Brake Upgrade Options for the AWD 3000GT/Stealth or "If it won't brake, fix it"
by Jeff Lucius
Topics
Introduction
Stock Brakes
Braking Torque
Brake Fluid
Brake Lines
Brake Pads
Brake Bedding
Rotors and Calipers
Synopsis
Acknowledgments
Introduction
Before you go out and spend possibly $2000 to $3000 or more for the "ultimate" stopping power for our cars, consider first that the stock brakes on all years of Mitsubishi 3000GT VR4 and Dodge Stealth R/T Twin Turbo are some of the best on production cars for the time period (1991-1999). The stock brakes are entirely adequate for street use but can benefit from better fluid, stainless-steel braid-protected brake lines, and Metal-Matrix or carbon-blend brake pads. For track or other performance use, an investment in a rotor and caliper upgrade should be seriously considered.
In the table below are some performance results for stock brakes on new production 3000GT/Stealth (3S) cars from popular car magazines. Braking distances from 60 to 0 mph approaching 110 feet are considered outstanding for street cars. For comparison, Road & Track presents these selected 60-0 mph stopping distances in their Dec 2000 and Dec 2004 issues (with 10' significant distances): Porsche 911 GT1, 98'; Lotus Elise, 105'; Mitsubishi Lancer Evolution, 108'; Ferrari Enzo, 109'; Ferrari 360 Modena, 110'; Mazda RX-8, 110'; Volvo S40 T5, 110'; Subaru Impreza WRX STi, 111'; Lexus IS 300, 113'; Mercedes-Bens SL500, 113'; Dodge Viper SRT-10, 114'; Audi TT Coupe, 116'; Chevrolet Corvette, 116'; Acura NSX, 117'; Ford GT, 117'; Volkswagon Toureg V8, 117'; Porsche 911 Turbo, 119'; Nissan Skyline GTR, 120'; BMW M Roadster, 121'; Panoz Esperante, 122'; Saleen S7, 125'; Lamborghini Diablo VT, 126'; Ameritech McLaren F1, 127'; Chevrolet Camaro SS, 129'.
3S Stock Braking Performance
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Magazine |
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Issue |
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Car |
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60-0 mph |
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80-0 mph |
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100-0 mph |
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Sports Car Int'l |
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9/90 |
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'91 TT |
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120 ft |
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214 ft |
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Road & Track |
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9/90 |
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'91 VR4 |
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135 ft |
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237 ft |
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Motor Trend |
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3/91 |
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'91 VR4 |
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117 ft |
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Sports Car Int'l |
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12/93 |
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'94 VR4 |
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123 ft |
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223 ft |
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Sports Car Int'l |
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8/95 |
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'95 VR4 |
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123 ft |
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Road & Track |
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8/96 |
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'97 VR4 |
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347 ft in 4.5 s |
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Sports Car Int'l |
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10/96 |
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'95 Spyder |
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134 ft |
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Motor Trend |
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5/97 |
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'97 VR4 |
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116 ft |
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Road & Track |
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1999 spec. |
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'99 VR4 |
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122 ft |
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218 ft |
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The Road & Track 9/90 issue also tested for brake fade on their '91 VR4. Cool brakes required 15 lb of pedal effort to execute a 0.5 g stop. After six of these 0.5 g stops in a row, pedal effort remained at 15 lb; that is pretty good for street brakes.
The Road & Track 8/96 issue published the results of their 0-100-0 mph competition. The stock '97 VR4 did not do so well overall except that it had the best 100-0 mph stopping time of 4.5 seconds (347 ft). No car in the competition decelerated faster. The four-wheel ABS system contributes to this success. Other cars did stop in a shorter distance in this competition: HKS-modified Supra in 326 ft (5.1 s), BMW M3 in 334 ft (4.5 s), and Mercedes-Benz C36 in 340 ft (4.9 s). The Porsche 911 Turbo stopped in 363 ft (4.6 s). Interesting that the Supra had to be modified to be in this competition. I wonder how well a modified VR4 would have done?
The objectives of braking system improvements are to reduce stopping distances (especially from high speeds), tune the feel of the brakes (subject to the driver's preferences), and improve thermal control (heat dissipation and temperature sensitivity). Though the tire-to-road interface always limits rapid deceleration, I don't discuss tires here. Quite often a switch to stickier tires may be the easiest way to improve your stopping distances. However, if repeated application of the brakes causes fade in the application that you are using your car, whether it is the street or the track, then thermal control must be a priority. Some of the information presented here is applicable to any automobile. However, I have concentrated on improvements for the AWD 3000GT/Stealth braking systems.
