Vintage R&T Road Test: 1975 Mercedes 300D – “The Astonishing 5-Cylinder Motorcar”


(first posted 10/29/2016)     Prior to the arrival of the MB 300D, and the soon-to-come VW diesel, oil burning passenger cars required substantial sacrifices, most of all in performance, along with noise and vibration. A MB 240D took some 28 seconds to, ah, slowly increase its speed from rest to 60 mph. Mercedes rightly saw a problem: there was no obvious way to upsell traditional MB diesel buyers, since they tended to be diesel die-hards. And Mercedes also wanted to increase the attractiveness of their diesels to potential (gas) six cylinder customers, especially since the gas six was losing performance due to emission changes. The solution was rather brilliant: add another cylinder to the 240D engine, make some other changes to improve its smoothness and driveability, and launch the first mass-produced 5 cylinder “motorcar”. Astonishing indeed.


As the article makes clear, there were more changes than just adding a cylinder. The 300D was the first Mercedes diesel to use a new governor system to control the diesel injection pump, which improved response to the accelerator pedal. And due to careful balancing and special motor mounts, it was deemed to be closer to the six in its vibrations than the four. Mercedes’ success with their five was soon followed by Audi doing the same thing with their engines. The Germans had found a solution to satisfying the power-hungry Amerikaner by adding one more cylinder.


Of course, objective performance was still leisurely; 0-60 in 20 seconds. But in normal daily driving, that wasn’t really all that bad. The 300D would keep up with traffic quite nicely, and once up to speed, did very well on the highway. R&T got theirs up to a whopping 98 mph!

One of the engineers at KSCI bought one of these, and I remember being pretty impressed in one or two rides with him. It was clearly smoother and more powerful than the 240D, which I had exposure to. And of course, all of its other very outstanding qualities in therms of ride, handling, brakes, and feeling of profound solidity were all just accentuated by the new 5 cylinder diesel. The 300D was a significant breakthrough vehicle for MBZ, until the even much better turbocharged 5 cylinder came along. Then suddenly the normally aspirated 300D felt just like a slug in comparison.