When a carmaker decides to develop a radically different platform, its test mules for the upcoming vehicle can't be based on a previous model. That's why back in 2008, McLaren Automotive's development cars were two Ferrari 360s, and an Ultima GTR. Porsche found itself in a similar situation in the early 1970s.
In 1972, Porsche knew that its first V-8 would sit in the nose of a luxurious gran turismo, the 928. The company known for its rear or mid-engined, air-cooled sports cars was heading into uncharted territory. The 928 debuted at the 1977 Geneva Motor Show, but before it could get there, Porsche had to buy and chop up a few Audis, a Mercedes-Benz and an Opel to use as development mules. As Julius Weitmann's and Rico Steinemann's on the 928 explains, the car's development began with a Mercedes-Benz SL, another front-engined, V-8-powered German luxury tourer.
Codenamed V1 and fired up in July, 1972, the SL was used by Porsche to develop the 928's transaxle. The car retained its original engine and three-speed automatic gearbox up until 1974, when Porsche upgraded it with its 4.5-liter V-8, and fitted a new front axle.
Mule V2 was an Opel Admiral sedan. It had a 928 chassis with an Opel drivetrain, and was later upgraded with passive rear-wheel steering—which Porsche called a Weissach Axle—to sort out handling issues.
Then came V3, the first of three Audis, a 100 Coupe S, which was originally powered by a 1.9-liter four-cylinder producing 115 horsepower. Porsche altered its wheel housings, relocated the suspension mounting points, and added some massive fender extensions, so the Audi could take the V-8 drivetrain and wider tracks of the 928. This car took part in two extensive test runs in Africa, as well as one up Mont Ventoux in the south of France. In 1975, Porsche upgraded it with an automatic gearbox.
V4 is perhaps the most interesting 928 mule of them all. Instead of wider fenders, the second Audi prototype was simply cut in the middle and widened by 4.3 inches. Not unlike the 928-powered Nordstadt Super Rabbit of 1978.
First run in July 1974, V4 was a proper 928 underneath the Audi costume. The 5.0-liter V-8 with Bosch K-Jetronic fuel injection, five-speed transaxle, and entire chassis were identical to the then-upcoming 928. Unfortunately, this prototype was near Hamburg in 2011.
V5, Porsche's final Audi 100 was very similar to V4, and was used to test the production rear axle at the end of 1974, although still without the control link. Porsche also built a bare chassis mule, named Munga after Auto Union's DKW Munga off-roader, which beat Porsche's own Type 597 back in 1956. Equipped with a carbureted V-8 and the racing gearbox of a 908/3, open-top fun was surely had in Weissach.
The Mercedes, the Opel, the Audis and all those modified 911 test beds were followed by real 928 prototypes. K1 was a plastic-bodied chassis used for a number of mostly static tests. W1 was the first real 928, and one the engineers put a lot of miles on at Weissach, Ehra-Lessien and Ventoux. It also did some winter testing in Turrach, Austria before Porsche sacrificed it for a crash test. After W1 came W2, built for endurance tests. W3 was the most important of them all, as its great handling convinced Porsche shareholders that the 928 project was still a grand idea. As for W4 through W12, I feel like I shouldn't spill all the beans—for that you should read the .
All in all, Porsche engineers put around 600,000 real test miles into its various 928 prototypes, and another 600,000 if you count all their condensed-time endurance runs. Looking at these images from Algeria, one thing is for sure: if Porsche ever needs someone to put more miles on a widebody V-8 Audi 100 S, we at Road & Track will be the first to volunteer.
Also, if you happen to find yourself in Porsche Museum's not-so-secret warehouse, keep an eye out for Porsche's two (?) remaining Audi 100 S Coupes. If they ask for a password, try "Ingolstadt."
Update: 4/9/2019: We got a few extra photos from , who says they were shown to him about two decades ago by the president of the Audi 100 Coupé S Club:
The plot thickens.