A Mighty Mercedes:
The 300 SEL 6.3

by Robert DePue Brown

Daimler-Benz has produced many Mercedes models which have, in their respective ways, achieved great success on the tracks and in daily driving. The Blitzen Benz, the 540K, the 770K, and the "Silver Arrows" are some that immediately come to mind.

However, few Mercedes have possessed more extraordinary qualities than a little known model of the late 1960s -- the 300 SEL 6.3.

0 to 60mph in 6.5 Seconds

First, study the performance results: 0 to 60mph in 6.5 seconds; 0 to 80 in 12 seconds; 0 to 120 in 31 seconds. A power to weight ratio of 12.9 pounds per horsepower. A top speed of 137mph (220kph).

A very fast car, by any standards. And startling when you realize that the 6.3 weighs over 3800 pounds, has a four-speed automatic transmission, air-conditioning, air suspension, power brakes on all four wheels, power steering, acres of highly polished walnut trim, and deep, deep crushed velour upholstery.

This is the car that has been called, quite rightly, "the executive road racer."


This is the factory version of the 1971 Mercedes-Benz 300 SEL 6.3, a car referred to as "the ultimate in Q Cars."

The 6.3 is a curious crossbreed of a car -- and unlike most hybrids, wildly successful. It combines the basic body shell and frame of a 300 SEL Mercedes sedan of the late 1960s with an engine from a 600 series car. The 300SEL was no slouch on the boulevard, with sophisticated engineering and an efficient, if relatively docile 3.0-liter engine. But an enterprising Mercedes engineer wanted to see what would happen if he took this eminently successful car and put into it the heart and the power plant of the 600 Mercedes limousine. The single 6.3 300SEL produced in 1967 was this test car. The engineer took great pleasure in inviting guests out for a ride in an "ordinary" 300SEL -- that turned out to have extraordinary performance.

9 to 1 Compression Ratio
The 600 series engine was designed to propel a car of awesome size at virtually sports car speeds. Designated Engine M-100, it generated 300 gross horsepower at around 5200rpm. An eight-cylinder engine of "V" design, it had a bore/stroke of 4.06/3.74 and a total piston displacement of 386 cubic inches. The compression ratio of 9 to 1 told a lot about its brute power. (This was later detuned to 8 to 1 for US models.)

The idea of planting this monster engine in what most Americans would call a medium-sized car was not an engineering feat to be taken lightly. As Graham Robson remarks in The Magnificent Mercedes, "It was at the time, the ultimate in 'Q Cars' and led to one German magazine publishing a cartoon of a Daimler-Benz engineer vainly trying to thrust an aero-engine into the body of a 300SEL while saying between gritted teeth, 'It's just got to go in...'"

But they did get the 600 series engine in the 300SEL engine compartment -- but only just. When Don Garlits, one of America's greatest racers, first lifted the polished hood of a 6.3 and peered into a space totally jammed with blocks, cylinders, ducts, compressors, and air cleaners, he exclaimed, "Oh, the poor mechanics." Garlits' initial inspection of the 6.3 revealed a bewildering array of innovations, such as an inner fender panel switch which, for safety reasons, turned off the auxiliary cooling fan when the hood was lifted for engine inspection. Such attention to details was characteristic of the 6.3.

The first 100 engines were virtually hand-made originals. Over the production run of the engine, four different fuel injector pumps were fitted- Models X, W, Z, and Y. "Y" was the last version -- designed with a double solenoid to accommodate the problems caused by US pollution standards.

The high performance sedan was first shown in Europe in March of 1968. Its debut in the United States was at the Laguna Seca race course in June of that year. Rudolph Uhlenhaut - one of the legendary figures in Mercedes history - was personally on hand to demonstrate the car's superlative road-holding ability and maneuverability.

As Car and Driver commented when it reviewed the car, "This automobile is the most stimulating, desirable 4-door sedan to appear since the Model J Duesenberg." Road and Track called it simply, "The greatest sedan in the world."

Only 6500 Units Built
Among the 6.3's various qualities was its initial high price, $14,000 (in 1968) which climbed to over $16,000 before the model was discontinued (in 1972) -- and its rarity. Not very many people have seen; let alone owned the 6.3. In all, 6,526 units were manufactured at Sindelfingen - 1 unit in 1967, 1,094 in 1968, 2,578 in 1969, 1,797 in 1970, and 670 in 1971, and 386 in its final year in 1972.

So 6.3s are rarely seen at automobile shows -- or anywhere else for that matter. But there is one man who has owned as many as eight 6.3s at one time. Karl H. Middelhauve, an expert mechanical engineer and furniture plant designer, who divides his time between the United States and Germany, probably knows more about the 6.3 and has owned more of them than any other car collector.

