View PhotosFrank Maresca|Car and DriverFrom theDecember 1981 issue of Car and Driver.At Car and Driver World Headquarters, Mercedes automobiles are somewhat controversial. We all love driving them, and certainly their keys don’t collect dust on the car board, but we just can’t agree on whether they are really the world’s best cars. They do almost everything so well that they’re obvious contenders for that title, but Mercedes is technically a conservative company, and some of us feel that its cars are not quite at the cutting edge of technology, where the leader should be. So to fuel our ongoing verbal fires, we ordered up a Merc with the latest updates, a 1982 300CD Turbo Diesel.For 1982, the turbocharged three-liter, five-cylinder diesel engine has replaced both its naturally aspirated antecedent and the injected 2.8-liter gasoline six in all North American editions of the W123 series. This is certainly a logical move, since it combines the gasoline engine’s performance with fuel economy better than that of the naturally aspirated diesel. Mercedes probably would have taken this step earlier, but was hampered by limited turbo-diesel production capacity. Now that Mercedes is geared up to crank out 190 blown oil burners a day, it’s proceeding with its master plan to convert virtually its entire fleet, at least in America, to diesels. The extent to which this has already been accomplished is amazing: diesels are expected to account for fully 80 percent of Mercedes’ 1981 U.S. sales; they made up just 15 percent in 1974. View PhotosFrank Maresca|Car and DriverThe obvious motivation is fuel economy, but here again the office is divided. For what these cars cost, fuel economy can’t possibly be a serious economic consideration. And despite the high level of refinement that Mercedes has wrought, the 300CD’s engine is still cruder than a gasoline powerplant. It puts forth a throbbing vibration at idle, emits various malodorous vapors, and sets up a low-speed clatter that discourages taking advantage of the 300CD’s considerable side-glass area. Mercedes is nonetheless committed to big cars for the foreseeable future, and diesels offer the only currently known technology for extracting reasonable fuel economy from such machinery.
With the turbo, Mercedes has progressed far beyond the merely reasonable. For a 3535-pound car loaded with power options, 27 EPA city mpg is outstanding. Not only is this a 3-mpg improvement over the non-turbo small Mercedes we tested recently, but the high efficiency is accompanied by a drastic performance improvement as well. The 0-to-60-mph time has been sliced from the 20-second range to 12 seconds flat, and the top speed is up from 90 to 105 mph. If you want a big car with good performance and excellent fuel economy (price no object), the Mercedes turbo-diesels are the only way to go. More Mercedes Diesel Reviews From the Archive
- 1981 Mercedes-Benz 300SD: Expensive Efficiency
- 1981 Mercedes-Benz 300TD: A Quantum Leap
- Tested: 1984 Mercedes-Benz 190E 2.3
Apart from the new engine and its related powertrain changes (a stronger transmission and a taller final-drive ratio), the turbo 300CD is virtually unchanged from its predecessors. Introduced in 1978, the W123 coupe is a permutation of the small W123 sedan series (new in 1977), but with some sporty flair by virtue of its two doors, hardtop roof, 3.3-inch shorter length, and 1.7-inch-lower roofline. The resulting body is still very proper and understated, and not at all in the boy-racer vein, but it does have a bit more pizazz than the sedans. Despite the reduced dimensions, the coupe is still very comfortable and commodious, with room for four adults and lots of luggage. And with all the windows down and the sunroof slid back, it’s about as open as a nonconvertible can get. View PhotosFrank Maresca|Car and DriverThe underpinnings of the coupe are identical to its sedan brothers. The front is suspended by unequal-length control arms and the rear by semi-trailing arms, all supported by coil springs. Steering is via a power-assisted recirculating-ball box, and a solid disc at each wheel dissipates the speed as required. From Mercedes, we’ve come to expect this hardware to be biased toward good handling (particularly on rough roads) and excellent high-speed stability, at the expense of some low-speed ride harshness. Generally, that’s what the 300CD offers, but it’s not quite that simple. The 300CD is a good-handling car, but not remarkably so. Any of several American sedans, equipped with their respective handling suspensions, can easily keep up with the 300CD on smooth pavement, while offering comparable or even better ride quality. The Mercedes mythology of sedan comfort with sports-car handling is a promise not kept in this 300CD.
