RyanPerrella

I have wondered about this long enough, time i get a definitive answer. When I bought my car the ad for it stated that the car had GTS brakes on it. I remember measuring the rotor diameter and they seemed like S4 rotors so i paid it no mind.

Ive since always had pad issues and noticed that they never seemed to work correctly. (See Pagid wear pattern below) Last time i did the brakes I bought S4 pads but have always had problems because the spring never would come in contact with the tops of the pad. It seemed the pads were smaller. They are Performance Friction carbon metallic pads and just always assumed they were an inferior pad maker or something. Well I have a new theory now and just realized that the pads are fully seated in the caliper yet i still have this 5mm rust ring around the disc close to the hub. (see pic) It appears to me that i have larger calipers (GTS) and still have S4 rotors. As odd as this sounds i now believe this to be the case unless someone else can put this idea to rest once and for all.

I wouldnt have paid any attention to this unless i had these pad issues of them not fitting tight against the spring clip and didnt have such an obvious rust ring around the disc. I would imagine the GTS discs have a slightly larger hub or something or perhaps they just dont have a machined rotor surface at the point that there is rust on my rotors.

Here are some pics please let me know what those of you familiar with GTS brakes think. Another thing i noticed is that in the PET the S4 brakes have the caliper bolts (that hold the two caliper halves together ) starting from the outside or wheel side of the caliper. The GTS diagram has the bolts starting from the opposite end. The calipers on the car have the bolts starting from the inner fender side or can be said to be like the GTS diagram.

They look like the GTS rotors with the Porsche stickers as opposed to the cast in porsche like the early S4's. But i know later calipers had this same look. Another thing that makes me think the calipers arent original is that the finish on them is still very nice. The rear calipers are more grey colored and look like the clear coat is flaking off so they dont look like they are of the same vintage as these fronts. So i believe the front rotors were replaced at some point. Why and when i dont know but if they were replaced i think its safe to assume that they would have been replaced with the newer larger GTS calipers.

It also appears that i can fit a larger disc inside this caliper. (see pic) I know GTS discs are only slightly larger then the S4 discs so i could imagine a GTS sized rotor will fit without issue.

Would love to get some opinions on this. If its agreed that they are GTS calipers i will be ordering the 993 TT compound rotors and lindsey racing aluminum hats so that i can use the compound design rotor opposed to the single piece 965 GTS rotors.

If a GTS conversion was done i could see them putting on GTS rotors and pads and at a later time when brakes were replaced, being that the car is a GT a GT rotor was ordered and reinstalled without the installer realizing that the previous rotor was larger. But this is all speculation at this point.

I am thinking i will pull out the S4 pads and compare them with the pagid's which were on the car and see if the pagids have a larger footprint. Are GTS pads taller, IE more square then the S4 pads. I know they are supposed to have a larger contact area and figured the pads would be longer but being taller would also increase the surface contact area. Can someone confirm or deny this difference in S4 and GTS pad size.

Also, Is there a Porsche part number someplace on the calipers? If i could find a PN i could tell if they are GTS or S4, but i cant seem to find a PN anywhere, although they could be on the suspension side of the caliper.

Jim M.

Ryan, measure the distance from the casting land where the brake bleeder port mounts to the same location where the crossover tube mounts. The GTS/Big Red caliper is 7 5/8" from edge to edge. That's the only way I've ever been able to tell. The casting numbers have never been a help to me and there isn't a P/N on them that I've ever found.

JHowell37

Those are GTS calipers. The word "Porsche" on the S4/GT calipers is in raised letters created during the casting process.

Jim M.

Also the GTS/Big Red rotor is 322mm diameter while the S4 is 304mm. Hope that helps. I think if you had GTS calipers with S4 rotors the pad would be above the edge of the rotor and you would have a lip on the pad instead of the usual lip on the rotor.

RyanPerrella

I found a thread in which Cobalt mentions pad dimensions of 132mm x 48mm for S4 and 132mm x 58mm for GTS.

So I measured my old Pagids and got 57.65 (see pic) Seems to be a GTS pad if i am measuring it correctly, which i may not be.

Then i pulled the performance friction pad (AKA Autozone pad) and got 54.54mm which isnt in spec for either GTS or S4

see pics below

Also when side by side the appear to have the same sized backing plate and pad seems identical.

Could this autozone pad be made to fit GTS and S4 cars possibly? They made an inbetween so as to service both cars and not have to make 2 pads? I know this sounds odd.

Jim M.

GTS pads are 58mm wide (high) and the S4's are 48mm wide.

RyanPerrella

I thought so too but I think mid 89 the caliper changed to this type without the raised lettering. Angela's 90 had the same type writing on those front calipers.

Jim, look at the Pagid pad, there is a huge lip

Jim M.

Your picture clearly shows a lip on the pad so you do in fact have S4 rotors.

RyanPerrella

Rob Edwards? What do your calipers look like. We have the same year so should be identical, are your calipers raised letters or stickers?

RyanPerrella

Ok so do I in fact have GTS calipers?

Sorry, this is EXCITING!

except for the fact that in their current state i have a complete mess of a braking system, but now i know how to fix it!

Jim M.

Yes you have GTS calipers with S4 rotors.

dprantl

Um... ok this is weird. I just went out to measure my front calipers and they are 7 5/8". The calipers on the rear are just over 6" and so are the fronts on my wife's 968. That means my '91 GT also has GTS front calipers Could it be that all late '91 GT's got GTS front calipers?

Dan
'91 928GT S/C
'86 928S 5-spd w/LSD *sale pending*

RyanPerrella

Allright good deal

Now I think i am going to shoot for 993TT rotors but need to find hats to get the offsets correct. It was pointed out to me to look at the Lindsey racing website as they make adapters that allot the 993 TT rotors to be mounted to 951S cars. The way the thread was going i was assuming that the 951S and 928 require the same hub dimensions. Their hats are $299 for the pair which when combined with the cost of 993TT rotors doesn't make them that much more expensive then the GTS rotors so i am told.

Does anyone know of any other supplier of these rotor hats that can make this disc conversion work? I would love to spend $200 for hats, but if the only option is $300 then i can live with that in the grand scheme of things. (Maybe I should post this on the 951 board?)

What are my used rotors worth? I think i saw them new for $115 each for Zimmerman's but i dont know if the rotors are Porsche or what.

RyanPerrella

Are you having the same type of rusting and pad issues on your car that i describe?

My car was said to have had the upgraded brakes so i would think mine weren't stock.

My car is a 90 BTW

dprantl

Er, I just got my car recently so I haven't noticed any issues. You would have to ask Sebastian, he was the previous owner. Didn't notice yours was a '90, my bad.

Dan
'91 928GT S/C
'86 928S 5-spd w/LSD *sale pending*