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When building my 2008 for touring car-type racing I studied a lot of the posts about the lack of durability when the factory Brembo brakes are fitted with competition pads and used hard. It struck me as odd that the 135i's had this problem whereas the E90 335i and M3 models did not despite the latter being equipped with the technically inferior single-piston sliding calipers. A comparison of the respective specifications led me to the conclusion that the 135's problems was likely to be largely caused by its rotors being 4mm thinner (26mm vs 30mm for the 335i/M3). I believe that the smaller air gap between the rotor faces results in a greater transfer of the heat generated when competition-type pads are used to brake harder and more frequently.
As my car is also a daily driver (with front cage and race seat removed) and my racing budget is small I was reluctant to go down the after market BBK route. I was also conscious of wanting to maintain a setup that is road legal, offers relatively cheap replacement costs and avoids wheel fitment problems. To test my theory I purchased some used 335i front rotors/calipers for $US 50 per side plus freight. This is a straight bolt-on and comes with the bonus of a 10mm increase in rotor diameter to 348mm. In relation to the hydraulic side of things there is a difference of just under 5% in piston area that causes a slight increase in pedal effort but no change in pedal height. As the rear brakes do not work as hard as the fronts I fitted a set of Race Brakes stainless steel caliper pistons and their silicone seals/dust boots. In conjunction with some uprated brake fluid and a set of race pads (usually Pagid RS29) I have now been racing (not track days) hard for nearly 3 years without any performance or durability problems or supplementary aids like cooling hoses. A small improvement to be done soon will be the replacement of the rubber bushes used in the sliding calipers with brass items to reduce uneven pad wear. I should mention that the Pagids work well even in Winter lows of -7c and only get slightly noisy after a lot of road use in the 2-3 month break between race seasons. At some tracks the summer air temp reaches the low 40's c. Combined with my uprated twin oil cooler setup the 335i front brakes enable me to easily do races up to 33km in length at maximum effort.
In summary this has proved to be a cheap solution that works well and provides easy access to either factory or after-market rotors. The only downside for some would be the loss of the bling associated with the factory Brembo calipers compared to the dull industrial look of the 335i caliper.
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