W116 W116 W116 W116 W116

Mercedes-Benz 300SD

The benevolent potentate

Road & Track, August 1978

WHILE PROFIT is the ultimate reason every automobile is built, there can be a variety of reasons why any one model is put into production in favor of another. These days governments, especially ours, play a big part in such decisions and the federal hand has never been heavier than with its Corporate Average Fuel Economy (CAFE) standards. As you probably remember, this is a schedule of fuel mileage figures that each corporation selling cars in the U.S. must meet—the numbers starting with 18.0 mpg in 1978 and working up to 27.5 mpg in 1984—on a sales-weighted average of all the cars sold by that one company. The alternatives to not meeting the standard are two, one being that an automaker stop building low-mileage models to bring the average 'up to the prescribed level, or pay a fine that's based on how badly you miss the mark ($5 per tenth of a mile per gallon over the target).


As you've no doubt noticed by all the commotion from Detroit, meeting the CAFE standards is neither simply nor cheaply done, even if, like our large auto manufacturers, you have a line of subcompact and compact cars. Imagine, then, the plight of companies such as Mercedes-Benz, whose business is tied to selling luxury cars. First, Daimler-Benz has the problem that the cars they sell in the U.S. are generally well-outfitted and items such as air conditioning mean weight. Then again, Mercedes aren't light cars, but robust, the difference between robust and heavy being that in the former the added weight goes into something useful, like a stronger, safer structure. A heavy car contains dead weight that does nothing for the benefit of the car or driver. For a further definition of heavy, compare a 1977 General Motors' intermediate to its smaller 1978 counterpart.


One of the alternatives is to just pay the CAFE fine, but there is still the proposed "gas guzzler" tax. If, as one proposal suggests, that penalty would be in the area of $400, Mercedes-Benz could just add it to the car's price and the buyers probably wouldn't complain. Another plan, however, puts the fine closer to $4000 and that's just too much to add to the price of an automobile with no added benefit. Another option is for Daimler-Benz to start producing a series of smaller cars, and rumors have it there will be smaller Mercedes in about 1982 to supplement the current line. There is, however, a limit on how small D-B can go before the cars they build aren't really Mercedes and might do the firm and its image more harm than good.

One obvious point through all this is that Daimler-Benz must do everything it can right now to get as much fuel mileage as possible from its present line, which leads us, at last, to the Mercedes-Benz 300SD. This is Mercedes' attempt to get high mileage from one of the larger S-class cars and, in fact, the S in SD signifies that, the D being there for the obvious reason. The engine is the 5-cylinder from the 300D with some significant modifications for the Garrett AiResearch turbocharger. The extent of that work is discussed in the accompanying story, but the result is to raise horsepower from the 300D's 77 at 4000 rpm to 110 at 4200 rpm and torque from 115 lb-ft at 2400 rpm to 168 lb-ft at the same rpm.

The transmission backing the turbo diesel is the same 4-speed automatic used with all non-V-8 Mercedes, but with several appropriate changes. The torque figure for the turbo diesel is 19 lb-ft above the 49-state version of the Mercedes twin-cam 6-cylinder, so the transmission needed calibration changes to match the diesel's torque curve, plus some beefing up to cope with the extra power. While the other 4-speed automatic-equipped Mercedes start in 2nd gear unless you put the lever in L, the 300SD starts in 1st gear because of the engine's torque and 3.07:1 rear axle ratio (which happens to he the same as that in the 4.5-liter V-8 models, while the 280SE is a 3.69:1 and the 300D a 3.46:1). To reduce the 300SD's tendency to creep ahead when the car is supposed to be stopped, the transmission goes to 2nd and then shifts to 1st as soon as you step on the accelerator. Except for that rear drive ratio change, the rest of the driveline is identical to that of the 280SE.

The remainder of the car is also identical to the 280SE, with a few minor exceptions we'll mention shortly. That would make this car the 112.8-in., short-wheelbase version of the W116 chassis, the 450SEL's wheelbase being 116.7 in. This 6-year-old chassis has been explained in R&T many times before and it features 4-wheel independent suspension, 4-wheel disc brakes, Mercedes' rugged structure and a grand variety of details we've praised in the past, and still regard just as highly.

