11. 1979 Mercury Cougar XR-7
The 1979 Mercury Cougar XR-7 came equipped with a V8 engine that was underpowered from the factory, hiding its real capabilities. Enthusiasts quickly discovered that aftermarket ignition systems and upgraded exhaust components could dramatically enhance performance. These modifications transformed the XR-7 into a true sleeper muscle car, surprising many rivals on the street. Explore more about the Cougar’s evolution at Hemmings.