Quote:
Originally Posted by weehe126
If the new rears are smaller why wouldn't you keep the stock rears? Or am I missing something.
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My apologies, in my post I was comparing to the 335i hardware because that was the cheapest OEM upgrade for my 325i. Though 13% less piston area than the 335i caliper, the rear Cayenne caliper has only 5% less piston area than the 325/328i caliper.
For the cost of upgrading the rear to 335i calipers and rotors, the Brembo units from the Porsche seemed to make more sense. The slight decrease in piston area can be compensated for with an appropriately sized master cylinder; with a .7" front MC and a .625" rear the brake balance should be pretty close to stock and pedal force should increase by around 10%.
Two pistons per pad spreads the applied load better and allows for a slight increase in cooling. The increased pad surface area also improves brake torque and the increase to the overall rigidity of the caliper itself will help. Another important factor in the decision was the ease of pad changes, pulling a pin and swapping pads is a lot easier that removing the caliper. The fixed caliper will also require less maintenance and has less moving parts (no slider pins to worry about). Oh, I also had the vehicle in front of me so it was easier to snag all four calipers!
That's my logic at least.
EDIT: It should be noted that the even the StopTech kit ($2500 USD) for the rear of the E9x (ST40 caliper) area offer almost zero increase to piston area over the 335i brakes (assuming a 28mm and 34mm pistons, from my research). The benefit from the kit comes from increasing the mean pad height and the other points noted above.