The old engineer's joke that "speed doesn't kill, rapid deceleration does" applies to all vehicles capable of attaining dangerous velocities. Since we have been concentrating on improving Project STi's acceleration and handling, we have definitely achieved the ability to reach deadly velocity on the track. In our first foray onto the grounds of Willow Springs raceway we noted that perhaps we were starting to drive beyond the capabilities of the stock brakes due to the higher speeds that we were able to attain on the straights.
The STi is blessed with excellent, large, competition-worthy Brembo brakes from the factory. However, these Brembos are somewhat hamstrung by the requirements that the STi be a civilized street car. Street drivers usually don't tolerate any sort of squeaks and squeals that high performance brake pads are prone to. Thus the brake pads that the car is equipped with from the factory are compromised to be less than optimal in hot braking performance for quietness and effectiveness when cold. Usually a brake pad works the best within a given temperature range. A pad that works best when hot, often works poorly when cold and vice versa; and Subaru compromised in the conservative cold direction.
When driven on the track, the stock brakes were not horrible since stock brakes often fade away to nothing when pushed hard. We did notice that the brakes did fade, losing about 25% of their effectiveness then stabilizing for the duration of the track session. Although this was not terrible for OEM brakes, it was not exactly confidence inspiring either. Although we could have simply installed a super duper mega huge brake kit, we decided that we did not really want this approach for several reasons.
The first reason is that the 2004 STi is prone to suffer what is called brake caliper knockback. The 2004 tends to wear its wheel bearings. This causes the wheel and brake rotor to wobble around when subjected to cornering load. This wobble knocks the brake caliper pistons back into the caliper. When the brakes are reapplied, the brake pedal height is now lower than usual because the pistons have been knocked back into the caliper and must be pushed back out to contact the rotor. In extreme cases the pedal may drop to the floor and must be pumped to regain braking power. Subaru recognized this problem and the 2005 and later models have bigger hubs and wheel bearings to eliminate this.
If a larger brake rotor is installed, the effects of knockback are greatly magnified and the brakes are more sensitive to any sort of run out in the hub bearings. Since the STi is blessed with the equivalent of a racing brake upgrade stock, we decided to work around the stock rotor size to help reduce the effects of knockback. This evolved swapping the stock Brembo rotors with some highly effective ones from Disc Brakes Australia. When traveling to Australia several months ago in Part Two of our project (tough job, I know), we drove a highly modified STi at the track, Whiteline's infamous P25. No matter how hard we abused the brakes on the P25 they worked consistently with no signs of fade. Driving the P25 around on the streets during the week demonstrated that the brakes also worked well when cold, with little noise or fanfare.
When queried about the brakes' excellent performance, Jim Gurieff, Whiteline's Technical Director, gave much of the credit to the rotors on the P25 supplied by a fellow Australian company DBA, or Disc Brakes Australia. The DBA rotors have many unique features that are truly functional and help the brakes work better over a wide range of conditions. Impressed as we were by the P25, we ordered up an identical set of rotors for Project STi.
Part of the secret to the effectiveness of the DBA rotors is their patented Kangaroo Paw venting system (if DBA was from Wisconsin I guess it would be called the Cow Hoof system). The Kangaroo Paw design features a series of 144 diamond and teardrop-shaped pillars and columns, individually spaced between the rotor faces. This system provides two key advantages. One advantage of this design is the ability to cool the rotor faster than before. This works on the same principal as a turbocharger impeller, designed to draw air through the rotor. As the disc rotor rotates, air is moved through the rotor by centrifugal force. Aerodynamic turbulence generated around the rotors individual pillars and columns helps improve the removal of heat from within the internal areas of the rotor. Whiteline offered some data showing a rotor surface temperature drop of over 100 degrees generated during track testing of the DBA rotor.
The second advantage offered by the Kangaroo Paw rotor is increased dimensional stability across the disc friction face, achieved by the equally spaced pillars and columns. This helps prevent rotor warpage and pedal pulsation during use. These advantages increase the DBA rotor's efficiency by as much as 20 percent when compared to conventional rotor designs. A 20 percent increase in disc rotor efficiency, translates to reduced brake operating temperatures, brake fade, disc rotor stresses, disc thickness variation, run-out and shudder.
We replaced our front rotors with DBA's top of the line 5000 series rotor. The 5000 series rotor is a two piece design featuring a lightweight aluminum center hat with a corrosion resistant black anodized finish. The aluminum center hat reduces unsprung and rotating weight by several pounds per rotor. This improves acceleration, braking and suspension performance. The rotor itself is available separately so the hat can be reused should the rotor wear out. The rotor is positively held to the hat with aerospace NAS bolts. Although the 5000 series rotor comes in a cross drilled configuration, we opted for a more competition worthy slotted surface.
When brakes are used at high speeds, the brake pad material vaporizes and the pad can hydroplane on a layer of gaseous vaporized brake pad. This makes the brakes much less effective. This type of brake fade is often encountered, especially with new unbedded brake pads. Holes drilled into the surface of the rotor give the gas somewhere to go, reducing the hydroplaning effect and eliminating this type of brake fade. The trouble is, although drilled rotors look sexy, they often crack between the holes. Thus for track use, many feel that a slotted rotor is better. The slots act like tread on a tire and give the gas a place to go. Additionally, the slots do not have any tendency to crack. For this reason we decided to go with slotted rotors. Slots also tend to reduce the brake pads' tendency to glaze and help wet performance as well as they give water somewhere to go, thus helping reduce the pads' tendency to hydroplane on a layer of water.
For the rear, we opted to use DBA's 4000 series rotor. This is a one-piece rotor made necessary by the STi's use of a drum-type parking brake that is integrated into the rear rotor. This makes the use of an aluminum hat impractical. The 4000 slotted model has a unique 6x6 Wiper Slot design for exceptional venting of a vaporized brake pad. Unlike many aftermarket rotors, the 4000 series has a coating of heat resistant paint on the center part of the rotor, so if you have open wheels you don't see a nasty rusty-looking rotor.