The reasons for this stem from physical laws that not even Mercedes has overcome, as well as the use of some argument-fueling hardware. The physical problem is the difficulty of combining a supple ride (which implies long, soft suspension motions) with precise and tight handling (which requires exactly the opposite). It’s impossible to segregate these two functions such that neither interferes with the other, and Mercedes hasn’t helped its cause by using a semi-trailing-arm rear suspension and smallish Pirelli P3 tires.View PhotosFrank Maresca|Car and DriverIt’s true that semi-trailing arms are used by some of the world’s most prestigious carmakers on some of their most expensive machines, and it was a considerable step forward when Mercedes first used them to replace low-pivot swing axles in 1968. Semi-trailing arms are a simple, rugged, and cheap independent-rear-suspension system, but today there are better designs available. The major problem with the design is a substantial cornering-attitude sensitivity to throttle position. Mercedes has masked this effect with a heavy dose of understeer, but this merely moves the sensitivity out of the hair-raising zone, and at the expense of roadholding. Although the 300CD cannot develop the cornering forces of a Porsche or a BMW, it does tend to stay pointed in the desired direction more easily. Still, the sensitivity is always there. While running at about 90 mph on the Interstate, our 300CD demonstrated a certain uneasiness in the curves. Even though it was well below its cornering limits, lane changes could be quite easily effected using the throttle alone. View PhotosFrank Maresca|Car and DriverThis problem isn’t severe enough to be dangerous, and it did take place at quite high speed, but it’s a phenomenon unexpected in a car from top-speed heaven, and it’s tiring on an extended trip. It’s a pain you don’t have to contend with in, say, an Audi 5000 Turbo. Not helping the suspension cope with these problems are the Pirelli P3 tires. They’re certainly fine tires for what they’re intended to be—namely, Pirelli’s bottom-of-the-line radial rubber. From any standpoint other than low cost, they are unremarkable, and the price of the tire should hardly be an overwhelming consideration on a $32,000 car. Stickier rubber could mask some of the suspension problems, as Porsche has found on the 911. View PhotosFrank Maresca|Car and DriverAll of these criticisms disappear on rough roads. When keeping the tires pressed to the road is far more important than suspension-geometry subtleties and stickiness, the 300CD excels. No combination of bumps, dips, holes, and ridges can faze it. On a high-speed jaunt down the worst lane of New York’s potholed Henry Hudson Parkway, our 300CD was nothing short of incredible. It just glided along serenely, while all around it, lesser (and slower-moving) machines were being launched laterally and skyward. The credit for this exemplary performance must go as much to the 300CD’s absolutely solid, rigid body structure as to its suspension. Giving the suspension components a stable platform from which to operate makes their task considerably easier. This solidity is unique to Mercedes, and it gives the capable driver both security and confidence.Equally evident, and contributing to the solid feel, is the heaviness of the 300CD’s controls. The steering wheel, for example, is huge, and despite the resulting leverage, it requires considerable effort to turn, as if it were connected to some very weighty, though well-oiled, pieces. The throttle requires a heavy foot, and all the switches move with a solid, audible click and a definite detent. Nothing seems flimsy.View PhotosFrank Maresca|Car and DriverThis solid execution is carried on throughout the car. The seats are large, fairly hard, and supportive. The door handles look as though you could lift the car with them. Other than the star, nothing is attached with small pieces. Everything just seems to grow naturally from the car’s basic mass. The general impression is that you could go through anything with the 300CD and it would just turn around and ask for more.No one argues about this aspect of the 300CD or its basic excellence. It’s just that Mercedes are so good in general and have been for so long that we view them apart from other cars. For