As with most successful racers, Yunick was a master of the
grey area straddling the rules. Perhaps his most famous exploit was his #13 1966
Chevrolet Chevelle, driven by
Curtis Turner. The car was so much faster than the competition during testing that they were sure that cheating was involved; some
aerodynamic enhancement was strongly suspected, but the car's profile seemed to be entirely stock, as the rules required. It was eventually discovered that Yunick had lowered and modified the roof and windows and raised the production car's floor (to lower the body). This car has many legends about it, and they were definitively debunked by the 2019 Dinner with Racers episode on Amazon Prime TV. Since then, NASCAR required each race car's roof, hood, and trunk to fit templates representing the production car's exact profile.
Another Yunick improvisation was getting around the regulations specifying a maximum size for the fuel tank by using 11-foot (3 meter) coils of 2-inch (5-centimeter) diameter tubing for the fuel line to add about 5 US gallons (18.9 liters) to the car's fuel capacity. Once, NASCAR officials came up with nine items for Yunick to fix before the car would be allowed on the track. The suspicious NASCAR officials had removed the tank for inspection. Yunick started the car with no gas tank and said, "Better make it ten," and drove it back to the pits. However, the story was not true It is also claimed that he used a
basketball in the fuel tank which could be inflated when the car's fuel capacity was checked and deflated for the race.
Yunick also used such innovations as offset chassis, raised floors, roof spoilers, nitrous oxide injection, and other modifications, often within the letter of the rule book, if not the spirit. "All those other guys were cheatin' 10 times worse than us," Yunick wrote in his autobiography, "so it was just self-defense." Yunick's success was also due to his expertise in the aerodynamics of racing cars.
In another incident, Yunick showed up for a race with stock fender wells still installed on his Chevelle, even though the rules stated they could be removed. After the car qualified well due to improved aerodynamics, fellow competitors complained. Yunick replied, "The rules say you MAY remove them. They don't say you HAVE to." After qualifying, Yunick promptly cut out the fender wells. After further complaints to NASCAR, Smokey said, "The rules don't say WHEN I can remove them."