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By Shifted
#47157
I figured it would be useful for others to know the specific brake rotor sizes. This would be useful if you're looking for non OEM parts (like two piece rotors) or figuring out the upgraded sizes for front and rear.

Porsche makes a lot of different brakes sizes, without a lot of part number cross overs. However, they will often make a rotor in the same size, but one is solid and the other is cross drilled. They won't show the part numbers as fitting other vehicles, but they do. So, if you have a GTS (or have upgraded to big red/black front calipers), you can not only fit the solid GTS front rotors, but you can also fit the cross drilled 964 turbo front rotors.

The hat heights are my calculation to the nearest 5mm since that's what you'll find for most two piece rotor hats. However, if you're having them custom made, you may want to get them made to the nearest mm.

 
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User avatar
By maddog2020
#47536
Brembo's GT Brake kit for the 928 is the Ferrari F50 front brakes and bolt right up with their spacer. Front rotors are 355mm
Hat Height is 45 like the front GTS and the Rotor Thickness is32mm overall height is 78mm

:order:
Last edited by maddog2020 on Tue Jan 05, 2021 1:28 pm, edited 1 time in total.
User avatar
By Shifted
#47539
So, the front brakes end up being slimmer discs than stock? A hat height of 45 and a thickness of 28 only results in about a 73mm overall height, but you said 78mm. Which of the specs is incorrect? Are you sure that the discs aren't 32mm? That would be more normal for front rotors.

What size are the pistons? I assume that it's a 6 piston caliper, three per side?
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By Shifted
#47541
Since you're bringing calipers into this, please feel free to create a table with the various caliper specifications. That would include piston counts and dimensions, which years they fit on, and what modifications are needed to make them fit on other years. I think that would be really helpful for those who are looking to upgrade their calipers.
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By maddog2020
#47588
corrected the thickness info to 32mm. they are 4 piston calipers (think about when the F50 was designed and sold) They will bolt to any 928 that has the 86.5 and later spindles. And you do need to adjust the brake shields a tad bit. You also have to do that on the 993tt rear dust shields.
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Last edited by maddog2020 on Tue Jan 05, 2021 3:38 pm, edited 1 time in total.
User avatar
By Shifted
#47592
That's interesting. Without knowing the piston sizes it's impossible to gauge their impact on the brake system balance and pedal movement. Looks like they are in the $3k to $4k range. And they have similar upgrades for the rear. Very interesting. Wish we had more information on the calipers, though.

Putting together the specs on calipers for all years, including upgrades, would be useful. Just because something can be put in there, doesn't mean it's a good idea. My primary concern would be the effect on front/rear balance. I believe that 15 or 20 years ago when people were upgrading the 928 fronts to big black/red calipers and the rear 993 turbo brakes, people were having issues with the brake balance and had to change the proportioning valve to resolve it.

The risk of affecting braking is a big enough deal that I would want to be sure of the piston sizes before installation.
User avatar
By maddog2020
#47616
I had the piston data years ago and I'm sure I can request it from a brembo dealer. In this instance the piston were not much larger, if I recall correctly but it was the pad surface area and the rotor cooling surface area where these come into play.

Several guys on RL did this upgrade with calipers from a ferrari salvage with excellent results. If I recall correctly the F50 fronts and the 993tt rears were almost identical in the balance and ratios for the GTS front and rear using the GTS brake bias. There was a guy on RL that put F50 calipers and rotors on all 4 corners, and the braking performance was "like the finger of God" when you hit the brakes.

I think when I bought mine the cost was in the $2500 range and I sold my nearly new GTS stuff for pretty good money, so the offset was around 1K.

If you upgrade the rear to the 993tt then to realize the gains you have to change the brake bias. If you upgrade both and the ratios are similar then you can achieve phenomenal better braking. ask some of the guys that changed their brake bias 1st on their GTS and then added the 993tt rear brakes. they ended up with too much rear brake and resulted in trail braking on turns and bumpy corners in the wet.

The goal is to improve the braking without upsetting the balance.

the table above needs to have pad area and rotor area to be relevant. Caliper piston size isn't the whole story. also none of the rotors above are solid. They are all vented.
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