The following steps (in order) should be considered when improving your braking system.
1.Improved brake cooling.
2.High-performance brake fluid.
3.Steel braided brake lines.
4.High-performance brake pads.
5.Larger rotors and calipers.
6.Master cylinders.
There are no commercial brake-cooling kits for our cars that I know about. Improved brake cooling for 3S cars generally takes the form of homemade combinations of venting, heating, and roof-drain parts that direct cool air from the front of the car onto the rotors and calipers. Also, I am not aware of any aftermarket master cylinders that are available. The master cylinder converts the force generated by the brake pedal assembly into hydraulic fluid pressure. The stock master cylinder seems to provide a good balance between fluid capacity (sufficient for our ABS system) and hydraulic pressure (1.133 times pedal force). So I will concentrate on items 2 through 5 above. But first, I'll summarize the stock setup and present the concept of braking torque.
Stock Brakes
The front brakes for all years of the AWD models use ventilated, iron-disc rotors with 4-piston, rigid, iron calipers. The outside dimensions of the calipers changed in 1994 because of the larger rotors but the internal parts (such as the pistons) and the brake pads did not change. The rear brakes for AWD models up to 1993 use ventilated, iron-disc rotors with single-piston, floating, iron calipers. Starting with the 1994 model year, the AWD cars have rear brakes with twin-piston, rigid calipers. The drum-type parking/emergency brakes are incorporated into the hat of the rear rotors. The master cylinder is the
2
same for all model years, with a 26.9 mm (1.06") inside diameter, as is the vacuum-type brake booster, with an approximate 7:1 boosting ratio of the brake pedal force. Mitsubishi recommends DOT3 or DOT4 brake fluid for all years. Though not directly related to the brakes, the following wheel information may be relevant when upgrading. All wheels (all sizes, all years) on all models (FWD or AWD) have a 46 mm (1.81") offset with 5 lugs in a 114.3 mm (4.5") pitch circle diameter. The relevant "performance" dimensions of the stock brake systems are summarized in the table below. Note that rotor sizes are the actual outside diameters. The service manual specifications are "effective diameters", which is likely near the middle of the brake pad.
Stock Dimensions
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Front |
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Rear |
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Rotor |
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AWD year |
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Rotor |
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Rotor |
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Cylinder |
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Rotor |
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Cylinder |
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O.D. mm (in) |
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Thickness mm (in) |
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ID mm |
O.D. mm (in) |
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Thickness mm (in) |
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ID mm |
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1991-1993 |
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295 (11.62) |
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30 (1.18) |
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40.4 x 2 |
284 (11.19) |
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20 (0.79) |
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38.1 x 1 |
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+ 42.8 x 2 |
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1994+ |
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314 (12.36) |
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30 (1.18) |
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40.4 x 2 |
295 (11.62) |
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20 (0.79) |
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38.1 x 2 |
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+ 42.8 x 2 |
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The 3000GT VR4 and Stealth TT utilize a 4-wheel, 2-channel ABS braking system, as shown in the illustration below. The master cylinder contains two pistons which effectively divide the brake lines into two separate hydraulic systems. One system is the left-front and right-rear brakes, and the other is the right-front, left-rear brakes. This is done for safety in case a leak develops in one of these systems. These two hydraulic lines go from the master cylinder to the ABS hydraulic unit (HU). The HU controls the four output brake lines as two channels. One channel for each side of the car. However, because of the use of select-low valves between the two rear wheels and between two wheels on one channel (same side of car), a total of 3 wheels are controlled hydraulically at the same time. In order to prevent lock-up of the rear brakes during heavy braking, a proportioning valve is used on the two brake lines going from the HU to the rear brakes. For the hatchback AWD models, the output pressure will be the same as the input pressure until there is 533 to 604 psi pressure in the rear brake lines (called the split point). At pressures greater than this, the output pressure to the rear brakes will drop 34 to 42 percent. For the convertible AWD models, the split point is higher at 704 to 775 psi and the pressure drop is lower, 25 to 34 percent. The preceding is for North American and European models. The proportioning valve for vehicles bound for Australia was yet a third model. It had a split point in the range of 391 to 462 psi and a pressure drop of about 25 percent.
