Typically you want to be able to fit 15" wheels when building a rally car. 16" rally tires are very expensive and hard to get. The option that most people choose are the JDM 4-piston front calipers and the JDM 2-piston rear calipers, also known as the 4-pot and 2-pot brakes. These will fit certain rally specific wheels like Methods. Six Method wheels will run you about $1000 and a set of JDM 4-pot and 2-pot brakes will be around $500. We haven't even bought pads and rotors yet! If you are more budget oriented like me, then you probably don't want to be spending money on either of them. My solution is dramatically cheaper and performs just as well - use your factory brakes!

This guide was based on 2002 WRX brakes, but according to NASIOC, it should also work for the following Subarus since they use the same rotor size:

02-04 Legacy GT

05-09 Legacy non-gt

01-13 Outback

02-14 Impreza WRX (including 08, which uses 2-pots)

03+ Forester

Baja

BRZ/FRS/GT86

Things you need

Angle Grinder

Metal Grinding Wheel (~40% life needed)

High Temperature Paint

If you don't have an angle grinder, I recommend buying from a quality tool brand like DeWALT. Here's a really affordable one that won't let you down: DEWALT 4-1/2-Inch 11-Amp Angle Grinder.

They also have great grinding wheels as well: DEWALT Metal Grinding Wheel (5-Pack) .

I recommend a high temperature paint to coat the calipers after you finish. I used a 500°F engine enamel paint because calipers get very hot during racing conditions.

Grinding your calipers

15" wheels clear the factory rear brakes, but 15" wheels do not clear the front brakes. What we are going to do is grind the top of the caliper while it's on the car with an angle grinder. There is a lot of material on top of these calipers, so grinding it down several millimeters isn't going to cause any problems.

Jack the car up and take off the front wheels. Bring out one of your 15" wheels so you can use it for test fitting.

The top front of the caliper is the first part of the caliper that collides with the wheel if you try to put it on. Start grinding there and as the wheel starts fitting better and better, start grinding further back.

You can tell where it rubs by the scratches left on the inside of the wheel. Grind down on the caliper and check every so often on your progress. This will not be quick. I think it took me over an hour to do both, maybe two, mostly because I was being overly cautious on the first one. Once the wheel fits over the caliper, try bolting it on and rotating the wheel. You do not want it to bind on the wheel at all. You actually want to keep grinding until there is about 2mm of clearance as a safety margin to account for heat expansion.

When you are done grinding, go ahead and paint the top of the caliper, it will rust really badly if you don't. You can pull the caliper off and paint it entirely if you really want to make it look good. I already repainted my calipers before doing this modification, so I just coated it again while it was still on the car.

And there you have it! That's how you can fit factory 15" wheels on your WRX without changing your brakes. I am not sure which 15x6" Subaru wheels will not work for this modification, but if the minimum diameter of the inside of the wheel is greater than or equal to 13" then it will probably work because it will not require more grinding.