Image Credit: Broad Arrow Auctions.The E39 BMW M5 already had one of the great performance engines of its era, a naturally aspirated 4.9-liter V8 producing 394 horsepower. Apparently, that wasn't interesting enough for Ferdinand Piëch.Around the turn of the millennium, the Volkswagen Group boss was overseeing experiments with unusual engine configurations that would eventually lead to some of the company's wildest creations. Somewhere along the way, engineers needed a car in which to test an experimental W10.Rather than build their own prototype from scratch, they started with one of the best performance sedans available. An E39 M5 had its BMW V8 removed and replaced with Volkswagen's secret 5.0-liter W10.The resulting one-off became Piëch's personal transportation during development, and somehow it survived. Now, this extraordinary BMW-VW mashup is heading to Broad Arrow's Zoute Concours Auction on October 9 with an estimate of €250,000 to €350,000.A 473-HP Volkswagen W10Image Credit: Broad Arrow Auctions.Broad Arrow says Volkswagen built only three examples of the experimental W10 engine. The naturally aspirated, all-aluminum 5.0-liter unit installed in the M5 produces 480 PS, or approximately 473 horsepower, along with 592 Nm (437 lb-ft) of torque.That's around 79 horsepower more than the M5's original S62 V8 produced. Better still, the W10 is paired with a six-speed manual transmission.The prototype wasn't exactly burdened with electronic intervention, either. Its traction control, stability control, and ABS were deactivated, while a standalone ECU handled the experimental engine.According to Broad Arrow, the W10 project helped explore the W-engine architecture that Volkswagen would subsequently take much further. Piëch's obsession with unusual cylinder arrangements ultimately gave the world engines such as the Volkswagen Group's W12 and, most famously, the Bugatti Veyron's quad-turbocharged W16.Piëch Apparently Drove It HimselfImage Credit: Broad Arrow Auctions.The M5 was originally built on March 2, 1999, and its exterior gives virtually nothing away about the craziness going on under the hood. It's finished in Carbon Black Metallic with a black leather interior, making it look remarkably close to a normal E39 M5.Sabine Willeke, who led the engineering team behind the project, says Piëch used the car as his personal transportation during development. He drove it on temporary registration plates while the W10 was being tested and refined.Its total mileage is unknown because the factory instrument cluster was replaced by a Stack display without an odometer. Aside from that and some additional gauges on the center console, the cabin is surprisingly conventional.It Even Comes With A Spare W10Image Credit: Broad Arrow Auctions.Only three W10 engines were built, according to Willeke, and this is the only one installed in a road-going car. One of the other engines is still in Germany, while remarkably, the third is included with the M5 as a display engine.The current owner acquired the prototype roughly a decade ago and subsequently registered it in Slovakia. It isn't merely a static piece of Volkswagen history, either, as the owner has continued driving it and had it serviced as recently as 2025.Broad Arrow expects the M5 to fetch between €250,000 and €350,000 when it crosses the block. Considering you're getting an E39 M5, Ferdinand Piëch history, and two of the three W10 engines ever built, finding a comparable car to check the estimate against will definitely be a challenge.If you want more stories like this, follow Guessing Headlights on Yahoo so you don't miss what's coming next.
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