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The other issue has already been recognized that under hard/extreme/repeated use, the weak link in the brake system is heat capacity.
We all know that if your OEM brakes can exceed the grip provide by the tires then you typically do not need any more braking power. For the most part our brake systems are not designed to provide greater amounts of brake torque, but rather maximize the efficiency of the system and improve temperature management. While the stock brakes will lock up the tires on the first few hard stops, after a few more you will induce fade and your stopping distances will continue to lengthen.
The first few steps that most people take are braided lines and a high performance brake pad. This is good for providing a slightly firmer pedal feel and getting a pad that has a wider temperature range to match the intended driving style. For a handful of people the wider temperature range is just enough to get them away from the possibility of pad fade. For others it simply gives them a few more aggressive stops before the fade returns due to a heat soaked disc and boiling fluid.
The next intermediate step, would be introducing more airflow to the system in the way of brake ducting. Now you have larger amounts of fresh air being introduced to the system and you have found a way to lower the consistent operating temperature. This is a great step and something that you see on practically 99% of true race cars due to the efficiency. The problem here is that (1) I can't think of anyone off hand that has developed a proper duct kit for the M3, and (2) the attempts that I have seen do not come close to being able to deliver enough airflow to lower the temperatures enough to eliminate the chance of fade. At this point you have already spent about $500 on front and rear pads and brake lines.
Now you're thinking about a caliper upgrade.
A caliper upgrade will undoubtedly deliver a few improvements to the M3.
(assuming it's developed with quality manufacturing and proper engineering)
It will more than likely be stiffer and lighter than the OEM sliding caliper. With the appropriate piston sizes the pedal feel will remain similar, but response and modulation may be improved. In a best case scenario it will have a larger pad shape which will provide more swept area for longer pad life (Note: The new pads will be more expensive.), and with the proper pad you will still have a friendliness to the disc. The larger pad volume can also (even though very minor) act as a heat sink for a tiny bit of added heat capacity. Keep in mind I am speaking in terms that relay how this kit would be marketed to the consumer. This is regardless of whether it is realized in actual use or not.
You've probably spent $1600 - $2000 depending on which brand jumps at the chance to develop the caliper only upgrade, + the money you already spent on pads and lines. At this point you have not really done anything that has significantly added heat capacity or reduced operating temperatures. Besides the improved look behind the wheels, you are back in the same place as before with fade issues. If by chance this caliper upgrade provides increased braking forces either by larger piston area or increased coefficient of friction (based on pad selection and caliper efficiency) it will lead to adverse effects in heat management by increasing the braking temperatures at a faster rate.
The most common question I have been asked in the last few months is why did Brembo not develop a 355mm brake kit to attempt to fit under factory wheels. The short but very direct answer is heat capacity. The OEM disc is a 360mm, with a tall annulus, and thick walls. That creates a disc that alread has significant amount of heat capacity even though during extended track use it becomes not enough. A 355mm (and even our 365mm) disc would have been a reduction in heat capacity as compared to the OEM disc. Rather than creating something purely based on aesthetics and an obvious market demand, and without ssuming that our improved metallurgy, vein design, and wider airgap would be enough improvement on it's own to maintain consistently lower temperatures, FEM (finite element modeling) proved otherwise in the early staged of our R&D.
The good news is that we did our best to make sure it would fit the OEM 19's so at least there's an inexpensive option for 1 set of wheels. If you're a serious track guy chance are you'll eventually end up with 2 sets of wheels, one for the track and one for street. There are a few 18" wheels currently (with more coming soon due to our influence with the top wheel manufacturers) that fit around our 380mm system that can become your track wheels. All of these that come to mind are lighter than the OEM wheels making them the better choice for your track tires. The 19's become your street tire and you're braking problems are solved.
It's not a cheap endeavor, so I'm sure most of you will be hoping that the first stage of pads and lines work out. We did make sure though, that our BBK option will be the final step in your braking needs.
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