I am going to be attempting this rear brake upgrade with custom 7050 Aluminum adapter plates (That I will be machining) for the e36 M3 Rear Calipers on the the stock 318ti trailing arms. I will post plenty of pictures and CAD drawings of the adapter when I get it to work.
From the limited info available on this custom brake upgrade this is what I have discovered:
Stock e36 318ti Rear Rotor: 
Stock e36 M3 Rear Rotor: 
Stock e85 Z4 3.0L Rear Rotor: 
Stock e82 128i Rear Rotor: 
The above images depict the dimensions of the rear rotors of each of the bold-lettered vehicles. When talking OEM rear rotors that are vented BBK upgrades for our cars and that would allow for the 318ti E-Brake shoes to still work there are two that seem to fit the bill (That I was able to find). These are the e85 Z4 3.0L rear rotors and the e82 128i rear rotors. As you can see from the dimensions they each have similar hat heights to the stock rotor, and although not depicted they both use 160mm diameter E-Brake shoes (Same as the 318ti). Our Shoes are 5mm wider in friction material than either the e82 or e85 e-brake shoes, but that should not effect the functionality of the parking brake.
I am going to choose the e82 rotors as I have the e36 M3 rear calipers, and looking at the thickness and diameter of the 128i rotor, it most closely resembles the thickness and diameter of the M3 rotor, which means that the application will be an almost perfect pairing for the piston travel of the caliper and the bias should be nearly identical to the M3 (with matching front brake calipers/rotors/brake servo/and master cylinder of course).
I believe that the aftermarket rear BBK upgrades from IE and the bracket available from 300mm.de were designed around using the Z4 rotor as they are advertised as 294mm x 19mm vented upgrades which corresponds to the diameter and thickness of that rotor, so this I presume would be another viable rotor choice, but as it is a tad thinner than the 128i rotor (an consequently even thinner yet than the M3 rotor) I don't believe it to be the better choice to pair with M3 rear calipers. If you got the Z4 calipers to match they would be a good upgrade option as well over the stock 318ti rear brakes.
To be clear, any of these proposed upgrades are not direct bolt-ons and would require some or all of the following: - modification of the 318ti backing plate to either remove or flatten out the lip on the perimeter of the backing plate that used to fit the stock 272mm diameter rotors as it would certainly interfere with the larger rotors.
- Possibly cutting the backing plate to make clearance space for any custom carrier adapters and/or calipers installed that are larger than the stock calipers.
- Possible Custom rear brake line to reach the new calipers.
- And of course a custom bracket/adapter to mate the new caliper's carrier to the 318ti hub's mounting points and move the caliper the correct distance away from the center of the hub to fit the larger diameter rotor.
Hope this info helps all contemplating a less costly rear brake upgrade than the Z3M sub-frame swap. I should have this project done some time in early May of this year and will be able to contribute real tangible documentation to back up this research proof of concept.
All credit for the idea of using the e36 M3 caliper paired with the e82 128i rear rotors should go to "Marv17." To the best of my knowledge he has already done this swap successfully and is the reason why I am even pursuing this project.
Cheers
