A one-off BMW E39 M5 powered by Volkswagen’s experimental W10 engine will go to auction at Broad Arrow’s Zoute Concours sale in Knokke-Heist, Belgium, on October 9, 2026. The prototype carries an estimate of €250,000 to €350,000, or approximately $285,000 to $400,000.

The car is the only road-going vehicle ever fitted with a Volkswagen W10 engine. Beneath its Carbon Black Metallic E39 M5 bodywork is a naturally aspirated, all-aluminum 5.0-liter engine producing 473 horsepower and 437 pound-feet of torque.

The secret Volkswagen W10 project

Volkswagen chairman Ferdinand Piëch ordered the engine swap in 1999 as an unofficial engineering project. Engineers removed the standard S62 V8 from a 2000 BMW M5 and installed the experimental W10, using the BMW’s existing six-speed manual transmission.

According to Broad Arrow, the project was conceived when Volkswagen and Audi lacked a high-performance sedan capable of challenging the M5. Instead of creating a separate test mule, the engineers used an E39 M5 chassis dated March 2, 1999. The finished car looks largely like a standard M5 from the outside, despite its unusual mechanical layout.

The project remained outside Volkswagen’s formal research and development structure and never progressed beyond the prototype stage. Sabine Willeke, who led the engineering team, told Broad Arrow that just three W10 engines were built. One remains in Germany, another is displayed alongside the M5, and the third powers this car.

473 horsepower from 10 cylinders

The W10 delivers 79 more horsepower and 68 more pound-feet of torque than the S62 V8 it replaced. Its architecture places 10 cylinders on a single crankshaft, while a standalone engine control unit manages the powertrain.

The conversion also required extensive changes to the car’s electronics and controls. Traction control, stability control and anti-lock braking were switched off. The original instrument cluster was replaced by an aftermarket Stack racing display, which has no odometer, so the car’s true mileage cannot be verified.

Broad Arrow describes the W10 as a direct precursor to the quad-turbo W16 that later powered the Bugatti Veyron. Willeke has also said the prototype served as Piëch’s personal transportation during its development, with the project reportedly costing close to €2 million, or about $2.3 million.

A rare M5 with registration complications

The current owner acquired the car roughly a decade ago through engineering contacts and registered it for road use in Slovakia. The owner drives the prototype rather than keeping it in storage, but the extensive conversion means it still lacks a certificate of conformity.

That paperwork will become the next owner’s responsibility. The combination of undocumented mileage, nonstandard electronics and registration work makes this a very different proposition from a factory-original E39 M5. Its auction result will show how collectors value unusual engineering provenance against originality.