Vintage R&T Review: 1978 Mercedes 300SD – Mercedes Shows GM How To Make A Fast, Efficient and Durable Big Diesel Sedan

The great irony of Olds’s introduction of its new big diesel V8 cars is that it came out just shortly before the only other large diesel sedan: the Mercedes 300SD. Like the Olds, the big Merc’s engine was developed specifically with the US in mind, to meet the demands of CAFE as well as the growing demand for import diesels. In fact, the 300SD was not even sold in Europe, as a diesel S Class would have been hard to swallow there, where diesels still had a lowly image of taxis, farmers and cheapskates.

But the US market was different in many ways, and the 300SD, especially in its next generation, the W126, became a bigger hit than Mercedes ever expected. And with its excellent performance, triple digit top/cruising speed, and 25+ mpg efficiency, that was not really all that surprising. Never mind its durability, which was presumed to be great then, but turned out to be utterly legendary.

Also not so surprisingly, the approach Mercedes took was quite different than Olds, as well as the outcome.

Of course Mercedes had a major head start in passenger car diesels, having built them since the 1930s. But the approach to building a more powerful one was quite new, at the time: turbocharging. That became increasingly ubiquitous, but up to this time it had not been done in a regular production car. The benefits compared to a larger, heavier naturally-aspirated diesel like the 5.7L Olds V8 were numerous: it was considerably lighter, generated about equal hp from only 3.0L, and had increased efficiency, as forced induction invariably does, by making the effective compression ratio higher as well as reducing internal friction compared to an equally powerful NA engine.

0-60 came in 12.7 seconds, but because of being a bit slow off the line, that number does not do justice to its mid range performance. The 0-80 sprint took 24 seconds. The 0-60 beat the Olds by almost two seconds. And its top speed, which it could safely cruise at, was 110 mph. Given how slow diesels invariably were up to this point, this was all a revelation.


Of course the 300SD was three times as expensive as an Olds diesel, but that was the reality back then, regardless of what engine was under the hood. But what was interesting is that Mercedes diesels cost less than their gas counterparts, and in the case of the 300SD, it was faster than the 280SE. No wonder diesels soon made up over 80% of US Mercedes sales. Meanwhile, Olds charged some $750 more for their diesel.

Here’s an accompanying article explaining the engineering of the new turbocharged Mercedes 5 cylinder diesel.