Additionally, the 05+ sti has a different bolt pattern (5x114.3 vs 5x100) so those rotors are not compatible with other models.
If your car has rear drum brakes, a disc swap is fairly more involved. I'm not entirely sure on the process but there are more details farther down.
I would also be wary about trying to exchange stuff with the 08 Imprezas. The rear backing plate could be completely different.
So, if you're wondering what brakes you have and what other brakes are out there, I've made this big list of most of the different brakes found on modern Subarus. The information came from a lot of places like brake catalogs (click the rotor size for spec sheets provided by DBASteve), FSMs, cars101, and various threads/FAQs on nabisco and scoobymods. This is mostly just about US model cars. Generally brakes were the same around the world but there are a few little special exceptions that are linked to later on.
Disclaimer: While I have tried to make this post as accurate as possible, there are some mid year revisions. I also can't be positive all the year ranges listed are correct, especially for pad shapes. In some cases, it is necessary to call a dealership with your VIN number or physically compare parts.
Smallest Subaru front brakes, found on FWD non-abs Imprezas (93-96). The caliper and bracket are similar to those for early 260mm brakes with a shorter bracket and they use the same pads. These things fit under 13" wheels.
93-96 Impreza 2WD.
This is the most common front rotor size before 2000. It was found on many Imprezas and Legacies and uses a single piston caliper and bracket. Somewhere between 1996 and 2000 the caliper design changed depending on model so while the rotors are all the same, calipers don't interchange between brackets. These fit under 14" wheels.
90-99 Legacy non-turbo/-GT/-Outback
93-01 Impreza AWD (non RS)
This uses a two piston sliding front caliper and there are three (or more) caliper/bracket designs. One for the 91-94 Legacy SS/TW, an early Legacy 2.5GT/Impreza RS bracket (~96-02), and a Later RS/GT bracket (02+). Post ~98ish calipers are interchangeable between brackets, and will also exchange onto the taller wrx brackets (as long as they are both of the same style). I have also heard that you can even put the later 1-pot calipers on these brackets. They fit under 15" wheels.
91-94 Legacy turbo
96-02 Legacy GT/Outback
00-04 Legacy non-GT
98-07 Impreza RS/TS/2.5i
This is the WRX front rotor. It uses a two piston sliding front caliper or the Subaru fixed 4-pot. There are two sliding caliper bracket designs: One is the same as the 98-01RS, and the other started use sometime in 02/03 (varies depending on model). The 4-pots are only on the 06-07 WRX in the US, though they were on many older non-US STis and WRXes (the black calipers with Subaru across them). Generally 16" wheels are required with these brakes but there are some 15" wheels out there that work. Also 16x6.5 Subaru wheels do not clear 4-pots.
01 Legacy GT LTD
02-04 Legacy GT
02+ Impreza WRX (including 08, which uses 2-pots)
There are three WRX DBA rotors, here is a description from Steve at DBA[
650 was the original light weight WRX replacement rotor
4000 - Was developed for production car racing with the 4 pot WRX caliper. (before the USA WRX was released). The 1999 to 2001 WRX had a sintered ABS tone ring mounted on the hub so the mounting bell profile could not be altered to suit the USA caliper. This one is for the rest of the world.
4650 - Is a modified 4000 to suit the 2 pot USA WRX caliper. Only the mounting bell shape is different to clear the support bracket on the USA caliper. Still don't know why Subaru spec'd the two pot caliper.....
What that means is the 4000 rotor will not work with the 02-05 brakes, and maybe not on any car with the abs sensor in the knuckle. It would be best to get the 650 or 4650. The 650 is 24mm thick (same as OEM), and the 4650 is 25.4mm thick (mmm.. beefy. Might have to leave out the shims at first).
Added bonus points: This rotor is also available in 5x114.3 to make it possible to swap an 05-07 STi to the subaru 4-pots for rally. Not sure where to find that rotor but I would start with rally shops.
