Lo he visto en el FAQ, os lo copio:
Z3 V12 desarrollado por Motorsport BMW. Solo hay uno.
Over the years, BMW and its various subsidiaries and sub-brands such as the BMW Technik GmbH (currently known as BMW Forschung und Technik, or Research and Technology) and M GmbH, have quietly developed an array of prototype models that never saw the light of day when new, but which are gradually coming out of hiding.
Among the many secret development prototypes that have been released to the public eyes are the M8 Coupe, 850Ci Convertible, E30 M3 Pickup Truck, E36 M3 Compact, E46 M3 Touring, E34 M5 Two-Door Cabriolet, Z1 Coupe and Z18 Buggy - just to mention a few.
Today, BMW's fabled M GmbH division dropped three photos of an M Roadster prototype of the Z3 series powered by a V12 engine. The company doesn't provide any information other than that the study was built "years ago".
Given that the Z3 series was sold from 1996 to 2002, it’s very likely that the V12 shoehorned under the hood of the roadster is based on BMW's M73 5.4-liter engine used on the E38 750i/iL sedans and E38 850Ci coupe of the mid-1990s to early 2000s and which was rated at 322hp (326PS) and 490Nm (361 lb·ft).
At the time, the fastest Z3 model was the production M Roadster fitted with the E46 M3's 3.2-liter naturally aspirated inline-six delivering 321hp (325PS) and 353 Nm (261 lb·ft) in EU specification (315hp in North American trim).