Some facts from Wikipedia:
The Mercedes-Benz 300 SEL 6.3 started out as a private venture in 1966 bycompany engineer Erich Waxenberger, which culminated in the world's first "Super
Saloon" or Q-car.
Waxenberger's principle was simple: Take the powerful 6.3 litre V8 Mercedes-Benz
M100 engine from the luxurious Mercedes-Benz 600 limousine, and fit it into the
regular Mercedes-Benz W109 S-Class model which only had 6-cylinder engines at
that time. The result was a nearly 2-tonne saloon with performance similar to
most dedicated sports cars of the era. It is said that Rudolf Uhlenhaut, when
invited to test drive the prototype, opened the hood at the first red light to
find out how the big engine and its supporting equipment had been squeezed in
Surprisingly, the rather conservative company went ahead and launched the car
into the marketplace at the Geneva Motor Show in March 1968, in order to make
better use of the M100 engine production facilities. The 6,500 build of the 6.3
outnumbered the 2,700 build of the 600 by far.
What set this car apart from its contemporaries in the late 1960s though, was
that it could cruise at over 200 km/h (124 mph) with 5 occupants in complete
comfort within the body styled by Paul Bracq. Later, the company also fitted
new, smaller V8 engines into the W109 series. The 300SEL 4.5 was only available
in the USA, while the 280 SE 3.5 Coupé could also be ordered in Europe.
In the 1970s, the Mercedes-Benz 450SEL 6.9 was a successor to the 6.3, with even
more modifications and power.