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Discussion Starter #1
My current setup is this:
Front 991S calipers with the Giro 350mm upgrade
Rear 981S OEM calipers with the Giro 325mm upgrade
My rear calipers are shot. Even tried re-sealing and I'm still getting air. The pistons didn't look too good when they had the calipers apart and they told me that the new seals might not work. It was a gamble.

Due to heating issues in the rear I am exploring options for upgrading the rear brakes for heat managment purposes since I have to buy new calipers anyways.
I recently installed the Guard diff. and started driving with TC off. That should help reduce rear braking somewhat, but will it be enough?

Option 1: Put 981S PCCB calipers in the rear (powder coat them red) and install the Giro 350mm rotor for this option
Option 2: Install the 991S rear calipers and the Giro 350mm rotor. I have not been able to find anyone who has done this so I don't know if this will fit. Part numbers are all different so its a trial and error approach.
Option 3: Simply replace the existing calipers with new ones and hope that the diff. and TC off is enough.

If anyone has successfully installed the 991S rear calipers on the 981S would love to hear from you.
 

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I'm surprised to hear that the rear brakes are wearing faster than the front brakes. Do you have torque vectoring and is that a possible cause? The fact that you have installed a guards diff and are turning TC off will certainly help. If your TC has been kicking in often, then sure that can be a contributing factor in the rear brake wear.
I'm also waiting for someone to jump in on your option#2 above.
 

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Discussion Starter #3
Actually, I didn't say the rear brakes are wearing faster, they aren't. It's just that before I changed the diff and was using TC, the rear brakes still got plenty hot, even if they outlasted the fronts. I'm just trying to figure out, since I have to buy new calipers anyways, if it makes sense to upgrade the rear so that everything lasts longer. I agree (hope) that the Guard and non-TC will keep the brakes cooler and maybe the OEM calipers will be just fine. We'll see what we come up with. I called both Giro and Suncoast, and to their knowledge, no one has tried putting the 991S calipers on the rear yet.
And yes, I do have PTV. Matt at Guard believes that with a more aggressive Diff. that PTV won't engage as much. That should help too.
Thx
 

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Wow--I'm surprised.
Sounds like the rear caliper internals got cooked good. Given that here is ducting on the rears I guess the the rear brakes are working a lot.
Might be a bit late for this but I think the heat tape might be a good idea to see how hot things are getting back there. Have you tried
any IR devices on the caliper to see how they might be.

Before you spring for the 991 rear calipers, I think it might be worth it to go with stock replacements just to see if the combo
Guard diff. and TC of has positve impact on caliper temps. Also, how about Ti shims; aren't they supposed to keep heat out of the caliper?
 

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Discussion Starter #5
Good thoughts Mike. I haven't used temp strips but I did use the pyro from time to time but I didn't keep records. At the time I was more concerned about the fronts. That has been solved with the 991S setup. Because of more mass and more importantly, 48 vanes vs 30 on the 340mm, the fronts are nice and cool. I have asked a few folks about the TI shims and they get mixed reviews. I'm not real eager to be the one to experiment with buying the 991s stuff for the rear and then find out it doesn't fit. Unless I hear from someone who has done it, or my dealer is willing to do the experiment on their dime, I will probably do as you suggest and stay with OEM calipers and hope the other changes make the difference.
 

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Whatever you do, keep in mind that you already changed the brake F&R bias with your present caliper setup. I also wonder how much your high altitude location has to do with your situation.
 

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Discussion Starter #7
Also good points amdeutsch. I wish I understood brakes better than I do. I haven't experienced anything negative in terms of bias from my current setup. Perhaps if I had done this months ago, I might never have experienced this issue with the rears. As it was, I was learning, I was probably braking too hard, PTV was engaging a lot, as was TC. I do know that some are switching to PCCB calipers all around which use 350mm rotors. What I have learned from my research that the size of the rotors matters less to the bias than does the total piston area of each caliper. Rotor size (diameter, thickness, and number of vanes) matters more to cooling which is what I"m after.
 