Some of those minor differences between the 280SE and 300SD are to lower the government certification weight to the 4000-lb class and include aluminum for the hood and the partition between the passenger compartment, plus a smaller fuel tank (21.7 gal. versus 25.3 gal.); the 300SD will get an aluminum trunk lid later. Other changes are made because of the diesel, such as a sound-absorbing mat on the underside of the hood and an idle speed-adjusting knob and a glowplug warming light on the instrument panel. And so no one will confuse a turbo diesel with a 280SE, there is a "300SD" on the left of the trunk lid and the words "Turbo Diesel" on the right.

You won't have to worry about confused 280SE drivers because they'll be aware that the 300SD just out-accelerated them, so they should have a good view of that trunk lid. Yes, diesel fans, we have a new fastest-production-diesel-to-60-mph king, as the 300SD did it in 12.7 seconds, outrunning the Oldsmobile diesel V-8 by 1.7 sec and the Diesel Rabbit by 3.1 sec. As one staff member put it; "No longer do diesel drivers have to be intimi-dated at the thought of freeway on-ramps and even slight inclines." Right off the line the acceleration does drag a bit, but the turbo makes itself known around 2000 rpm and by 2500 you're really feeling its effect. Actually the only time you sense the lag is with the throttle down hard right off the line. The rest of the time the combined effects of the turbd.ietup, the car's weight and automatic transmission mask the lag and it really feels more like a second, but subtle, passing gear than a turbocharger. In a way, the 0-60 mph figures don't do justice to the 300SD, because it's the car's mid-range passing response that is most impressive. Diesel drivers don't have to plan their moves well in advance and then just be patient. Rarely will you find a diesel car that has recommended tire pressures for 100-mph driving, but the 300SD will top that figure by 10 mph.

Inside the car you still have to wait for the glowplugs to warm the cylinders, though with the new glowplugs it is somewhat quicker. Also, the glowplug warming light is now also a warning light and will flash if one of the glowplugs should fail. Don't touch the throttle, turn the key and there's the familiar rattle, though certainly subdued. Accelerate away and the noise sub-sides and the boost climbs. The sound at cruising speeds is something like M-B's 6-cylinder, though one staff member also thought it sounded like a quiet Hollywood muffler on a car two lanes over; another said the exhaust note heard in the back seat was slightly rough. There's a very muted whistle from the turbo, but you have to listen closely to hear it. On the sound meter the 300SD's readings were virtually identical to those of the 450SEL we tested last May.

The 300SD's brakes were of the sort we expect in a Mercedes, pulling the car down from 60 mph in 146 ft and from 80 mph in 245 ft, with plenty of control. These distances, by the way, slightly undercut those of the 450SEL.

Ah, but what about that all-important fuel economy number? In our test of fuel economy, the 300SD averaged 25.0 mpg, which is on a par with the Oldsmobile Diesel also tested for the May issue. That's quite a trick when you consider that the 300CD we tested in January 1978 got 24 mpg and the 300SD is using fuel at a faster rate. What happens, though, is that the turbo diesel manages to use the fuel more efficiently, with the compression ratio effectively raised by stuffing the extra air into the cylinder.


With that sort of performance and economy from the 300SD, you probably expect to see virtually all Mercedes on diesel and turbos soon, but that's not what Daimler-Benz has in mind. Mercedes considers this a "timely" car necessary to "dieselize" their fleet as a hedge against CAFE, but they aren't about to put their whole future in diesels. First, there's the problem of how people will react to diesels and if Mercedes went too heavily that way and the buyers didn't, the results could be disastrous. Next, a diesel engine is an expensive project, more so than gasoline engines.


They also don't feel the turbos are the solution and that there will always be a place for normally aspirated engines. Besides, if D-B began to turbo all their engines, they would begin to get a great deal of redundancy between engine lines—a turbo six duplicating the 3.5- or 4.5-liter V-8--and that would be very costly.

In the U.S., though, the turbocharged 300SD makes a great deal of sense and, as we say in the sidebar story, we expect to see the turbo diesel engine in still more models, which may be, like the 300SD and 300CD, built for America only. They are unique here, because the position of the diesel cars here is different: It is a workhorse powerplant in Europe, where it is used more in taxis and delivery vans, while here it started as the car of the rational intellectual, and has gained a great deal of social acceptance in the upper stratas.

Instrument panel
Mercedes' 300SD has the usual S-class instrument panel, but adds the idle adjust screw on the left and a glowplug warming light on the right.

Diesel buyers here aren't preconditioned to think of their cars as dull utensils, and if they are in the market for a 300SD, these buyers aren't poor either. After sorting through all the dollar/ Deutsche Mark parity problems, Mercedes-Benz has priced the 300SD at $23,878 on the east coast and $24,096 out west.