BMW M3 turns 25

But one of today's key models in the M family was almost an afterthought back in the 1980s.  According to what has now become a BMW legend, after production of the M1 mid-engine sportscar was discontinued, the then CEO Eberhard Kuenheim casually mentioned to Motorsport GmbH engineers in 1985 that the 3 Series needed a sportier engine.

Kuenheim subsequently commissioned a design for a successor to the M1.  It became known as the M3 project with German car magazines speculating about the car's handling and performance.

Kuenheim's expectations were in good hands.  The key data revealed a 150kW sportscar that would punch way above its class with a top speed of more than 230km/h and a zero to 100 km/h sprint inside 6.7 seconds.

However, drivers had to wait until mid-1986 to get behind the wheel of one of the most dynamic BMW 3 Series cars.  The job of building the M3 fell to Motorsport GmbH's managing director of technical development Paul Rosche.

Rosche had already demonstrated its expertise with the legendary 5 Series driven by M engines as well as developing the Formula 1 turbo engine that powered Brazilian Nelson Piquet to win the World Championship in the Brabham BMW in 1983.

The new 3 Series engine had something in common with the racecar - the crankcase.  It originated from volume production and actually formed the basis for the four-cylinder 2.0-litre engine.

Four cylinders meant less weight and high torque, an ideal platform for a sports engine in the projected displacement class.  Naturally enough, the normal series production four-cylinder was far too tame for a sports engine.

A comprehensive power boost was called for to turn the plucky daily workhorse into an athletic and sporty power unit.  BMW engineers increased the displacement to 2.3 litres and added four-valve engineering.

However, there was also another reason for the decision to opt for a four-cylinder engine rather than the 3 Series six-cylinder engine.  The longer crankshaft in the big engine started to vibrate much earlier than the shorter four-cylinder shaft.

The design engineers therefore designed the crankshaft drive of the BMW M3 with sufficient torsional stability to achieve more than 10,000 revolutions a minute.  Compared to the normal production four-cylinder engine, this represented an increase of more than 60 per cent.

The rated speed for the road version of the BMW M3 was still significantly below the critical range at 6750 revolutions a minute and therefore offered sufficient scope for further development.  Rosche remembers the work on the engine.

"We started work immediately,'' he says. "One advantage was that the big six-cylinder engine originally had the same cylinder gap as the four-cylinder engine. "We therefore cut two combustion chambers off the four-cylinder head of the M88 and bolted a panel over the hole on the rear side.''

This meant that the new four-cylinder engine had a second forebear.  The six-cylinder engine that had initially created a sensation in the M1 and had then transformed the M635CSi into one of the fastest coupes in the world.

"Whether you believe it or not we had created an outstanding four-cylinder engine for the 3 Series within the space of two weeks,'' Rosche says. 

"Under the development name S14, this engine was to generate headlines in sport and in volume production over the years to come.  One Sunday, I drove to von Kuenheim's flat and gave him the car for a test drive.  When he came back he said: `Good, I like it'. And that's how the M3 came into being.''

Subsequently, the BMW M3 became the most successful touring car in European motorsport history.