Mercedes, we break out the platinum yardsticks, and the arguments continue.Technical HighlightsThe 300CD’s turbo-diesel engine is the same five-cylinder unit that was first introduced in naturally aspirated form in 1974, then turboed in 1978, and uprated in 1980. Its design is very conventional for a Mercedes diesel, with an iron block, a non-crossflow iron head, and in-line valves operated by a chain-driven single overhead camshaft. The bottom end is very rugged, with a nitrided forged-steel crankshaft, forged rods, premium bearings, and elaborate pistons with internal oil-cooling passageways and a pressed-on steel ring carrier. This ruggedness reflects the normal, conservative Mercedes engineering practice, but the engine concept as a whole is a perfect illustration of why turbochargers work so much better on diesel than on gasoline engines.The turbocharger not only drastically improves the 300CD’s performance, but also improves its fuel economy. Its EPA city rating jumped from 24 to 27 mpg, and the highway figure went from 28 to 33 mpg. No gasoline turbo can match these gains. Turbocharging a gasoline engine brings either a performance increase or a fuel-economy improvement—never both benefits at the same time. The explanation for this stems from the basic combustion differences between the two types of engines. Gasoline-engine combustion is susceptible to detonation; turbocharging, with its attendant increases in combustion temperature and pressure (all gases increase in temperature when compressed, whether in a turbocharger or in a cylinder), aggravates this problem. The solution is to reduce the compression ratio, but an unfortunate side effect is reduced engine efficiency under all operating conditions. Diesel combustion is quite different. Only air is compressed by the piston during the compression stroke, and it’s compressed to very high temperature and pressure levels (much higher than in a gas engine). A spray of fuel is then introduced by the injection pump directly into the cylinder. After a very brief delay, during which the initial droplets of fuel are heated by the compressed air, the fuel ignites. Further combustion is then controlled by the rate at which the fuel is sprayed. There is no possibility of uncontrolled combustion (detonation), because there is no premixed air-fuel mixture to ignite prematurely. Therefore, the compression ratio can stay high, and efficiency is maintained. Another factor in favor of the diesel is that its combustion process is actually improved by turbocharging. The delay between initial fuel injection and the onset of combustion is reduced because the compressed-air temperature is higher (because of the turbocharger’s compression). This allows more efficient injection timing and improves overall efficiency by about 5 percent. The turbocharger’s ability to pump in large amounts of air also helps emissions. By allowing the turbodiesel to run leaner, even at high power settings, particulate formation is reduced. The final step is to take advantage of the increased power with taller gearing; this is where the majority of the improved fuel efficiency is realized. Gasoline engines also do this, but the improvements are usually more than offset by their reduced compression ratios. When it comes to turbocharging, diesel clatter, not engine knock, is the true sound of economy.SpecificationsSpecifications
1982 Mercedes-Benz 300CD
Vehicle Type: front-engine, rear-wheel-drive, 5-passenger, 2-door sedan
PRICE (ESTIMATED)
As Tested: $32,300
ENGINE
turbocharged SOHC diesel inline-5, iron block and head, port fuel injection
Displacement: 183 in3, 2998 cm3
Power: 120 hp @ 4350 rpm
Torque: 170 lb-ft @ 2400 rpm
TRANSMISSION
4-speed automatic
CHASSIS
Suspension, F/R: control arms/semi-trailing arms
Brakes, F/R: 10.9-in disc/11.0-in disc
Tires: Pirelli Cinturato P3
195/70SR-14
DIMENSIONS
Wheelbase: 106.7 in
Length: 187.5 in
Width: 70.3 in
Height: 54.9 in
Passenger Volume, F/R: 50/34 ft3
Trunk Volume: 13 ft3
Curb Weight: 3535 lb
C/D TEST RESULTS
60 mph: 12.0 sec
1/4-Mile: 18.4 sec @ 75 mph
90 mph: 32.6 sec
Top Gear, 30–50 mph: 5.8 sec
Top Gear, 50–70 mph: 8.0 sec
Top Speed: 105 mph
Braking, 70–0 mph: 218 ft
C/D FUEL ECONOMY
Observed: 18 mpg
EPA FUEL ECONOMY
Combined/City/Highway: 29/27/33 mpg
C/D TESTING EXPLAINED
Source: caranddriver.com
