Sports Car Brake Upgrade
Now that you’ve read and understand everything about brakes (right?), it’s time to explore four different brake improvement projects. In other words, it’s time to grab the wrenches and head out to the garage to bolt on some high-performance brake hardware. Each upgrade scenario has its own set of unique conditions and constraints, but by referencing these four chapters, you’ll be better prepared to address any challenges your own vehicle may throw at you.
To kick things off, we’re going to get our hands dirty (literally) walking through the installation of two-piece rotors, high-performance brake pads, four-piston fixed calipers, and stainless steel brake hoses behind the front wheels of a modern sports sedan—the quintessential big brake upgrade. It’s important to note that these same steps and considerations apply to practically any vehicle going through a front disc brake upgrade. Whether you are working on a late-model Camaro, an Acura Integra, an Audi A6, or a Ford GT, the fundamental sequence of events remains the same.
However, in parallel with selecting and installing the upgraded components, we also take the time to address system-level characteristics such as gain, balance, and pedal feel. For this reason, we frequently reference equations from Chapters 1 through 4 in order to juggle these sometimes conflicting requirements while optimizing overall brake system performance.
Representative of many compact sports cars on the market today, the BMW 330i is a competent performance machine. Like several other vehicles of this nature, this particular example is used primarily for commuting and driving around town, but on weekends it is driven to the limit at high-speed driving schools and racetrack lapping events. Consequently, a high-performance brake system upgrade was desired that would balance the needs of street and track use.
Although its six-cylinder engine produces only a modest 235 hp, this car’s 3,300-pound curb weight contributes to significant brake system temperatures when driven aggressively. Being a BMW, there is certainly autobahn DNA in the brake system design, but like any production passenger vehicle, its stock brake system hardware is biased toward high-speed cruising more than toward generating fast lap times around a road course.
In fact, that last statement is true for practically any street-driven vehicle. Whether you drive a turbocharged sport compact, a high-performance V-12 exotic, a or a modern muscle machine, brake systems designed for street use simply don’t cut it when driven hard on track.
Although increasing the BMW’s brake system thermal capacity quickly became the primary objective, we wanted to make this improvement without making any sacrifices when driving around town. In other words, a dedicated, race-only solution with dual master cylinders and aggressive brake pads was not a viable option.
Therefore, the plan was to replace the brake rotors and calipers with pieces more suited toward high-performance driving while retaining the stock apply system hardware. An equally important objective was to accomplish this task while maintaining stock brake system gain and brake balance. Without the constraints of sanctioning body rules and regulations, our only limiting factor was our checkbook.
Picking the Right Parts
Even though the vehicle’s front-to-rear weight distribution is a respectable 50/50 at rest, under 1.05g of deceleration, the weight distribution shifts to approximately 80/20. Typical of most production-based vehicles, this suggests that the rear brakes are probably not the primary area of concern. As a result, we elected to focus on the thermal mass and performance of the front brakes while leaving the rear brakes alone.
Fortunately, a complete front big brake upgrade kit is available for this vehicle from StopTech. While the kit consists of upgraded front rotors, calipers, brake hoses, and brake pads (smart parts), the icing on top is that the individual components are designed to be compatible from a gain and balance perspective as well (smart system).
As with any thermal mass upgrade, the most important step is to select and install new front rotors. The stock 330i rotors measure 12.8 inches in diameter and 1.0 inch in thickness and are tucked underneath stock 18-inch diameter wheels. In theory, a pair of 14.0-inch diameter rotors would have fit inside the stock 18-inch diameter wheels, but since we wanted to run 17-inch diameter wheels at track days, we selected 13.1-inch rotors that are 1.3 inches thick.
To further improve rotor cooling, we chose friction discs with differential vanes to replace the straight-vaned stock parts. We also went with a two-piece design to reduce radial brake pad taper at elevated temperatures, with the added benefit of reduced rotational inertia. Finally, we chose to have the friction discs slotted. (Although the additional leading edges reduce brake pad life on the street, in this application their on-track performance benefits justify the compromise. Plus, the slots provide the desirable high-performance look, too!)
Our project 330i came with floating, cast-iron front calipers containg single 2.2-inch diameter pistons. They unbolt from the brackets as shown. We swapped them for four-piston aluminum fixed calipers to increase efficiency and to reduce weight. (Randall Shafer)
The stock 330i floating calipers are made from cast iron and incorporate a single 2.2-inch diameter piston. It didn’t take very long to decide to upgrade to a pair of aluminum, two-piece, multi-piston fixed calipers. After all, decreased weight, increased efficiency (better pedal feel), and optimized pressure distribution across the brake pad backing plate (reduced taper wear) are desirable both on the street as well as on the track.
To take full advantage of the multi-piston geometry, StopTech implements differential piston bores of 1.3 inches and 1.5 inches in this application to reduce longitudinal brake pad taper. While this design increases brake pad life on its own, the calipers also use larger brake pads than stock, providing an increased amount of friction material for longer brake pad life.
Note that because the StopTech calipers are a universal design, 330i-specific mounting brackets are required to attach them to the stock suspension uprights. Fortunately, they were included in the kit, alleviating the need to fabricate custom brackets.
Hoses, Fluids, and Pads
While filling up our shopping cart, we also decided to upgrade the stock rubber overmolded brake hoses with high-performance stainless steel brake hoses. Although there was no reason to upgrade based on thermal needs alone, the reduced compliance would provide improved brake pedal feel.
For brake fluid, we chose Motul RBF600, a high-performance DOT 4 blend. A dry boiling point of 594 degrees F indicates that brake fluid fade shouldn’t be an issue when new, and a wet boiling point of 421 degrees F suggests performance should be maintained even with a significant amount of moisture adsorbed into the fluid. (In spite of these quality numbers though, brake bleeding on track days is still a good idea to keep the brake pedal as firm as possible.)
Like many performance street cars, this one will be primarily driven on public roads, so we wanted to select brake pads that would perform well at low temperatures. However, well-rounded cars like this also see the track, so high-temperature robustness was just as necessary to prevent brake pad fade. To satisfy these conflicting needs, we ended up going with two sets of brake pads—one material for around-town driving and another material for hot laps on the weekend.
For street use, we chose a semi-metallic brake pad friction material to replace the stock non-asbestos organic material. Why? To allow for moderately increased thermal performance during spirited driving while maintaining acceptable levels of wear and noise. On the track though, the street brake pads will be replaced with a dedicated racing compound specifically designed to deal with the heat generated under green flag conditions. (Unfortunately, the elevated levels of rotor wear, brake dust, and noise from the race pads would make them unacceptable on a daily basis.)