3
Braking Torque
Torque is a force acting to rotate a body about an axis. Torque can be directed to increase or decrease rotation. Torque is the magnitude of the force times the perpendicular distance between the line of action of the force and the axis. Braking torque (TB)B then is the force applied by the caliper to the rotor (FC) times the effective radial distance the brake pads are from the center of the wheel (RE). The force applied by the caliper is determined from the pressure in the hydraulic lines (PH) times the total piston area in one half of the caliper (AP) times the brake pad coefficient of friction against the rotor (µ) times 2 (there are pads on both sides of the rotors). The complete expression is below.
TBB = FC x RE = [PH x AP x µ x 2] x RE
The pressure in the hydraulic lines (PH) is determined from the pressure exerted by the driver on the brake pedal, the length of the brake pedal "arm", the brake booster, the size of the brake master cylinder, and the condition of the brake fluid.
Please note that ultimately it is the friction of the tire acting against the road surface that actually stops the car. If there is little friction, such as "balled" tires on ice, the car will not stop well regardless of brake torque. On the other hand, if tire-road friction is good a car can come to a stop (eventually) with no application of the brakes (let's imagine the drivetrain and its "drag" have been removed, as well as aerodynamic drag).
The upgrades and maintenance discussed here are designed to optimize braking torque. Keeping the brake fluid free of water and from excessive heating maximizes transfer of pressure from the master cylinder to the calipers. Stainless-steel braid-protected brake lines reduce pressure transfer time from the hard lines through the soft lines to the calipers. Good brake pads provide the optimum friction coefficient, which is heat dependent, as well as assist in heat dissipation/absorption management. Rotors can increase brake torque if they have a larger effective diameter, and can assist in heat dissipation. Finally, better calipers provide optimal management of heat transfer to the brake fluid, and of force multiplication using piston area, reduced pad vibration, reduced mounting flex, and optimized offset loads.
4
Before proceeding, let's look at how the braking torque is biased in the factory system. For all AWD models, both front brakes have a total of 5441 mm2 of piston area and both rear brakes have a total of 2280 mm2 of piston area. When pressure is the same in all hydraulic lines, that is, when the ABS is not activated and pressures are below the split point in the proportioning valve, there is a front bias of 5441/2280 or 2.38:1, or equivalently, the rear brakes pads get about 42% less pressure than the front pads. The difference in effective diameter of the rotors further increases the front bias to an overall bias F/R of 2.5:1 for 1991-1993 AWD models and 2.58:1 for 1994 and later AWD models. When braking hard enough to exceed the split point pressure in the proportioning valve, overall front bias will increase to about 3.85:1 for hatchback models and to about 3.38:1 for convertible models. For track use, there may be some advantage in swicthing to the convertible's proportioning valve (higher split point), or if a person could find one, the proportioning valve used in vehicles exported to Australia (lower split point). For example, if the front brake bias has been increased through the use of larger rotors and same-as- stock or greater piston area, then an increase of rear bias (about 12% or so) to balance this could be achieved under hard braking by using the convertible's or Australian proportioning valve on the hatchback.
Brake Fluid
The Haynes Automotive Reference Manual defines brake fluid as "A compounded liquid for use in hydraulic brake systems, which must meet exacting conditions (impervious to heat, freezing, thickening, bubbling, etc.)." Well, being impervious to heat is certainly the goal! The car's brakes convert the kinetic energy of the car into thermal energy, or more simply, they heat up as they slow the car down. The heat (energy) generated by the brake pads and rotors heats up the brake fluid in the calipers. If the fluid heats up enough, it will boil creating vapor in the system. Vapor in the hydraulic system (whether it is from brake fluid, water dissolved in the fluid, or air) is bad because it is compressible, reducing braking efficiency, possibly to the point of loosing the brakes entirely. This is called brake fluid fade.
WARNING. Brake fluid is poison. Keep it away from skin and eyes. Do not allow brake fluid to contact painted surfaces.