This uses a two piston sliding caliper similar to the other 2-pots, although brackets and calipers aren't compatible between other 2-pots. 17s are required with them.
05+ Legacy GT
Tribeca (5x114.3 bolt pattern)
STi rotor that goes with the 4-piston Brembo calipers. There are two rotors, the 04 has a 5x100mm hub bolt pattern, 05+ uses 5x114.3, although caliper mounting remains the same so you could use 05+ calipers on a 5x100 car with 04 rotors. There were some changes to the caliper over the years that involved some sort of stiffening, so from what I can tell an 07 caliper would be slightly better than an 04.
It seems the 08 STi still uses these brakes but the calipers are black with an STi logo.
A note about the SVX:
The SVX has brakes that are about the same size as WRX brakes and the SVX has 5x114.3 hubs. However, up front, the rotor hat offset (how close or far the rotor surface is from the hub) is different from that on other Subarus. The knuckle also locates the caliper bracket in a different position. That unfortunately means that you can't just buy some 05-07 STi brembos or Tribeca brakes or something to upgrade your SVX. I'm pretty sure that in the rear things are compatible with other Subarus.
The most common Subaru rear rotor. It uses a single piston sliding caliper, of which there are a few variations. Early Legacy sedans (non-turbo) and Imprezas have a smaller piston than Legacy wagons and turbos, and there are a few bracket/caliper revisions.
90-99 Legacies/Outbacks with rear discs non-turbo
93-03 Imprezas with rear discs
04+ non-Sti Imprezas
06+ non WRX or STi imprezas
Foresters with rear discs
I think this is on non-turbo 05+ Legacies, and 05+ Outbacks as well.
This is well known as the H6 rear rotor. The caliper is the same as the ~99-04 calipers so many people buy these rotors and the right brackets for a cheap upgrade.
00-04 Legacy/Outback non-brighton
Bonus points: SVX bracket + this rotor in 5x100 = old style caliper H6 upgrade (probably).
This is the Legacy turbo rear rotor. The caliper is similar to the other calipers but is wider to accomodate the vented rotors. I hear it was also found on the v1 WRX (93-94).
91-94 Legacy turbo
290x18mm (170mm parking brake)
290x18mm (190mm parking brake)
This is a new rotor size for Subaru (in the US anyway). It uses either a 2-piston opposed caliper or a 1-pot slider. The 2-pot will not bolt up to other subarus without somthing like the Kartboy brackets or having a new backing plate pressed onto the spindle. There are two versions of this rotor, one is for cars with an R180 rear diff which has bigger parking drums.
05+ Legacy GT (1-pot)
06-07 WRX (2-pot)
SVX (5x114.3) (only in JDM-land)
Some older non-us spec-b Legacys and wrx/sti models.
Note: The LGT uses a different brake line than older Legacies and Imprezas. So if you want LGT rear brakes there could be an issue there. The line mounts to the caliper at a different angle and from what I've heard Impreza lines will leak. On top of that, the LGT line (and all 00+ Legacy/outback rear lines) is some funky multi-part piece with two sections of flex hose and a hard line that attaches to the trailing arm thingy that won't work on an (pre-08) Impreza. I'm not sure what the solution is to that problem but I think there are some people out there with LGT rear brakes on Imprezas.
316x20mm (5x100, 170mm parking brake)
STi brembos. There are now three versions of this disc. One for the 04 STi, one for 05+, and one with a 170mm parking brake drum that will fit on any other Subaru with either a 2-pot backing plate or kartboy brackets.
Hey that's an odd size now isn't it. I was previously wrong with my thinking the Tribeca uses the same rotor as an STi. It does have the same bolt pattern (5x114.3) though. AFAIK, it uses a 170mm parking brake but don't quote me on that.
So, what does that mean? The calipers and brackets should work on an SVX. Unfortunately no front brakes are interchangeable onto an SVX because of the rotor hat back-spacing.
