BLUF: Porsche Cayenne calipers on an E90, using M5 and 135i rotors, to fit under a 17x9.5 ET35 Apex ARC-8. As of 27 Mar 19 this is a work in progress/project. If you came here for final results, check in later.
I've got a 2006 E90 325i I am turning into a track car (lapping/time attack, maybe some wheel-to-wheel). I don't like the idea of throwing money at problems that can be better solved with patience and creativity, so I decided to look into building my own big brake kit.
Calipers
I locally sourced a set of Porsche Cayenne calipers (2006 with 18Z front) for cheap and figured they would be a good starting point. Compared to the stock 335i caliper the front piston area is basically on par with the StopTech ST60 (~7-8% increase), though pad selection isn't as good. Rear piston area drops off 13% compared to the 335i caliper, but only 4.5% compared to the 325i caliper. Since this is going to be a track car I can get around most of this issue with master cylinder sizing and brake balance adjustment, however I may swap these out for a different rear brake that is closer to the ST40 in overall piston area (which is roughly the same as the 335i rear).
Only modification I am hoping to do to these is moving the cross-over tube to the other end of the caliper since these will need to be run on the opposite side of the hub compared to the Porsche fitment. I know p0lar has milled these for radial mount but I am hoping to not have to resort to that; maybe I'll hit a roadblock and won't have a choice though, who knows.
EDIT: I hit a roadblock, the caliper ears interfere with the mounts on the upright... something has to give, so I'll be modifying the calipers.
Rotors
Finding reasonably priced rotors was a pain in the ass; I spent hours on Brembo Europe's online catalog (life saver) trying to find what would work. Initially I was going to use the Porsche rotors, re-drill them to a 5x120 pattern, and run a hub-centric spacer to make up the hub register difference; since rotors are fairly inexpensive for the Cayenne, surprisingly. Instead, after a few hours of research, I decided to go with E39 M5 front rotors for the front of the E90 and F25 X3 front rotors for the rear of the E90. These were as close to the Cayenne rotor diameter/width I could find while still having a BMW bolt pattern/hub register.
The M5 rotor should sit about 3mm more inward than the 335i rotor due to the increase in rotor height. Pad swept area covers all but 10mm of friction surface (so what, I'll have a lip, rotors will probably crack/warp before they ever wear that much anyway). The M5 rotor is 2mm narrower than the Cayenne rotor, so I will be running 1mm shims on the back of the pad; this will also help with keeping heat away from the pistons/seals/fluid. Some people use titanium backing plates for weight savings, but stainless steel is cheaper and a better insulator of heat. In the rear, 135i front rotors will be 6mm thicker and 38mm greater in diameter than the 325i rear and will sit inboard an additional 6.8mm compared to the stock brakes. Since I will need to make a hub-centric ring to properly locate the rotor, if I need to make it act as a spacer too, it's not a big deal.
Going to likely run Centric blanks from RockAuto or Zimmerman blanks from FCP Euro (lifetime replacement). Slotted are nice and all, but it's going to be (for not) a fair-weather car so I'm not too worried about wet rotors. Less likelihood of cracking as well. Plus they are cheaper.
Pads
One downside to this setup is that motorsport pad selection isn't all that great on the front calipers (because it's for a truck, basically). Carbotech are well reviewed and fairly priced, so I'll probably go with them. I can get Pagid RS29 for the fronts if I want to spend another $240CAD for the pair over the Carbotech XP10.
Whole Package
Having components that all work on paper is one thing; but putting it all together is another story. Ideally, I don't want to machine the caliper much/at all. I don't want to go to a radial mount, I don't want to mill the face of the mount ears, etc. I don't like running custom parts on a racecar because that means you need a bunch of custom spares... I'd much rather run off-the-shelf parts in case something breaks.
I was worried about my self-imposed wheel size limitations. Keeping costs down was the goal for this project, so I wanted to be running 17" wheels and tires. As such, this whole package must fit underneath an 17x95.5 ET35 Apex ARC-8. Thankfully, this wheel has some of the best barrel clearance I've ever seen on an aftermarket wheel! Quick calculations and template making leads me to believe that this will work. From my math, the 18z caliper in the front over an M5 rotor will not stick out any more than the ST60 does (Apex has confirmed that the ST60 355mm kit fits under these, so I used that kit as my fitment benchmark) so spoke clearance should be good. Barrel clearance looks good too after some cardboard aided design (see pics).
Next step is to buy some Centric/Zimmerman blanks and start designing a bracket that will locate the caliper. I'll keep everyone in the loop!
Thanks for reading.
TLDR
Front Brakes- 2006 Porsche Cayenne front calipers (Brembo 18Z) for a 350x34mm rotor
- Piston sizes: 30mm, 34mm, 38mm
- Piston area: 8.522inē
- Difference in piston area from 335i: +7.73%
- StopTech ST60 difference in piston area from 335i: +7.48%
- E39 M5 front rotors (345x32mm)
- Zimmerman: 34112229527 & 34112229528
- Carbotech XP10: CT1014-XP10
Rear Brakes- 2006 Porsche Cayenne rear calipers (Brembo) for a 330x28mm rotor
- Piston sizes: 28mm, 30mm
- Piston area: 4.100inē
- Difference in piston area from 335i: -13.02%
- StopTech ST40 difference in piston area from 335i: +0.21%
- E8x 135i front rotors (338x25.8mm)
- These will require a hub-centric ring to be cut to properly locate the rotor on the 75mm rear rotor boss
- Zimmerman: 34116778648
- Carbotech XP8: CT978-XP8
Misc. Info- No milling of caliper or upright; not converting to radial mount - This has changed, either the caliper or upright needs to be modified... I chose to modify the caliper to a radial mount
- Probably not running dust shields (racecar)
- No e-brake (racecar)
- Dual master cylinders, no booster/ABS (racecar)
- Must clear Apex ARC-8 17x9.5 ET35 barrel and spokes
- May swap rear calipers for something that doesn't drop piston area so much - Decided this won't be an issue with the various ways to adjust brake balance on a racecar
- 1.0mm stainless steel shims for front brake pads - Will be making 1.0mm shims for the rear as well now that I'll be using 135i rotors
- Under $2000CAD ($1500USD) front and rear