During an awards trip I was on in September or October 1990 we were shown a slideshow teaser and given some information on some interesting new product in the pipeline. Most notable was the 500E, and the 140 chassis S-class.
One of the speakers said that the main reason for the Porsche assembly was that the car was too wide for the normal 124 assembly apparatus, and that Porsche was just up the tracks across town, had the perfect facility, and that the facility was underutilized at that time. He said that the facility in Bremen that built the 129's could accommodate the width but that would've introduced a bunch of other complications, not the least of which was the 129 was selling strong right out of the box and it had just been introduced. They could not build those fast enough. I remember thinking at the time "yes and you're probably making a lot on 129's and probably losing a fortune on that ridiculous hot rod 124. I was not a fan of the concept until I got to try the resulting product. He also said that there was a chance some of the equipment at Sindelfingen was going to be adapted and that they would bring the production fully in-house. Obviously, some of that came to pass and we saw the result in the 400E, the standard production plant obviously at that point accommodating the different floor pan and front console essentially straight from the 500...
So I think referring to that 500E as a "widebody" is perhaps fair enough, but to me that's really a "pre-MB AMG" term. I kind of like it though...
Somebody had asked if Porsche was in any way involved in the design and/or development of the car and the response was one of those "we can neither confirm nor deny" kind of things. That was no particular big deal to industry watchers because it was already well known that Porsche did an enormous amount of consultancy and development/endurance driving testing and so on for lots of different car companies, some local, some distant.
Remember Porsche was in a world of hurt right at that moment in its history. There was even a lot of talk about what corporate giant it was going to wind up in the possession of. The assumption that it was going to have to be sold was pretty much considered a foregone conclusion. Famously at around that same time a Mercedes bigwig made the statement in a German business publication that if Porsche was ever for sale "we will not allow anyone else to buy it".
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