Brake Disc sizing and balancing
 

I’m hoping for some help with a brake setup, I am in the process of upgrading the braking system on my RX4 while retaining the stock diff and 4x110 PCD.

I am using S4 FC discs up front, with new steel hubs and Wilwood callipers. The reason for the wilwoods is not so much performance, more so that I can use my wheels and the FC discs combined.

For the rear of the car, I was thinking to use S2 RX7 discs (GSL) as these will retain the 4x110 PCD. I have some GSL rebuilt callipers and all the brackets/cables etc and I am hoping these will work without to many modifications.

What I am not too sure about is the balancing of the brakes, what is the best way to do this, use the stock GSL proportioning valve?

And is there a ‘rule’ as to the difference of size between front and rear disc size?

Are the GSL rear caliper pistons similar or the same size as an FC? Are the rotors? If so you could try an FC prop valve.

If not just install an adjustable prop valve on the rear brake circuit, and you can dial in the brake bias where you want it.

Looking at the whole setup, I think the best way forward will be to just use an aftermarket proportioning valve, the Wilwwod units are around $80 so not such a large investment and like you said, I can set the car up how I like.

There is a size difference between the FC and GSL disc size, hopefully this will not be an issue.

You'll be able to account for the rotor size difference with the prop valve. I run larger rotors in front, and run a wilwood valve on the rear circuit as well. It works great. I will be putting larger rear rotors on in the near future.