To celebrate 50 years of the M division, BMW has been showing off once-secret prototypes in a series of videos, but the two shown in the latest video released Wednesday are among the most unusual yet. Once upon a time, BMW built an E34 M5 convertible and a Z3 M Roadster prototypes with V-12 engines. Both cars are now part of an M 50th anniversary exhibit at the BMW Museum.

It’s unclear why BMW M built a convertible version of a four-door sedan, but it did a thorough job. With no roof pillars to attach the seatbelts to, engineers relocated them to the seats themselves. The car’s top mechanism was also designed to preserve trunk

The E34 M5 was powered by a 3.8-liter inline-6, which sent 340 hp and 295 lb-ft of torque to the rear wheels through a 6-speed manual transmission. An open-air experience of that powertrain does sound appealing. This wasn’t the only oddball E34 BMW’s skunkworks built; it also installed the 6.1-liter V-12 from a McLaren F1 in an E34 M5 wagon.

In comparison, the V-12 Z3 M Roadster seems almost conventional. The prototype first broke cover in 2012, and is powered by what is likely an M73 engine; the 5.4-liter V-12 was used in the 7 Series when the Z3 was in production. It produces 321 hp, a big step up from the stock M Roadster’s 240 hp.

Fitting the V-12 into the Z3’s engine bay was easier than it might seem. While the car itself is tiny, the engine is roughly the same length as the stock M Roadster’s inline-6. It’s just that there are two banks of cylinders instead of one. The heavier V-12 likely affected the car’s weight distribution as well.

The video also includes a brief glimpse of the 1991 BMW M8 prototype, which is part of the same museum exhibit. Based on the E31 8 Series, it has a more powerful version of that generation’s S70 6.0-liter V-12 making 550 hp. BMW decided not to do a production version, however, and the prototype was stashed away. It was recently restored to running condition by BMW.

BMW M’s experimentation didn’t end there. It stuffed the engine from its Le Mans-winning V12 LMR race car into an X5, creating an SUV that could lap the Nürburgring in under eight minutes.

More recently, BMW also experimented with CSL versions of the M3, M5, M6, and M2. None of these made it to production, but BMW did recently revive the CSL badge for the M4.