In recent weeks, there’s a good chance you read about one of the most radical Volkswagen prototypes ever: an E39 BMW M5, ...

But the unremarkableness of this E39 M5 disguises something very, very cool. This one has had its BMW V8 swapped out for a Volkswagen W10. This is the only car ever to have this engine fitted.

One such design the company built but never entered production was a W10. The automaker was going after BMW and wanted to create an M5-crushing model with that engine. To test it, the company put ...