The DOT brake fluid classifications (49CFR571.116) include a set of minimum specifications that are guidelines for manufacturers as to how impervious their fluid is to heat. The dry boiling point (when the fluid is fresh and contains no water) is the temperature at which the fluid turns to vapor. The wet boiling point (measured when the brake fluid contains 3.7% water) is related to how easily the brake fluid will absorb water (or how hydroscopic the fluid is). The lower the wet boiling point, the more water the fluid will absorb. Absorbing water is bad, so a higher wet boiling temperature means better brake fluid. And, of course, a higher dry boiling point is good too.
The table below summarizes the DOT guidelines plus the relative advantages and disadvantages of each fluid type. The actual performance may exceed the DOT guidelines and should be printed on the container. For example, Pyroil Premium DOT 3 Brake Fluid states a minimum wet boiling point of 291ºF. DOT 3 and DOT 4 are polyglycol-based fluids and can be mixed with each other. DOT 5.1 can also be mixed with DOT 3 and DOT 4, even though it is based on a different chemical and has about half the viscosity. If DOT 5.1 is specified for an ABS system, do not add or use any other fluid type. DOT 5 is silicone-based (not less than 70% by weight of a diorgano polysiloxane) and must not be mixed with or contaminated by DOT 3, DOT 4, or DOT5.1.
5
Standard Brake Fluids
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DOT 3 |
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DOT 4 |
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DOT 5 |
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DOT 5.1 |
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Dry BP ºF (ºC) |
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401 (205) |
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446 (230) |
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500 |
(260) |
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500 (260) |
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Wet BP ºF (ºC) |
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284 (140) |
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311 (155) |
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356 |
(180) |
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356 (180) |
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Kin. Viscosity |
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1500 max @ -40ºF |
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1800 max @ -40ºF |
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900 max @ -40ºF |
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900 max @ -40ºF |
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mm2/second |
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1.5 min @ 212ºF |
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1.5 min @ 212ºF |
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1.5 min @ 212ºF |
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1.5 min @ 212ºF |
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Advantages |
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- Inexpensive |
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- Available at auto parts stores |
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- Does not attack paint |
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- Superior performance |
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- Readily available |
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- Less hydroscopic than DOT3 |
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- Does not absorb water |
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Disadvantages |
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- Attacks paint! |
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- Attacks paint! |
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- DOES NOT mix with DOT3, DOT4, DOT5.1 |
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- Attacks paint! |
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- Most hydroscopic |
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- More expensive than DOT3 |
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- Water may "puddle" in the system |
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- Expensive, hard to find |
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- Careful bleeding required |
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- Slightly compressible near its boiling pt. |
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- Very expensive, hard to find |
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Probably the best thing you can do to maximize the performance of your brake fluid (any type) is to change it regularly (at least once a year) to minimize the amount of water in the system. According to the Car Care Council and SAE field tests, brake fluid can become contaminated by water (2% on average) within 1 to 2 years, and in substantially less time in humid climates. Brake fluid (other than DOT 5) readily absorbs water from the air in the master cylinder. This cannot be prevented and changing (flushing) the brake fluid is the only solution. Alternating the use of different-colored fluids can aid in knowing when the flushing process is complete. The dry boiling point of DOT 4 is higher than the wet boiling point of the best brake fluid (Castrol SRF excepted). Speed Bleeder Products makes a replacement bleeder screw for the brake calipers that allows one-person bleeding. Speed Bleeders cost $7.00 each; you'll need 4 plus one for the clutch release cylinder if desired. For the Stealth and 3000GT turbo models, use Speed Bleeder model SB1010 for front calipers and rear calipers, except use SB7100 for rear calipers on cars with 3/92-5/93 production dates. SB1010 also works for the clutch release cylinder.
While our brake system will perform as designed using standard DOT 4, you may want to consider one of the popular high-performance brake fluids, especially for racing situations. Some of these are Ate's (Alfred Teves Engineering's) Super Blue Racing and "Super Gold" (same fluid but different color; also called Ate Typ 200), Motul's 5.1 and Racing 600, AP Racing's 550 and Ultra 5.1, and NEO's "Super DOT".
