The Opel Moonlightroadster was the first Opel vehicle with a straight-6. It used a 1790 cc six in 1933. It was a 12-valve engine with a very-undersquare 65.0 mm (2.6 in) bore and 90.0 mm (3.5 in) stroke, typical for the time. This engine produced 33.5 PS (33 hp/25 kW) and 100 N·m (74 lb·ft).
The 1956 Opel Kapitän was the next vehicle from the company with a straight-6 engine. This was a 2605 cc unit with 12 overhead valves. Bore and stroke were now oversquare for high power output at 85.0 mm (3.3 in) by 76.5 mm (3 in). A single Opel-designed carburetor and 7.8:1 compression yielded 91 PS (90 hp/67 kW) and 186 N·m (137 lb·ft).