Coulda Been: The BMW Z3 V12 Prototype

BMW's M division has turned up a special one-off prototype that even the carmaker itself can't account for: an orange Z3 with a V12 crammed into its engine bay.

The photos, posted on the brand's Facebook page this week, were accompanied by a short message: "We don't know why a V12 engine was built into a BMW Z3 years ago, but it still looks impressive, doesn't it?"

Impressive engineering indeed. The installation is so clean that it might almost be the favoured straight-six nestled under the roadster's clamshell bonnet.

Enthusiast site BMW Blog believes the car to be a prototype study to test production feasability - but like the V16 7 Series, M 3 Series Compact or E34 M5 Convertible, it went no further.

Others have suggested it was a 'skunkworks' project, such as the E90-based M3 Ute. US site Autoguide speculated that the engine is the M73 5.4 litre V12 from an old BMW 750i.