The modern Opel straight-six line began in 1968 with the 2.5 L (2,490 cc) 25S unit used in the Opel Commodore.
Still a 12-valve engine, it had a very oversquare 87 mm × 69.8 mm (3.43 in × 2.75 in) bore and stroke.
8.5:1 compression and a single carburettor produced 117 PS (86 kW) and 174 N⋅m (128 lb⋅ft), while 9.0:1 compression and dual carbs produced 132 PS (97 kW) and 186 N⋅m (137 lb⋅ft).

And 9.2:1 compression and Bosch D-Jetronic(Commodore A&B) later L&LE-Jetronic injection produced 150 PS (110 kW) and 186 N⋅m (137 lb⋅ft).

Applications
August 1968 - January 1972 Opel Commodore A
January 1972 - July 1977 Opel Commodore B
August 1977 - August 1982 Opel Commodore C / Vauxhall Viceroy
January 1978 - August 1987 Opel Senator A / Vauxhall Royale/Senator
February 1970 - December 1971 Ranger A
January 1972 - August 1977 Ranger B
The modern Opel straight-six line began in 1968 with the 2.5 L (2,490 cc) 25S unit used in the Opel Commodore. Still a 12-valve engine, it had a very oversquare 87 mm × 69.8 mm (3.43 in × 2.75 in) bore and stroke. 8.5:1 compression and a single carburettor produced 117 PS (86 kW) and 174 N⋅m (128 lb⋅ft), while 9.0:1 compression and dual carbs produced 132 PS (97 kW) and 186 N⋅m (137 lb⋅ft).
And 9.2:1 compression and Bosch D-Jetronic(Commodore A&B) later L&LE-Jetronic injection produced 150 PS (110 kW) and 186 N⋅m (137 lb⋅ft).
Applications
August 1968 - January 1972 Opel Commodore A
January 1972 - July 1977 Opel Commodore B
August 1977 - August 1982 Opel Commodore C / Vauxhall Viceroy
January 1978 - August 1987 Opel Senator A / Vauxhall Royale/Senator
February 1970 - December 1971 Ranger A
January 1972 - August 1977 Ranger B
Opel cam-in-head engine