The BMW E39 M5 was already one of the definitive performance sedans of its era, but one example became something considerably more unusual when Volkswagen decided to perform a secret engine swap to develop an engineering concept.

Built on March 2, 1999, this Carbon Black M5 was turned into an unlikely test mule for Volkswagen’s experimental W-engine program under then-Volkswagen Group chairman Ferdinand Piëch.

Rather than develop a dedicated prototype, engineers used the BMW’s RWD chassis and six-speed manual transmission as a ready-made test bed for the unusual 10-cylinder engine. At the time, neither Volkswagen nor Audi had a direct rival to the BMW M5, making the E39 an ideal platform for exploring the concept.

A W10 before the W12

The naturally aspirated, 5.0-liter W10 engine used Volkswagen’s narrow-angle VR architecture, combining two five-cylinder banks around a common crankshaft. Power output is pegged at 479 PS and 592 Nm, giving it significantly more power than the standard E39 M5’s 400 PS.

This was no simple combination of production engine components. The prototype featured a custom aluminum block, a fabricated intake system with carbon-fiber airboxes, an upgraded cooling system, dedicated ignition hardware, and a standalone fully-programmable ECU.

To achieve unfiltered testing data without computer interference and system incompatibility, the BMW’s factory ABS, traction control, and stability control were disabled. The original instrument cluster was also replaced with a Stack digital display, with additional analog gauges mounted in the center console. As a result, the car’s total mileage can no longer be verified.

The W10 project was carried out by SwR in Remscheid, Germany, with Volkswagen confirming its involvement in 2023. Engineer Sabine Willeke led the development, while Piëch reportedly drove the engine-swapped M5 on temporary plates and even used it as personal transportation during testing.

Just three W10 engines were built. One remains in the M5, another is included with the car as a display engine, while the third reportedly remains in Germany.

The W10 that never reached production

While the W10 never made it into production, the project formed part of Volkswagen’s broader W-engine development program, eventually producing the W8 and W12.  The engine concept would ultimately reach its most extreme form in the Bugatti Veyron’s 8.0L quad-turbo W16, producing more than 1,000 PS.

The M5 itself remained largely out of public view until 2023, when an incomplete W10 engine surfaced in Germany with details matching the unit installed in the BMW. The complete car later reappeared in running condition.

Acquired roughly a decade ago by the current owner, it is registered in Slovakia and has continued to be driven and maintained as a prototype. Now, this unusual BMW is heading to Broad Arrow’s Zoute Concours Auction in Belgium and is looking for a new caretaker.

What started life as one of BMW’s greatest V8-powered sedans became something entirely different: a Volkswagen test bed, a secret Piëch project, and the only known road-going BMW fitted with one of Volkswagen’s experimental W10 engines.

With one of three W10 engines still under its hood and another included on a display stand, this is a rare chance to own a piece of Volkswagen’s secret W-engine history. Hopefully, its next owner will do what it was built to do and drive it.