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I am having excellent results with the Rotora Big Brake upgrade and have not had even a second thought since putting them on. I have PTV and both front and rear pads are wearing nicely, no fade, no noise, and I use the same pads for street and track. They are really good.
 

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As far as brake bias is concerned you are exactly correct. "What I have learned from my research that the size of the rotors matters less to the bias than does the total piston area of each caliper."

In order to get a good match on new calipers, you need to keep the surface area of the pistons near their original sizes for front vs. rear. If they get any larger, then the front to rear ratio needs to stay the same. The piston sizes then can't get too much larger otherwise there will not be enough power from the master cylinder to put enough toque on the caliper pistons. The number of pistons doses not matter since 4 or 6 piston calipers just have much smaller individual pistons that add up to a similar area of one large OEM single sliding piston caliper. It is a delicate balancing act that needs to be orchestrated in the final choice of the hardware. You don't want to rely too much on the ABS fixing everything if there are problems.
 

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Discussion Starter #10
I haven't yet measured the total piston area of the 991s front calipers and compared them to the 981S. I am assuming the 991s has considerably more piston area based on the diameters of the repair kits. I have not been able to find sizes for the 981S front pistons, but the rears look like 28mm and 30mm. I think the 991S rears are 32mm and 32mm. But this is all based on repair kits I have found without actually measuring each caliper. I went with the 991s upgrade because they seemed to be quite popular and nobody was reporting issues. I did upgrade the MC to the 991s which has a 2mm larger piston so there is more than enough pressure there. If anyone has done the actual measurements it would be interesting to see that. It is possible that my car has too much front braking bias compared to the original setup but I can't really tell. When I don't have brake fade from my compromised rear calipers, it has awesome stopping ability.
 

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...I'am using the 997 GT3 brake calipers front and rear. The master brake cylinder is from the 991 GT3. Together with 350mm brembo racing discs (front& rear). I strongly suggest to code the psm to pccb to ensure the right bias.

Front
Rear
 

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Discussion Starter #12
Thanks for your input. Did you have to use spacers to make this work?

...I'am using the 997 GT3 brake calipers front and rear. The master brake cylinder is from the 991 GT3. Together with 350mm brembo racing discs (front& rear). I strongly suggest to code the psm to pccb to ensure the right bias.

Front
Rear
 

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You can also make your own titanium shims with this product as some one else suggested. Comes in three thicknesses and can be cut with metal shears. I guess you take the current shim and template it onto the sheet of titanium and cut them out. Maybe cut 4 as close as possible, then clamp them together to finish them off and drill holes if necessary. I bet this would help with the heat buildup that killed your rear brake piston seals. $16 is peanuts for this option if pre made ones do not exist. Here is an option for the grade 5 Ti sheets which is apparently what is needed for this upgrade. One 8x10 sheet should be enough for 4 rear shims. Not sure which is the best thickness to use, but you have .5, .8, and 1.0 mm to chose from.

Screen Shot 2014-12-02 at 7.51.38 PM.jpg
 

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Discussion Starter #15
Thanks everyone for your responses. After careful consideration, lengthy conversations with Mike at Girodisc, and some measurments, I have decided to install the 996T rear calipers along with the 350mm rotor available from Giro.
Just like the 991S brakes solved the front temperature issues, I think this will solve the rear. The 996T calipers have the same size pistons as the OEM calipers so there should be no issue with bias. In retrospect, I wish I had adopted a different approach when I first started doing this early this year. I was pretty aggressive, perhaps too competitive, whatever and as a consequence, I pushed pretty hard. Yes I had good track times for a newbie and the car is awesome, but perhaps dialing things back a little earlier on would have prevented some of this. If you have PTV and drive with TC on, and drive aggressively, you will be hard on brakes. But that's water under the bridge and time to move on. My plan is to keep the car out of the shop as much as possible for next year and just drive. One thing I have noticed is that front pads are lasting longer than they used to.
I should know in a couple of weeks if there is any issue with this change, but I think we are good to go.
 

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Glad you found an option to suite your problem. Stock Porsche calipers to boot. Well done. Let us know how they work out. After all of this you should be able to haul some butt down the straight, dive into the corner at max speed, and drop an anchor on the asphalt at the last second to make the apex with that set up. They may be more of us lining up to this in the future.
 

