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I'm running FD front rotors, FC 4-pot calipers, a WRX master cylinder, FC booster, Nissan Laurel C35 rear disks and pads, and an Edelbrock brake proportioning valve. Without the valve the rears locked up long before the front. You can run any type of brakes in the rear you like provided they don't lock up before the front, and it's very difficult to ensure that is the case without an adjustable brake bias valve.
Rear caliper pistons through all 3 generations have a 34.9mm/1.37" bore, so you're fine with whatever rear you use.
^what Jon said above applies. It's when you change the front calipers from the single piston to 4 piston that the proportioning valve needs to be changed. Grab it off the FC the subframe came from if you can.
OE Brakes that share a common MC bore are paired together to provide braking that is biased to the front of the car. This is also helped by the prop valve that is plumbed into most OE braking systems.
To answer your question, compare the piston diameters of the calipers that you have on the car to the FC rear caliper pistons that would be normally paired to the FC front calipers you are going to use. If the diameters are the same, then there are no concern because both calipers will have the same clamping force for the same line pressure. The basic rule is that a larger caliper piston will generate a higher clamping force than a smaller one.
Assuming that the "5-lug" front is using the T2 rotors, I do not recommend staying with the stock GSL rear rotors/calipers. You will end up losing braking because the fronts will do it all. Fronts will lock first. To make up the difference you would have to increase the bias to the rear. The attached pics show OEM rotor, caliper, and pad dimensions (pretty sure they all came off of RockAuto). I also have a spreadsheet showing the rotor torque. The bottom row shows the torque relative to GSL. The main assumption is that the pressure to the front and rear calipers is the same. That is not exactly correct because there is a proportioning valve in the stock system. For comparing the different rotor/caliper/pad combinations to each other, the assumption works fine. For instance, you can see that if you put T2 brakes on the front you will have about 20% more rotor torque at the front than with the GSL system. If you don't swap out the rears, you will end up with much more front braking than the stock setup.
I should have noted that I have experience with T2 front brakes with stock GSL rears. Not good. Actually really bad for racing. A bias valve might have helped, but didn't get that far.
^Thanks @Carl I figured you'd have the exact answer in detail
I just checked and I have the S5 NA front brakes with the SINGLE piston calipers and 5 lub hubs. The single piston calipers seem to be OK to use based on the above.
I think I still want the 4 lug hubs and rotors though to facilitate wheel fitment...
The car that originally got changed to T@ fronts, later got changed to 2nd gen 4 bolt, because of wheel fitment. The rear was modified to 2nd gen 4 bolt caliper and rotor. As you can see from the spreadsheet above, those fronts and rears match up well compared to GSL. To get the 2nd gen 4-bolt rear rotors redrilled, I just clamped the rotors face to face (centered of course) and used the 1st gen rotor as the drill jig. I have the design drawings for the bracket if you want. It is very easy to make. I made my first one out of 1/4" plastic sheet to mock it up.
I am emailing you a copy of a James Walker article on braking systems. It is very simple and clear about the engineering/physics of the braking system. Basically the only reason for us to upgrade to bigger brakes is to manage heat in the rotors and pads. I am able to manage the heat with good brake cooling ducts on the stock GSL system.
Thanks for the information Carl. I am looking at changing my front brakes from T2's to Willwood calipers and a bigger hat/rotor (11" to 11.75") to save weight. So you provided me with all the numbers I was going to have to look up!!
When you experimented with T2 calipers and GSL rear brakes did you maintain the OE prop valve? I am wondering if the removal of the OE prop valve and the installation of an adjustable valve for the rear brakes might have helped balance the car's braking performance. In the past on my Fox body autox car I gutted the OE prop valve and was immediately impressed with how much braking performance I got from the rear drum brakes once they was seeing the same hyd pressure the fronts were. Seemed to help with front lock up too. Of course this sort of thing really only works on a car with really stiff front springs and shocks/struts that can be adjusted to control weight transfer. NOT A STREET CAR MOD.
gtracer7-rx7 are the single piston 5 lug rotors 11" in diameter? Drilling rotors as Carl described is pretty easy. When I did mine, I bought a 4 lug FC rotor because it has the same hub bore as the 5 lug. Once you have one rotor drilled, it becomes your pattern for all of the rest.
We did not try removing the OE proportioning valve. That might have helped. I don't know how much the pressure is reduced by the OE prop valve, but I guess it could be measured. With that info, you could estimate whether or not gutting the prop valve would be enough. The T2 front/ GSL rear was really bad. Major loss in braking (and lots of flat spotted front tires).
gtracer7-rx7 are the single piston 5 lug rotors 11" in diameter? Drilling rotors as Carl described is pretty easy. When I did mine, I bought a 4 lug FC rotor because it has the same hub bore as the 5 lug. Once you have one rotor drilled, it becomes your pattern for all of the rest.
I'll measure over the weekend. Dropped the FB subframe and took both the FC and FB front ends apart to start measuring and mocking up.
Indeed it is. I measured and it is just under 11".
So I guess if I want to swap my S5 single piston caliper setup to 4 lug single piston, I'll need the hub, calipers and brackets (maybe just brackets...), and smaller rotors from the 4 lug.
Originally I ran the FC calipers on the front and stock rears... it was a clusterfuck to put it mildly. even a proportioning valve did not fix it entirely.
I then swapped to running FC calipers and rotors on the back with 314mm R34 rotors and 300ZX 4 pot calipers on the front. with the proportioning valve I was able to balance things out. that and i run different pads front to rear as well. higher temps on the back as they are not quite up to the ability of the fronts in that regards.
^^you should specify whether your experience was with the 4 piston calipers or the single piston, base model calipers of the 4 lug.
I'm going to assume that you are talking about the 4 piston calipers since that is consistent with Carl's info
10-11-21, 11:40 PM
Ok to run stock FB Rear Calipers with FC front calipers?