Middelhauve began his superb collection with a single dark blue 6.3, complete with alloy wheels, additional instrumentation including volt meters and road temperature thermometers, washing and wiping equipment for each of the car's four main headlamps, plus over-size European halogen road lamps. With the exception of the special equipment, the car was a standard 6.3, if any 6.3 can be called a standard model. It was originally owned by an executive of Daimler-Benz.

Middelhauve was trained as a mechanical engineer and the temptation to design a special 6.3 was irresistible. He found a handsome tan and cream 1966 Mercedes 300 series coupe and reviewed the possibilities of converting it to a 6.3 coupe with AMG, a custom conversion shop near Stuttgart AMG took on the assignment and in due time, the world's only 6.3 Mercedes coupe rolled into Middelhauve's driveway. The coupe was a brilliant success - it appeared to be an elegant GT machine with tan leather and tan sheepskin interior. But under the bonnet lurked the 6.3 engine, ready to blow virtually any sports car off the road. This AMG conversion is the only one of its kind in the world - although the factory produced four similar coupes for directors of the company. One of them has been restored and is now owned by another American collector.

This car incorporates AMG's "Step One" conversion - featuring tuning of the head, opening of the intake ports, high lift cams, and rocker arms that have been ground down and carefully weighed for easier movement The high compression pistons are manufactured by Mahle. The car develops 350bhp.

A 1966 Mercedes-Benz 300SE cabriolet in which AMG has installed the 6.3-liter engine and a five-speed ZF gearbox. A 1966 300SE coupe with the 6.3-liter engine. This car was done personally by Middelhauve.

A 6.3 Cabriolet
Such a success might have been enough for most talented automobile aficionados. But Middelhauve was just getting started. If a coupe with a 6.3 was desirable, how about a cabriolet with a 6.3? And for good measure how about a cabriolet with a 6.3 and a five-speed manual gear box? This conversion incorporates AMG's "Step Two" conversion which increases the compression ratio once more to 9.5 to 1. This produces 400bhp (320DIN ps). Again, high compression pistons made by Mahle are included. The thought was intoxicating. A suitable silver cabriolet was found and once again earnest discussions were held with AMG. Again, the result was brilliant - and unique. A silver 1966 SE cabriolet with red leather upholstery -- complete with the awesome 6.3-liter engine and to top it all off; a five-speed ZF gearbox which can be shifted with blinding speed. Exact details of the performance are perhaps best left unsaid in this age of the 'double nickel' -- but they are rather better than the factory specs. Interestingly enough, the car achieves quite remarkable fuel efficiency. When cruising at 80mph at 3000rpm in fifth gear, the car gets over 16mpg.

Then a 6.3 Station Wagon
Enough? No. Middelhauve's 6.3 legend was, in a way, only beginning. There were reports that Mercedes was beginning to plan for the production of a station wagon - the only factory production station wagon in its history. Of course, the vehicle would be a basic Mercedes T model, with the option of one or the other of four engines for Europe and a 3.0-liter diesel engine for the United States only. But why not a 6.3-liter gasoline wagon? Now, there was a bold idea.

Middelhauve just happened to have another 6.3 sedan, a car that had suffered some minor body damage, but was inherently sound. Falling back on his own mechanical ability, he drew up a set of blue prints showing how the sedan could be converted into a 300 SEL6.3 station wagon. Off to Germany went the car and the blueprints. Karl supervised production and within a year, a glistening 6.3 silver station wagon was added to his growing fleet. The car is an exceptionally attractive vehicle, with smooth, flowing lines, and larger than the current 300 TD wagon which is now almost a common sight in some suburban neighborhoods. No fear of the 300SEL 6.3 wagon ever becoming common -- it is the only one of its kind in the world.


This 1971 300SEL 6.3 sedan has been converted to a station wagon: it is the only known example of a 6.3 wagon.

Were there any more worlds to conquer? The answer was yes. How about doing a 6.3 conversion in your own garage - without the help of the Mercedes factory, without the support of custom conversion shops in the old country? Yes, that would be the next challenge. The basis for this seemingly impossible undertaking was a beautiful metallic blue 300SE coupe, complete with all the lovely refinements that have made many collectors speak of these cars as the last of the "coachbuilt" Mercedes. The instrument pods are Macassar ebony; the upholstery is supple black leather. The car carried all of the special "300" level trim and accessories.

A 1966 300SE coupe also fitted with the 6.3 engine.

