kalitta rebuilt the 429 for his dragster and Funny Car.)
Over the years there has been a lot of discussion as to the Boss 429’s power potential. Likely for insurance reasons, Ford published a 375hp rating for the motor, which most felt was a gross understatement—500 horses was more likely, some even said 600. The spec chart in Thawley’s story listed peak horsepower at 570 at 5,400 rpm. That number probably came from Ford, and may have been the most honest appraisal of the engine’s potential before the legal department cracked down.