08 WRX rear rotor, which uses a 190mm parking brake and 1-pot sliding calipers. The bigger parking brake makes me wonder if the caliper bracket is not like the older sliders, and if they made the backing plate to accept a 2-pot for simplicity. That would make it much easier to swap rear brembos but I'm really stretching things. Someone's going to have to take it apart and try.
Some cars have rear drums that are like 9." I don't really know anything about Subaru drum brakes so it would help if someone filled that stuff in.
Some of them are: Legacy and Impreza Brighton models, Some 95-99 Legacy L models, some foresters and I think 02+ Impreza TS and OBS. (I'm not sure exactly)
To swap out drums, at the least you will need new parking brakes, backing plates, discs, calipers, and brake lines. It is usually best to get a whole knuckle/hub/parking brake assembly out of a car with rear discs. If you want to put rear discs on your front wheel drive Subaru, you might have to parts from a 90-94 Legacy, since they might be the only FWD Subarus with rear discs and the FWD rear hubs/spindles/struts/etc are different.
Installing rear Subaru 2-pots or Brembos on a non 2-pot car:
The Subaru and Brembos have the same backing plate, which is different from all the other Subaru rear disc backing plates. So calipers don't swap between the two. Additionally, the STi uses a 190mm parking brake drum. Pretty much everything else uses a smaller 170mm drum. That means if you take a set of rear brembos and put them on your WRX, the parking brake will not work.
However, there are Subaru 2-pot rotors (290x18mm) with a 190mm parking brake hat and Brembo rotors (316x20) w/ a 170mm e-brake so you can put either setup on either car. There are also Kartboy brackets that will adapt the 2-pot calipers to any other Subaru, and since both rotors are available in 170mm parking brake variety, you can swap either one onto any other Subaru. Here's a thread about fitting the brembos to other cars with the new Kartboy brackets and DBA rotors.
Here is the rear brake redux, which has more information about the older rear brakes that came on STis and fancy Legacys in Japan.
Some D-plate numbers
The Friction Materials Standards Institute (FMSI) is an association that assigns numbers to different pad shapes that are used as a guideline for pad manufacturers to refer to, and bring a common numbering system to the brake pad aftermarket. Virtually every pad manufacturer will either use the FMSI number in their numbering system or have a readily available interchange to cross-reference their part number to the FMSI number.
I should note that the caliper bracket, and not the caliper, is what determines pad shape. So while there are a few different pads, in some cases the calipers are the same.
1990-1995 Legacy: D470
1996-1999 Legacy: D722
1993-1996 Impreza: D470
1997-2001 Impreza: D722
1991-1994 Legacy Turbo: DB1219?/ D563
1996-2002 Legacy: D721
2002-2006 Legacy: D929
1998-2001 Impreza RS: D721
2003-2007 Impreza: D 929
1/2002-7/2002 WRX: D 721
8/2002-2005 WRX:: D 929
See this article for clarification
1990-1999 Legacy: D1186
2000-2004 Legacy: D1379 or D770 (I think they're the same)
2005-2008 Legacy 2.5i/outback:
1993-1998 Impreza: D1186
1999-01 Impreza: D1379/ D770
2002 Impreza w/ rear discs: D1379/ D770
1/2002-10/2002 WRX: D1379/ D770
11/2002-2005 WRX: D1004
2003-2007 Impreza: D1004
2006-2007 WRX (and previous Black 4/2-pots)
Front: D 1001
Rear: D 961
I should probably put all the car/rotor/d-plate combinations in a spreadsheet or something. Outback should be the same as Legacy. I'll look into Forester and Baja stuff later I guess. There are a couple of things that I'm not clear on, like exatly when caliper/bracket designs changed between cars or which 00-04 Legacies got the 294mm front rotors. I might also be wrong about some of the forester front brakes. Also I've been meaning to add brembo or oem part numbers for the rotors or something like that.
Coming soon: Pad shape clarification courtesy of an EBC spreadsheet I was recently sent.