e36 vs e46 "rear subframe" issues

[Beej]

so did you end up doing the foam too? Looks like those plates are pretty nice.

Not yet. I actually read up a ton on the foam this morning...and I'm torn (ha).

The general trend (at least on M cars) is that the rear passenger and front driver mounts are the ones most likely to crack. Summing up a ton of reading:

Rear two mounts: you effectively can "fill" the cavity with the foam.
Front two mounts: the cavity is huge, there's no real way to "block" unnecessary sections, and there's no "top" hole to fill from (might be able to drill one?). So effectively, you can't really fill it. Since the foam isn't encapsulated or surrounded on all sides (or at least two opposing sides), I don't see how it can be effective in doing anything.

So I'm thinking aloud in hopes to get anyone's thoughts, since all we can really do is theorize. If you fill just the rear, I see two possibilities:
1) more energy transfers to other "non-solid" mounting points, perhaps causing them to flex/break more?
2) overall there will be less movement (since they're solid now), so maybe less stress overall?

There have been some instances where the entire rear axle carrier starts to break away from the car. I'm not sure if filling the cavities would make this more likely or not - again, where does the energy go after this weak spot is "addressed".

Plus you're blocking drain holes (not sure if they're from the sunroof or anything), and sitting water isn't anything to get excited about.

But I ain't no mechanical engineer.