Most BMW fans love the E60-gen M5 for its high-revving, F1-sounding V10 engine. In stock form, it made 500 horsepower and revved to 8,250 rpm, making it one of the most exciting road-car engines ever made. However, BMW M decided back in the day to try making an even crazier version–the E60 M5 CSL.

As part of this new video from BMW M, showing off some of the top-secret prototype M cars throughout the brand’s history, cars that never made it past the prototype phase, we get to see the E60 M5 CSL. It was as absolutely bonkers as you might imagine it to be. On the outside, its M-colored livery honored the 25 anniversary of the M5 at the time and it was the first ever M5 to have a carbon fiber roof.

Under the skin, the E60 M5 CSL proves how absolutely kick-ass it would have been. The 5.0-liter naturally aspirated V10 was juiced up to make 630 PS (621 horsepower) at a peak rpm of 8,750 rpm. So it actually revved closer to 9,000 rpm than 8,000 rpm, with a massive 5.0-liter V10. Absolutely nuts. It gets better, though. Rather than the clunky, frustrating SMG gearbox, the E60 M5 CSL was the first car to actually use Getrag’s seven-speed DCT (Dual-Clutch Transmission), which would later go one to be used in the E92 M3 and other subsequent M cars.

Not only was the E60 M5 CSL lighter, sharper, and more powerful than the standard car, it revved like a 911 GT3 and replaced the stock car’s crappy gearbox with a far better one. Had the E60 M5 CSL actually made it past the prototype stage and into production, it would have been, unquestionably, the coolest sedan in the world at the time. So cool, in fact, it would have negated all of the talk about its looks.