How a one-off BMW M8 prototype spawned the greatest engine of a generation

BMW E31 M8 prototype front 3/4

When the McLaren F1 arrived in 1993, seemingly from a time machine, it was an instant sensation that in no time was splashed on bedroom walls the world over. Gordon Murray’s design was timeless, exotic, and thrilling. The F1 was light, at 2,579 pounds. Ruthless and purpose-built. And nestled amidships was one of the greatest naturally aspirated engines to ever combust a drop of gasoline—BMW’s S70/2, a 627-hp 6.1-liter V-12 powerhouse of German engineering.

Built for the mid-engine F1, the S70/2 was never used in a production BMW. In fact, even the engine it was developed from—the S70/1—never saw the light of day. Why? Because BMW made a single S70/1 engine, specifically for a one-off prototype of a planned M8 for the E31-generation 8 Series.

At a BMW press event in California earlier this year, I got up close with the M8 prototype. (It was a rare opportunity—after the project died in the early 1990s the car sat in a basement in Germany for 20 years.) When I saw it, the M8 was parked in a closed-off, guarded room along with the upcoming X5M, X3M, and M8 (the modern one). The other cars might as well have been Dodge Avengers, because I completely ignored them once my eyes recognized the E31 M8 prototype’s bright red paint, low-slung silhouette, and unmistakable swagger.

BMW E31 M8 prototype
BMW E31 M8 prototype rear 3/4
BMW E31 M8 prototype engine
BMW
BMW E31 M8 prototype

Compared to a standard E31 8 Series, every part of the M8 prototype was designed to raise hell. Up front BMW engineers dispatched the regular 8’s familiar pop-up headlights in order to make room for the 6.1-liter V-12’s massive carbon-fiber intakes. The windows are made of lightweight Lexan, the body panels are composite, and the side mirrors are carbon fiber. For extra rigidity, the M8 prototype has a B-pillar, unlike the pillarless (and undeniably elegant) look on the production 8 Series. Scoops on the rear fenders, in front of the wheels, funnel air inward for additional oil cooling.

Inside the cabin there are no rear seats, because this is the type of car that gives children nightmares. The interior is all trimmed with Alcantara, the center stack is dotted with additional gauges (for oil pressure, oil temperature, and water temperature) not present on the regular 8, and the speedometer maxes out at 300 km/hr (186 mph). The chunky Kevlar race seats never would have made it to production, of course, but it speaks to the type of gearheads who built the M8 prototype—the same engineers that built the legacy of BMW’s M division.

The prototype came together in the early 1990s, when BMW M was called BMW Motorsport. In those days, it had nowhere near the size, product influence, and marketing muscle it enjoys today.

“Thirty years ago, real racing enthusiasts and engineers were the ones building M cars,” said director of BMW North America product and motorsport communication Thomas Plucinsky, who back then was a technical service trainer at BMW Canada. Don’t forget, when M developed the first-ever BMW M3, launched in 1985, it was a homologation special to meet production-model volume requirements for racing. The fact that it was unexpectedly successful in the marketplace was just a bonus.

So when renowned BMW engine designer Paul Rosche wanted to take the new M70 V-12 from the 750iL and turn it into the S70/1—a fire-breathing monstrosity to power a Ferrari-fighting M8 coupe—the idea was far from a sure thing.

BMW E31 M8 prototype engine outside of the car
BMW
BMW E31 M8 prototype

Regardless, Rosche got to work. Displacement increased from 5.0 to 6.1 liters. The M70 had a single cam for each cylinder bank and 24 valves, versus two cams per bank and 48 valves for S70/1. It also had 12 individual throttle bodies along with continuously variable valve timing. According to Plucinsky, who checked with his retired BMW engineer contacts, the engine made about 600 hp.

As to why the M8 project got mothballed, poor economic forecasts in the early 1990s were partially to blame. But Plucinsky revealed that there were other internal considerations at BMW that played a perhaps more significant role:

“When the [M8 prototype] was tested at Nardo, it easily surpassed 300 km/hr [186 mph]. At the time, the Board of BMW had made an agreement with Mercedes-Benz and Audi to limit the top speed of our cars to 250 km/hr [155 mph] and therefore decided that this high-performance car was not needed as part of the line-up. The project was killed.”

Yes, the M8 was just too damn fast for the world.

BMW E31 M8 prototype nose
BMW E31 M8 prototype steering wheel
BMW E31 M8 prototype wheel detail
BMW E31 M8 prototype rear

Still, the S70/1 engine wasn’t all for naught. Plucinsky notes that the McLaren F1’s S70/2 engine has the same bore spacing and continuously variable VANOS for the heads (also used on the S50B30 engine in the later E36 M3). Features like the individual throttle bodies and dry-sump lubrication are also shared. Clearly, however, the McLaren’s S70/2 was a modified design to suit the packaging requirements for the mid-engine F1 rather than the front-engine 8 Series. BMW also stuck a detuned, single-cam, 24-valve, 5.6-liter version of the V-12 in the 372-hp 850CSi, which would end up as the most powerful version of the E31 8 Series.

Confusingly, the 850CSi engine is designated internally as both the S70B56 and the S70/1. What gives? Nothing’s for certain, but Plucinsky suspects it was a rather creative project management strategy. “Knowing how Rosche worked,” Plucinsky told me, “the engine codes were probably confusing on purpose to hide the development costs of the M8 engine within the 850CSi project.”

Initial plans were for McLaren’s Formula 1 engine provider Honda to build a V-12 for the roadgoing F1. After Honda backed out, F1 designer Gordon Murray ran into Rosche after the 1990 German Grand Prix  and the two started to iron out a plan. If Rosche hadn’t found a way to build the S70/1 in the first place, it’s unlikely the McLaren F1 could have been completed in its three-year development—at least not with BMW power behind the driver’s seat.

Score one for the gearheads, who as a result helped develop the greatest supercar of a generation. Take that, bean counters.