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Thanks everyone for your responses. After careful consideration, lengthy conversations with Mike at Girodisc, and some measurments, I have decided to install the 996T rear calipers along with the 350mm rotor available from Giro.
Just like the 991S brakes solved the front temperature issues, I think this will solve the rear. The 996T calipers have the same size pistons as the OEM calipers so there should be no issue with bias. In retrospect, I wish I had adopted a different approach when I first started doing this early this year. I was pretty aggressive, perhaps too competitive, whatever and as a consequence, I pushed pretty hard. Yes I had good track times for a newbie and the car is awesome, but perhaps dialing things back a little earlier on would have prevented some of this. If you have PTV and drive with TC on, and drive aggressively, you will be hard on brakes. But that's water under the bridge and time to move on. My plan is to keep the car out of the shop as much as possible for next year and just drive. One thing I have noticed is that front pads are lasting longer than they used to.
I should know in a couple of weeks if there is any issue with this change, but I think we are good to go.
futurz - I'm coming to this thread a bit late, but I was surprised by the experience you had with your rear calipers. I have the same setup, although I'm probably using different pads. I'm guessing you're using Pagid yellows (if you mentioned what you're running, I missed it) and I'm using Carbotech XP-12s. The Carbotechs have more bite, but don't wear as long. I've done 35 track days in my CS, 20 of which have been with this upgraded setup. I haven't had any issues, other than the fact that my front rotors are now shot and need to be replaced. My rears are still well within tolerance. I'm not sure I would mess with your current setup - now that you have more track time under your belt I think you'll be fine. But as long as the new setup fits, I don't think it will do any harm. Good luck.

-Rich
 

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Discussion Starter #18
Rich - Your observations coincide with others and have led me to concede that a lot of this has to do with my driving style early on. Altitude may also have something to do with it as things don't cool off here as much. I have been using Pagid RS29's every since my first street pads wore off after two track days. I replaced the rotors after 9 track days. Since I had to replace the rear calipers anyways, I decided to upgrade the rotors as well for cooling purposes. I'm not planning on racing this car but i know some of the race shops are doing these kinds of mods on brakes. (Now I'm justifying) My tendency, when I start a new venture of any kind is to go all in. I just didn't realize what that would mean when it comes to tracking Porsche's.This year has been a learning experience for sure. We tend to learn more from our mistakes (if we want to) then our successes and that is the attitude I am taking. I'm looking forward to a year of just driving!
Thanks
 

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Discussion Starter #19
Update...
Well, we tried the 996 calipers and the 350mm Giro rotors and the offset was wrong. I will be checking with Giro in the am to make sure we have the correct rotors or if they have another version that would work, but at this point, unless they come up with a different rotor, it looks like I will be going back to OEM calipers and staying with the Giro 325mm rotor. I will definitely be turning off TC as that should help reduce rear brake intervention. Unfortunately, I don't believe there is any way to turn off PTV other than to minimize it by turning off TC, although I'm not sure that turning off TC will actually reduce the amount of PTV intervention. I have not seen that verifed anywhere. It's speculation at this point. If I had known I would be tracking this much and installing the Guard diff anyways, I would not have ordered PTV. Oh well.....20/20.
If something changes again, I will post.
 

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Futurz, was wondering how your brake mods are working. Did Giro come up with a solution for those 350 rears. How are things at the track.

I have been looking at my stock car and the brake ducts. It appears that it would be easy to go to home depot and buy some sheet aluminum. Then cut some customized strips and attach them on the bottom of the plastic ones on our cars. This would extend the ducts another inch or so from the edge of the plastic and grab and or direct some more air to the brake rotor and caliper. The fronts of course are much larger and the rears are smaller hence your previous problem. In looking under the car the rears don't even extend below the floor pan. Just a little extension with the aluminum could drop below this line and grab some extra cool air. The strips can be attached to the plastic with rivets or short stainless bolts, washers, and bolts, with loctite. If I get this done I will post pictures, but very busy now, so this will likely be a summer project. Probably costs $30-40 for all the parts and ones time.
 
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