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The Story of the Elusive E31 M8 Prototype, a 620-HP, V12-Powered BMW M Legend

The first production M8 blew people’s minds when it was launched in 2019. However, the concept of a high-performance, M-badged 8 Series dates back to the early 1990s, when the mad scientists from Bavaria created a monstrous one-off based on the gorgeous E31.
BMW E31 M8 Prototype 14 photos
Photo: BMW AG
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Encouraged by the success of the E24 6 Series launched in 1976, BMW decided to create an even more impressive line of grand tourers which would don the 8 Series moniker. Development began in 1981, and eight years later the car made its public debut at the Frankfurt Motor Show.

Bigger, more modern, and arguably better looking than the Six, the 8 Series was the first production car to be available with a V12 linked to a six-speed manual. At the time, it was also one of the only vehicles in the world to be designed using CAD tools, and combined with extensive wind tunnel testing, it had a drag coefficient of 0.29, as opposed to the 0.39 of the M6/635CSi.

It was unleashed on public roads in 1990 with the 5.0-liter M70B50 V12 under its hood. Initially able to deliver 296 hp and 332 lb-ft of torque, the motor was deemed way too underpowered by the M division engineers who had bigger plans.

With the blessing of the company’s decision-makers, they embarked on a top-secret mission to transform both the powerplant and the 8 Series into high-performance beasts capable of embarrassing the world’s best sports cars.

BMW E31 M8 Prototype
Photo: BMW AG
From 1990 to 1992 the wizards worked their magic, and the result was one of the most outrageous vehicles to ever don M badges.

Inside its engine bay sat a thoroughly redesigned version of the 8er's standard M70. Dubbed S70, its displacement was increased to a little over 6 liters and, unlike its far more subdued sibling, it featured four valves per cylinder, dual overhead camshafts for each bank, individual throttle bodies with roller valves, BMW’s VANOS (continuously variable valve timing) system, as well as carbon fiber intake manifolds.

With all these modifications, the epic M-engineered powerplant could produce up to 640 hp and 479 lb-ft (650 Nm) of torque, making it more than twice more powerful as the 1990 M70. The work that went into designing this engine would serve as the basis for the S70/2 which powered the McLaren F1.

To harness the full potential of the fire-spitting unit, the engineers went on to reinforce the car’s chassis in key points, widened the track, revamped the suspension, and fitted larger front brakes.

BMW E31 M8 Prototype
Photo: BMW AG
The M8’s bodywork was also comprehensively upgraded with weight saving and increased rigidity in mind. At the front, the pop-up headlights were deleted, and the beams were integrated into the redesigned bumper. Moreover, the hood gained a louver-style vent that enhanced engine cooling.

When viewed from the side, upgrades like wider wheel arches, aerodynamic mirrors, beefier side skirts, and huge vents on the rear quarters made it clear that this was no ordinary 8 Series.

To lower overall weight, fiberglass was used for the doors, hood, and trunk lid while the windows were made out of Lexan. Unlike any other Eight, the M8 got door pillars which increased the body’s torsional rigidity.

A similar recipe was used for the interior. Everything that was deemed unnecessary such as the air conditioning system, sound deadening, or the rear bench seat was stripped off. In addition, the original leather-wrapped front seats were replaced with lighter, race-spec buckets with Sabelt harness, additional gauges were mounted in the center of the dashboard, and a smaller M steering wheel completed the transformation.

BMW E31 M8 Prototype
Photo: BMW AG
Sitting on carbon fiber wheels and finished in bright red, the car tipped the scales at 3,197 lbs (1,450 kg), more than 882 lbs (400 kg) less than a stock 8 Series.

Since sales of the 8 Series were impacted by the global recession of the early 1990s, the Gulf War, and the subsequent energy price spikes, BMW opted against green-lighting production run. Instead, an M-engineered performance version called 850CSi came out in 1992 with a far more plebian, 375-hp S70 V12.

The only completed M8 was never shown to the public, and it ended up in a secret storage facility. For the next two decades, the company denied that it ever existed, despite emerging rumors, but, in 2010, a world exclusive feature in the February issue of BMW Car Magazine the E31 M8 was finally revealed.

The performance figures for this outrageous vehicle have never been released but, according to multiple sources, it was developed with the Ferrari F40 in its sights.

BMW E31 M8 Prototype
Photo: BMW AG
Developed with the aim of creating a BMW that was faster than a Ferrari, the M8 is said to be capable of accelerating from 0 to 60 mph (97 kph) in less than 4 seconds and attain a top speed of well over 186 mph (300 kph). However, BMW continues to state that comprehensive performance tests were never performed, so those figures have no official backing.

After its 2010 reveal, the prototype made some rare public appearances, the last of which occurred in March at the 2022 Amelia Island Concours d'Elegance.

Although 2019 finally brought us a series production M8 with a 617-hp, twin-turbo V8 in Competition-spec, we can all agree that it pales in comparison with the elusive 1990s one-off in terms of outrageousness.

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About the author: Vlad Radu
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Vlad's first car was custom coach built: an exotic he made out of wood, cardboard and a borrowed steering wheel at the age of five. Combining his previous experience in writing and car dealership years, his articles focus in depth on special cars of past and present times.
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