Raceshopper.com sells Ate Typ 200 (or Super Blue Racing) for $9.95/liter, and other brake fluids at some of the best prices you will find. Motul Racing 600 can be purchased at Porterfield for $18/liter. Other retailers sell AP Ultra 5.1 for about $20/liter and AP 550 for about $28/liter. Brake Man sells their racing brake fluid for about $25/liter. NEO "Super DOT" 610 sells for about $12 per 12-oz can ($34/liter) and has the highest wet and dry boiling points. Castrol SRF has very high boiling points as well, but at about $65/liter (raceshopper.com) seems to me to be a bit expensive; but not nearly as expensive as GS610 which sells for ~$84/liter! Ford Heavy Duty brake fluid is cheap ($6/liter) and has a very-high dry boiling point, making it a favorite for the weekend racer that changes brake fluid frequently. However, the DOT 3-rated wet boiling point makes Ford Heavy Duty less than desirable for street usage. Do not mix any of these fluids, some of which are DOT 4 and others are DOT 5.1, with DOT 5 siliconebased systems.
6
High-Performance Brake Fluids
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Motul |
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AP |
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Ate |
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Ford |
AP |
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Brake |
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Earl's |
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NEO |
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Motul |
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AP |
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Castrol |
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Motul |
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NEO |
GS610 |
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Ultra |
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Ultra |
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Typ |
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Hypertemp |
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Racing |
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Super |
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RBF |
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Super |
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HD |
550 |
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Man |
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SRF |
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5.1 |
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5.1 |
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200 |
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421 |
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600 |
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600 |
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600 |
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DOT 610 |
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Dry BP ºF |
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509 |
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518 |
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563 |
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577 |
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585 |
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585 |
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585 |
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590 |
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590 |
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593 |
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610 |
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536 |
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550 |
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610 |
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(ºC) |
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(265) |
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(270) |
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(280) |
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(288) |
(295) |
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(303) |
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(307) |
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(307) |
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(307) |
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(310) |
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(310) |
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(311) |
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(321) |
(321) |
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Wet BP ºF |
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365 |
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375 |
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293 |
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300 |
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421 |
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421 |
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421 |
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410 |
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518 |
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420 |
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421 |
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392 |
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290 |
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421 |
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(ºC) |
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(185) |
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(191) |
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(200) |
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(143) |
(145) |
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(149) |
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(216) |
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(216) |
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(216) |
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(210) |
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(270) |
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(216) |
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(216) |
(216) |
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Price |
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~$14- |
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$12-14 |
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$10-15 |
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~$6 |
~$28 |
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$25 |
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~$30 |
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$34 |
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~$18-21 |
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~$36-40 |
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$65-75 |
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~$30 |
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~$34 |
~84 |
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per liter |
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20 |
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Ate Super Blue or Typ 200
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Store |
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Price/liter |
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raceshopper.com |
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$9.95 |
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Strictly German |
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$9.95 |
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DaliRacing |
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$10.00 |
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SP Motorsports |
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$10.85 |
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Spec Miata Superstore |
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$10.95 |
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Cobalt Friction Technologies |
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$11.99 |
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Mach V Motorsports |
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$14.00 |
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Ate Super Blue and Type 200 - the best deal in performance brake fluid for the street at $9.95/liter.
Brake Lines
The brake fluid is contained in rigid steel tubing from the master cylinder to about 1 foot from the calipers. To allow for movement in the suspension, a flexible rubber-coated nylon hose connects the steel tubing to the calipers. These eight short hoses (two per wheel for the 3000GT/Stealth) can expand a small amount as pressure increases in the system. The expansion of these hoses increases the hydraulic volume of the brake system and causes a small delay in actuating the calipers. As the hoses age, their expandability can increase. To reduce this delay and improve "pedal feel" (increased firmness and reduced pressure), the rubber hoses can be replaced with nylon hoses covered by stainless steel braiding. SMC (using lines and fittings from Goodridge) supplies a steel braided brake line kit (part number 46200-M3000) for our cars ('90-'95). The SMC kit costs from $200 to $250. MVP Motorsports has advertised this kit for as low as $188. Pictures of the SMC lines mounted on my car can be seen at 2- brakelines.htm. New stainless-steel braid-protected brake lines are supplied with many of the combination rotor and caliper upgrade kits mentioned later in the web page.
Brake Pads
As mentioned above, the brakes convert the cars kinetic energy (its forward or reverse movement) into thermal energy (heat) in order to assist in stopping the car. Notice I said assist; this is because it is the friction of the tire acting against the road surface that actually stops the car! The brake pads are pressed against the rotor by the pistons in the calipers. The amount of force actually exerted on the rotor is
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