OM321

The first attempt at more power than the OM 312 could offer was the turbocharged OM 312 A, and it went nowhere: superchargers of the day were unreliable and hauliers would not have them. The unglamorous alternative was to open the bore from 90 to 95 mm, leave the stroke alone, and rate the result at 110 PS at 3,000 rpm. Turbocharged versions did arrive two years later, at 126 and 132 PS, once the technology had settled. The L 321 lorries built at Mannheim from June 1957 carried 5.5 tonnes; in LP form they belonged to the first Mercedes-Benz forward-control generation.

Its real life was in a bus. The O 321 H/HL, unveiled in October 1954, was offered as a city bus, an intercity bus and a touring coach with roof-edge glazing and an optional sunroof, and it went to 95 km/h at a time when the 180 D saloon managed 112. Between then and December 1964 Mannheim turned out 18,083 of them, including chassis and knocked-down kits for overseas assembly — no European rival was close, and the model is what made Mercedes-Benz a serious bus manufacturer. The line ended less happily: stretched again in 1959 as the OM 322, the last pre-chamber engine of the family suffered badly from heat in its cramped head, and finished with a failure rate more than ten times that of its direct-injection successor in its first year.