The smaller engined version used the 1800cc Slant Four engine, and a twin-dial instrument panel, while the larger engined variant used the 2300cc Slant Four and had a 7-dial instrument panel. Both were available in 2 and 4 door versions, and had "Rostyle" wheels. Confusingly, it was also possible to buy Viva models with these same engine options, but with vinyl interior trim. The lack of coherency of the model range during that period was one of Vauxhall's main marketing problems.
The Magnum badge also superseded the Firenza badge used on the 2-door coupé model, which name was then used exclusively for the HPF "droopsnoot" model (see Firenza). The model range continued until 1978, when it was rebadged as the Viva GLS, in a bid to shore up the increasingly poor sales of the model. The Viva range was finally discontinued in 1979.
Performance was quite brisk, but fuel economy was never the Magnum's strong point.