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BMW's K4 was innovative and ahead of the curve in its day, in turn inspiring the likes of today's i8

During a tour of a French motor research facility in 1946, Autocar’s Paris correspondent, WF Bradley, happened upon two rare BMWthat had been seized from a Stuttgart research base at the end of World War 2.

The first was easy to identify – the 328 Berlinetta Touring that had been driven to BMW’s first and only victory in the Mille Miglia in 1940 – but the second was rather more curious. The work of prominent aerodynamicist Wunibald Kamm, the K4 was an experimental car built to investigate “streamlining, highspeed road-holding and economy of operation”, in the words of Bradley.

Bereft of solid detail, Autocar’s man gleaned what he could from the French engineers. “The body is both interesting and surprising, the surprising feature being an almost vertical rear panel,” he wrote. “The under surface is perfectly flat, the headlights are set into the wings, the windscreen is rounded into the side panels and the driving wheels are enclosed.”

It was certainly distinctive, broadly based on the luxurious 335 model and featuring front ‘suicide doors' that hinged off the B-pillars, as did the conventional rear doors.

The K4 had a drag coefficient of 0.196 at a time when contemporary saloons were around 0.5Cd, but experimenting with aerodynamics without the use of computational fluid dynamics meant research was a case of trial and error.

In order to make the car stable at speed, there had been experiments with fins. “A single fin, with or without slots, has little effect on stability and may even be detrimental,” wrote Bradley. “To be of any use, the fin must have excessive dimensions. Two fins without slots is no better. They have a tendency to deviate the vehicle from a straight course, and they have a lifting effect at the rear.

“The best results are obtained by a pair of slotted fins, really consisting of a fin and a rudder. The system is executed in Plexiglas.”

The K4 had a 3.5-litre six-cylinder engine, producing 88bhp. The engine was equipped with a dual carburettor, “part of which is used for normal running and the other part coming into action during the last movement of the throttle pedal”.

A four-speed gearbox driving the rear wheels was augmented by a planetary overdrive. “Road tests by the French revealed the remarkable advantages of the overdrive,” Bradley remarked.

Even though the engineers tested on roads comprising hills, heavy traffic and built-up areas, the K4 still returned impressive fuel economy. At 77mph and 1950rpm, it was measured at 24.4mpg; at 35mph and 900rpm it improved to 32mpg. “Certainly much better results could have been obtained on the German autobahnen,” mulled Bradley.

Due to the poor state of the K4’s tyres, the engineers were unable to test the car at its maximum speed of 90mph. The car also had automatic tyre pressure regulation to modulate rolling resistance, although some parts of the system were missing and the engineers were unable to verify its effectiveness.

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The fate of the K4 is unclear, but it’s likely it was dismantled by the French engineers curious to learn all of its secrets. The philosophy behind the car remains as important to BMW as ever, with 2009’s Vision Efficient Dynamics concept hailed as a modern equivalent to the K4 and in turn inspiring the i8.

12 April 1946

Previous Throwback Thursdays

16 May 1987 - Ford Escort XR3i Cabriolet

17 October 1981 - The £12,000 baby Aston Martin

16 January 1985 - The launch of the Sinclair C5

15 April 1960 - Porsche's four-cylinder roots

17 August 2004 - The Honda NSX's last hurrah

11 October 1986 - Hyundai's second UK market foray

15 March 1980 - Triumph's TR7 Drophead

13 February 1991 - Mercedes F100 predicts future car technology

16 April 1997 - A modern 'Blower' Bentley 

19 June 1991 - Volkswagen Polo G40 tested

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fatboyfat 16 June 2016


All the comments here are about the looks - this vehicle is 70 years old and yet it has better aero dynamics that some of the best and newest we have on the roads currently.
Just goes to show what could be done but I guess to pamper to the vanity of the motorist we have been held back all this time.
405line 16 June 2016

SSanyyong Rodius

Quite clearly ssanyyong used this as a blueprint for their rodius
Scratch 16 June 2016


I was thinking more like this was a test mule for the X6.
405line 16 June 2016

Years behind BMW

BMW had the good sense to give up on this type of vehicle design somewhere around the end of WWII..too bad as ssangyong hadn't realised...and gave us the Ssangyong rodius.