A 6.3 With Headers
The first task was to remove the 3.0-liter engine that had been so carefully installed at Sindelfingen. This out of the way, the engine compartment was re-sprayed in its original color. Then a cold-blooded examination of the compartment began with an eye for begging, borrowing, and stealing every single cubic inch and cubic centimeter of space so that somehow the monstrous 6.3 engine could be shoehorned into position. Most of the hardware in the engine compartment had to be repositioned or eliminated. Finally, it seemed that the task could be done. But that took away the engineering challenge. Wait a moment -- would the great 6.3 engine not perform even better if headers were fitted? This would allow the mighty engine to breath deeply and easily. Thus began the incredibly difficult task of fitting eight additional stainless steel tubes into an engine compartment that was already packed solid. For weeks, the 6.3 engine was suspended just over the compartment while the infinitely sophisticated welding and adjustment of the headers was carried out. Each time that it seemed as though all the pipes and the engine would somehow fit' there was disappointment -- some object failed to clear by an eighth of an inch. So again and again the headers were rewelded and refitted.

Finally, Middelhauve’s engineering talents triumphed over the seemingly unconquerable realities of space and volume. All the pieces nestled together -- and miracle of miracles, the whole idea worked. One contributing factor was the modification of the intake manifold to incorporate two venturis. This resulted in 160bhp at 2500rpm on the rear wheels, as measured by dynomometer tests (compared to 120bhp with the AMG conversion). The 6.3 engine with a full set of headers recalled all the racing triumphs of the Blitzen Benz and the "Silver Arrows." At idle, the engine sounds decidedly different from the normally well-tuned Mercedes powerplant, giving off a restless staccato beat that recalls more nearly the throbbing of a 427 Cobra than the purr of a Benz. The car is, as Middelhauve succinctly puts it, "Quite a firecracker. It's ready to jump when you put your foot on the accelerator."

Welding the individual exhaust headers on a 6.3 engine for installation into a 300SE coupe. Note the close fit of the 6.3-liter engine in this 1966 300SE cabriolet. Twin venturis and custom headlamp and roadlamp equipment can also be seen.

Middelhauve's collection and work with the 6.3-liter engine dramatizes one central fact: the 6.3 is one of the most powerful engines ever developed and applications to date have not taken it to its ultimate level of performance.

To get extended life out of this unique engine, it is possible to re-bore it, re-grind the crankshaft, and use one of the four available factory oversize bearing kits. To get more power out of the engine, special pistons can be used with all other engine replacement parts being from regular factory stock. The next step would be to open up the block from 6.3-liters to 6.6-liters. If the camshafts are reworked and higher lifters installed, and if the intake ports are reworked and polished, it is possible to raise the engine's performance up to the range of 400hp (DIN). Then, if the two standard cast iron exhaust headers are replaced with eight individual headers, still another 50 horsepower can be extracted.

Middelhauve's latest project is to go to the ultimate and increase the engine to a full 7.0-liters. This involves a number of sophisticated steps: custom connecting rods, reworking of the combustion chamber, reworking of the intake ports, and a special camshaft. No alteration is necessary to the fuel injection into the inlet ports, but a spectacular change is made to the air supply. The stock air intake venturi with 70mm butterfly valve is removed and an individual port with butterfly valve and stack is fitted to each cylinder. This requires, of course, considerable reworking of the linkage. The total effect of these custom modifications is to increase the engine's output to nearly 600hp (DIN).

That such modifications - and such performance - are possible underscores the fact that the Mercedes 6.3 engine is so strong and versatile that in normal stock form it is only producing about 30% of the power of which it is capable. Thus the engine rarely strains and is capable of extremely long life.

When Middelhauve's 7.0-liter modification is complete, it will be fitted into a body and chassis that has been especially prepared for it. These changes will include a strengthened transmission with heavier bands and components, special plating and hardening of the gears for the rear axle, and the machining of closer tolerances of axle components with new materials that were not feasible to use when the 6.3-liter engine was developed in 1967. Wheels with seven-inch rims will be used together with low-profile high-speed tires as Pirelli P-7s -- again an item that was not available to Mercedes engineers in 1967.

When this car is complete, sometime late this summer, will it be the "ultimate" Mercedes? Probably. It will have an engine that equals the capacity of the Grosser Mercedes of the 1930s with, of course, far more usable horsepower and torque. In any event, it will be far more than a match for the latest 3.8 and 5.0 models currently being produced.

The Middelhauve Mercedes collection is thus unique -- a remarkable combination of the original production 6.3 sedans in pristine condition, several hand-wrought custom creations from one of the world's leading conversion shops, plus a car that is the result of Middelhauve's own abilities and imagination. Are there more 6.3 projects ahead for this plant designer, mechanical engineer, devoted family man and father of four charming daughters? "Well, there is that blue 1967 cabriolet in the back of the garage...," muses Middelhauve as his mind begins to grapple with problems of spatial relations and engine torque.



Reprinted from CAR COLLECTOR September 1982
Text by Robert DePue Brown
Photos by Carl Whitney Bucks