25. Ford Mustang Boss 429
The mythical Mustang Boss 429 is a proper homologation special legend. Ford conceived it in 1969 as a pure racing engine they intended to use in the NASCAR championship. The Boss 429 featured a different engine architecture than the rest of Ford’s big blocks. First, it was much wider and had semi-Hemi combustion chambers. That helped it achieve higher revs and get better flow inside the head to produce more power and torque (via Supercars).
Factory rated at 375 HP, this unit truly produced over 500 HP and more in race trim. Ford decided to put this engine into the Mustang, creating a limited-production Boss 429. But NASCAR decided not to homologate it since the series only accepted intermediate and full-size cars.