First seen at the London Motor Show in October 1973,[1] the Magnum was an HC Viva with a larger engine, more luxurious interior, higher trim level and four rather than two headlights.

The Magnum model range continued until 1978: by then it was increasingly being overshadowed in the domestic market place by the Ascona-based Vauxhall Cavalier.

In Trevor Alder's book "Vauxhall — The Post War Years" mention is made of a HP Firenza Sport Hatch prototype which was a one-off and was painted in silver starfire, with a six-light version of the droop snoot nose and also sharing the Viva/Magnum estate body shell.

The HP Firenza project was proposed to produce over 2,000 cars a year, but production of this model ceased after just 204 examples were built.

There were only 1692 coupés produced from late 1973 to early 1975 with the majority sold later in the model's life due to the 1973 oil crisis.

Performance was quite brisk, but fuel economy was never the Magnum's (or the Vauxhall Slant Four Engine's generally) strong point.