The purpose of a brake upgrade for our platform is usually temperature management, not increased torque. The stock brakes with aftermarket pads are excellent but can exhibit fade due to overheating under repeated heavy use; the goal is to delay the onset of fade by keeping temperatures lower and/or reducing temperatures more quickly. Threshold braking represents optimal stopping power and is already possible with the stock brakes (exceptions may include wider, stickier racing tires).
When upgrading brakes, there are three primary variables to consider: rotor diameter, caliper piston area, and the hydraulic control system (master cylinder, ABS, combination valve). The stock hydraulic system is calibrated for the stock rotor diameters and caliper piston areas; if either, or both, are increased, the stock hydraulic system may be forced to operate outside of its design parameters, resulting in different and likely unfavorable behavior. It is undesirable, and in some cases legally impermissible, to make modifications to the stock hydraulic system, so careful consideration of the other two variables is critical.
Many front-only big brake kits sold for our platform offer superior temperature management but at the expense of overall braking distances; because larger rotors and pistons are more effective (i.e.- produce more torque at lower pressures), the brake bias shifts forward, reducing the relative contribution of the rear brakes.
Here are a few things to consider (not specific to any particular kit on the market):
With larger rotors, larger caliper pistons, or both, the amount of torque generated at a given pressure increases. The stock ABS system is calibrated to modulate torque by pulsing hydraulic pressure to the calipers. With larger rotors and/or caliper pistons, each pulse releases and reapplies more torque than stock, resulting in more violent brake actuation and potentially extending braking distances, setting an ABS fault, etc.
Smaller caliper pistons generate less torque at a given pressure. It seems logical that smaller pistons would allow the stock hydraulic system to accommodate larger rotor diameters (though huge rotors with tiny caliper pistons seems odd), and as long as the amplitude of the ABS pressure pulses is sufficient to effectively modulate braking, the brakes should release enough to prevent a skid.
For more information, see James Walker's technical paper on ABS and Big Brake Kits.
Under aggressive braking, weight shifts forward, reducing rear traction to the point where dangerous rear wheel lockup becomes a concern. To allow the rear brakes to contribute normally under light to moderate braking but not too much when weight shifts forward, a combination valve in the stock hydraulic system includes a proportioning function that reduces the rate at which pressure to the rear brakes rises above a threshold. Called a "knee point" or "split point," this pressure threshold is calibrated to a specific forward weight transfer determined by the factory for the stock weight distribution and suspension rates (the Spyder combination valve's pressure threshold is slightly higher than the coupe valve's threshold because the additional rear weight distribution permits the rear brakes to contribute more under slightly heavier braking before enough weight shifts forward to warrant attenuation).
The split point is critical even when stock bias has been maintained, yet it is seemingly overlooked by many who design and purchase big brake upgrades. When larger rotors are installed on a vehicle, the amount of torque generated at a given pressure increases. This increased stopping power causes weight to shift forward at lower pressures and can lead to rear-wheel lockup before the proportioning valve engages. Calipers with larger pistons increase torque at pressures lower still, making the vehicle dangerously unstable under even lighter braking.
For more information, see James Walker's technical paper on Brake Proportioning Valves and Bill Williams' detailed dissection of proportioning valve operation.
For those pursuing an upgrade, I suggest use of better (thickness, venting, etc.) rotors that are close to the stock diameter and better (more rigid, larger pads, etc.) calipers that have close to the stock piston area. I went with a StopTech four-wheel upgrade designed by Supercar-Engineering to use 332mm front rotors and 322mm rear rotors (described on my Modifications page). Unfortunately, the rear calipers selected for my kit necessitated excessive piston areas at all four corners to maintain reasonable front-rear bias, and the resulting hydraulic imbalance initially caused some of the problems described above. StopTech and SCE reviewed the design and provided me with adjustable proportioning valves to install between the ABS HU and the stock combination valve as an effective correction. This has been a valuable learning experience, and all SCE StopTech kits have been redesigned to preserve hydraulic system compatibility. I have no qualms about recommending SCE as the best source of balanced brake upgrades for our platform.
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Jim Matthews - Yorkshire, England, UK
*** Team3S, 3SI #0030, GTOUK #155 ***
http://www.the-matthews.com/stealth.html
Jet Black '94 Dodge Stealth R/T Twin-Turbo AWD AWS 6-spd
Active Suspension Damping, Active Exhaust Valving
Ek2MFG dual-flex downpipe, Blitz SS Blow-Off Valve
K&N FIPK, A'PEXi Super AVC-R Mk-I, A'PEXi Turbo Timer
MirageCorp hybrid data logger, Magnecore plug wires
Optima Red Top 830 Battery, Redline synth fluids
SCE StopTech 4-corner brake upgrade w/ adj prop valves
Street: stock alloys, Yokohama AVS ES100 245/45ZR17
Track: Enkei RPM-02, Yokohama A-032R Hcomp 275/40R17
G-Tech Pro: 0-60 4.79 sec, 1/4 13.16 sec @ 113.9 mph
Top Speed: 171 mph, Dyno: 367 SAE HP, 354 lb-ft torque