There are a few reasons to change/upgrade the brakes, some being better than others. Some of them include:
- Heat capacity
- brake feel
- brake bias
- bling bling
- pad availability
Heat capacity is the ability of the brakes to store and radiate heat. A brake rotor is really just a big heat sink, since the purpose of brakes is to convert the kinetic energy of the car to heat. A bigger rotor means more capacity for heat which means they will take more abuse.
With stock brakes, it doesn't take much to overheat the system.
The more you use the brakes, the hotter they'll get, and when a pad reaches temperatures above it's design capacity, it starts to not work very well. The pad compound exceeds it's maximum operating temperature and basically starts to evaporate. This is called outgassing, and the gases form a layer between the pad and the rotor that prevent them from contacting each other, so all of the sudden it's like "hey I can't stop and there's smoke coming out of my wheels." That's the origin of cross-drilled rotors, because back in the day all pads had a tendency to outgas, and they drilled the rotors on race cars so they would still work. With modern pads, working in their design temperatures. it's not a problem so drilled rotors are not really beneficial. There is still of course some stuff coming off the pads, and it can help clear dirt and water, and possibly delay the onset of fade. Still, not particularly worth buying and with heavy use they tend to crack.
More details on drilled rotors
The brake fluid in the system can also overheat. The rotors and pads will heat up the calipers and brake fluid (along with other things in the area). When the fluid gets too hot, it starts to boil. Compressible bubbles in the brake system means "hey I'm pressing down on the pedal and nothings happening."
Brake fade will not occur on the stock system with normal driving, but some hard mountain driving or a track session can overheat the brakes very quickly.
The first step to fix this problem? Better pads and fluid. This lets the brakes get hotter before the pads and fluid overheat, which means you will still be able to stop well, even if your rotors are glowing red. In 02 for example Gary Sheehan raced a WRX with stock brakes and race pads/fluid in the USTCC and did very well.
Gary Sheehan wrote:I raced for a full year on the stock rotors and calipers. We had them set up where they would not fade. Switching to Stoptech's made my life easier because they are more consistent, have a better feel and run at lower temps, thus not ruining everything around the braking system. But they did not drastically decrease my stopping distance. Probably a few feet.
There, straight from a race car driver:
You don't actually need bigger brakes. With the right pads and the right fluid you can race our cars and not run into fade. If you have a lot of power and are seriously tracking the car however, you might have glowing rotors, melt piston and slider pin boots, melt ball joint and tie rod boots, destroy wheel bearings, etc.
Now that I've told you why not to upgrade the brakes, I can talk about why you should. Brake feel is very important because it gives you better feedback in the pedal about what the brakes are doing. A stiffer pedal with less travel produces much better feel and is easier to modulate. That, generally,
will produce more consistent and shorter braking distance. A well bled brake system in good working order with quality pads is going to feel much much better than an old system on cheap pads. However, there are tons of variables when it comes to brake feel, such as caliper design, the suspension, bushings, and alignment, master cylinder, brake booster, and pedal ratio. The master cylinder is very important when it comes to feel. A larger diameter means the piston has to move less to move the same amount of fluid, but requires more force to produce the same pressure.
But you don't really have to worry about that too much. To start, get some good pads, bleed the system, and make sure all calipers slide freely. Better pads produce more grip on the rotor and compress less. That makes the brakes feel better.
When you get into changing calipers and brake boosters and master cylinders, you have to worry about what it will do to feel, pedal travel, and brake bias.
Upgrades:
So, if you're at the point of looking for a different brake setup, there are plenty of OEM options, but only a few combinations that I would recommend. Just putting on bigger front brakes will give you more heat capacity, but will increase braking distances because you've shifted the bias even farther forward.
turbo cars already have pretty good brakes. They're the same size as the Impreza RS but have vented rear brakes. The upgrade I would suggest for these is front Subaru 4-pots. You have bigger rotors, fixed calipers for better feel, and bias remains the same. Rear 2-pots would be a nice match but are not necessary.
For a non-turbo sedan there are few good combos that will both increase brake capacity and maintain good bias:
-Impreza RS/turbo sized brakes all around.
-WRX front and 00-04 Legacy or 05+ LGT rear
-4-pot front with RS, turbo, 00-04 Legacy, 05 LGT, or 2-pot rears.
The 4-pots put out less torque than WRX brakes, so IMO any of the rear brake upgrades will work with them. Just putting on the calipers with bigger pistons out of a wagon would be easiest and cheapest, 00-04 Legacy would actually move the bias back a bit, and 2-pots would be the best match.
If you're upgrading the fronts while leaving the rears alone I'd suggest only doing RS/Turbo sized brakes and maybe installing a wagon prop valve. Wagon proportioning valves have a higher split point so that at higher braking forces they receive more line pressure than sedans (and therefore more rear braking torque).
Wagons have bigger rear pistons than non-turbo sedans, so the RS/WRX etc rears are not an upgrade. Turbo gives vented rotors but will keep the rear brake torque the same. This means that putting on the RS/turbo fronts on an n/a wagon is going to move the bias forward a bit, but probably not enough to be a problem. I definitely recommend bigger rear brakes to go with WRX-sized rotors. 00-04 Legacy, 2-pots, and 05-09 LGT would all work great.
Pedal travel:
One thing to keep in mind while upgrading brakes is the pedal travel. When you increase the piston area, you increase the amount of fluid that needs to move to brake the car. Turbo cars have a bigger master cylinder, and when installing WRX or LGT or 4-pots or whatever, there won't be an increase in travel because the piston area stays the same.
With an upgrade from non-turbo brakes to the 2-pot fronts, you're going to see more pedal travel. I don't have that big of a problem with it but I intend to swap master cylinders. The only requirement I know of is that abs and non-abs MCs are different. For non-abs people, a fwd SVX L MC will work. With abs I think any 1 1/16" mc will work (WRX sedan or STi, for example), but if it's post 1999 you'll need the brake booster that goes with it.