General Motors banned its divisions from racing in 1963, which should have ended Pontiac’s performance program on the spot. Instead, a handful of engineers found a loophole in the rulebook and shipped a 389-cubic-inch coupe as an option package on a mid-size Tempest. Management capped it at 5,000 units because they were certain nobody wanted another muscle car. Read on to find out how badly they misjudged it.