F1technical.net

Can-Am started out as a race series for Group 7 sports racers with two races in Canada (Can) and four races in the United States of America (Am). The series was initially sponsored by Johnson Wax. The Series was governed by rules called out under the FIA Group 7 category with unrestricted engine capacity and few other technical restrictions.
The Group 7 category was essentially a formule libre for sports cars; the regulations were minimal and permitted unlimited engine sizes (and allowed turbocharging and supercharging), virtually unrestricted aerodynamics, and were as close as any major international racing series ever got to anything goes. As long as the car had two seats and bodywork enclosing the wheels, and met basic safety standards, it was legal.

"Racing runs by rules sometimes so restrictive that it's sometimes hard to tell where the book ends and the sport begins. Can-Am racing is a relief from the iron band of the rule book."
— Mark Donohue