Originally Posted by swamp2
To clarify, this makes the rumor/speculation:
F80 = 4 door M3, aluminum chassis not shared with F30
F82 = 2 door M4, aluminum chassis, not shared with F32
That's essentially it, yes swamp, as far as prevailing rumor goes at the present time.
However, let me say that "not shared" is likely too strong a word. What I think we will see is significantly more changes from the 3 (and new 4) series body shells in the transformation to an M vehicle than what we have come to expect. That is to say that there will likely be liberal use of aluminum in order to bring weight down. Maybe this means that instead of M just snatching already-welded body-in-white assemblies from their assembly line, the M transformation starts much earlier, with its own unique aluminum enhanced unibodies built from the ground up right along side the all-steel Series counterparts.
Secondarily, when it comes to the bolt-ons that attach directly to the chassis such as subframes and suspension components, I think M will go out of their way to see that they come up with something lighter than the 3(/4) series rather than ending up with something that weighs more as has been the case with all, or at least most, M3s in the past. E.g., lets see what we can do about making this:
A little more efficient. You may point out that there is already aluminum there, and indeed the same holds true for the 3 series as well. But there is also a lot of steel too, so there is room for some improvement, at least theoretically. Also this is just the rear, there is also the front too.
Note that this is all just my guesswork. But there is only so many ways to go light-weight, and if aluminum is to be a key ingredient for that recipe, then I think this is a very plausible scenario.