991.2 3.0 S Powerkit modified brake cooling
01-15-2018, 08:26 PM
#16
I’m not sure how you’re getting the numbers above. Would you mind explaining your process?
The specific heat of cast iron is .5, while SGL’s carbon ceramic rotors are .8. Given this the heavier cast iron rotors actually have more thermal mass, and would be expected to heat less than the ceramic rotors in the example above over a single quick stop. Ceramic’s advantage is in better cooling and higher peak operating temperatures. However it gains heat as fast or faster in the first place given its low mass, so your example doesn’t seem to compute...?
Last edited by RRDnA; 01-15-2018 at 10:19 PM.
01-16-2018, 02:06 AM
#17
The vast majority of modern brake rotors are grey cast iron, typically .54. Meanwhile the C/SiC of Porsche’s disks is .8 per the manufacture:
https://www.sglgroup.com/cms/interna...ml?__locale=en
This is why carbon ceramic rotors tend to run hot- their slightly higher specific heat is not enough to offset their significantly lighter weight. Running hot is actually a benifit from a heat-dissipation point of view, and unlike iron ceramic’s coefficient of friction doesn’t drop off at high temperature. The unfortunate downside is that at those temperatures where iron would be fading the carbon is oxidizing, eventually leading to a 20k+ rotor replacement bill...
01-16-2018, 03:38 AM
#18
Using 0.8 and 20kg carbon ceramic rotors - increase in temperature ~108 degrees C (200-100kmh) - max operating temperature = 900 degrees C
Using 0.56 and 34kg steel rotors - 91 degrees C (200-100kmh) - max operating temperature = 700 degrees C
Using 0.56 and 28kg steel rotors - 111 degrees C (200-100kmh) - max operating temperature = 700 degrees C
However its worth keeping in mind the PCCB rotors are very large 410mm front and 390mm rear - leading to very large surface areas which are important in radiative and convective cooling (approximately 1.5 times greater than the cooling area offered by the 330mm base brake rotors and noting that area is directly proportional to heat flow (q)).
Helpful table.
Using 0.56 and 34kg steel rotors - 91 degrees C (200-100kmh) - max operating temperature = 700 degrees C
Using 0.56 and 28kg steel rotors - 111 degrees C (200-100kmh) - max operating temperature = 700 degrees C
However its worth keeping in mind the PCCB rotors are very large 410mm front and 390mm rear - leading to very large surface areas which are important in radiative and convective cooling (approximately 1.5 times greater than the cooling area offered by the 330mm base brake rotors and noting that area is directly proportional to heat flow (q)).
Helpful table.
Last edited by RRDnA; 01-16-2018 at 05:35 AM.
01-16-2018, 03:26 PM
#19
Using 0.8 and 20kg carbon ceramic rotors - increase in temperature ~108 degrees C (200-100kmh) - max operating temperature = 900 degrees C
Using 0.56 and 34kg steel rotors - 91 degrees C (200-100kmh) - max operating temperature = 700 degrees C
Using 0.56 and 28kg steel rotors - 111 degrees C (200-100kmh) - max operating temperature = 700 degrees C
Using 0.56 and 34kg steel rotors - 91 degrees C (200-100kmh) - max operating temperature = 700 degrees C
Using 0.56 and 28kg steel rotors - 111 degrees C (200-100kmh) - max operating temperature = 700 degrees C
However its worth keeping in mind the PCCB rotors are very large 410mm front and 390mm rear - leading to very large surface areas which are important in radiative and convective cooling (approximately 1.5 times greater than the cooling area offered by the 330mm base brake rotors and noting that area is directly proportional to heat flow (q)).
Unfortunately despite this they will routinely exceed 550C during hard use, which is roughly the temperature beyond which the carbon strands begin to rapidly oxidize and "burn out" of the ceramic matrix that surrounds and protects them. So while the PCCBs can indeed be pushed to 900C, actually using that additional temperature capability will lead to incredibly short lifespans. In the worse cases on more powerful cars we've seen current gen PCCB rotors eaten at a rate of roughly $15 per track minute. Thus at current PCCB replacement disk prices I'm not sure the 900C operating temperature noted by SGL is a fair one to use. Personally I'd try to set 600C as an upper limit and use temperature indicating paint to insure I'm not exceeding that. If replacement rotors cost what they do for the ZR1 I'd feel differently.
Last edited by Petevb; 01-17-2018 at 02:57 AM.
01-17-2018, 12:14 AM
#20
Race Director
Well I certainly can tell you that the brakes on my 2017 GTS need help at the track. I have no problem keeping up with most stock GT3’s even passing some but they have much larger brakes thus less heat issues and longer lasting brakes. The power and handling the GTS has will over power the Carrera S/GTS brakes at Sebring.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
01-17-2018, 12:35 AM
#21
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Do the S/GTS use the same size pads as the GT3? I know Pagid RS29's are popular with the Porsche crowd and I used them on my Mustangs and liked them a lot. They need to be bed properly though. The calipers on my GT350 are very similar looking to ones on the GT3. The GT350's use 395mm rotors.
01-17-2018, 01:27 AM
#22
Race Director
Do the S/GTS use the same size pads as the GT3? I know Pagid RS29's are popular with the Porsche crowd and I used them on my Mustangs and liked them a lot. They need to be bed properly though. The calipers on my GT350 are very similar looking to ones on the GT3. The GT350's use 395mm rotors.
01-17-2018, 01:34 AM
#23
Well I certainly can tell you that the brakes on my 2017 GTS need help at the track. I have no problem keeping up with most stock GT3’s even passing some but they have much larger brakes thus less heat issues and longer lasting brakes. The power and handling the GTS has will over power the Carrera S/GTS brakes at Sebring.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring..
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring..
A couple additional ideas, not sure if they are good. GT3s use a more sophisticated rear brake duct solution. Unfortunately the suspension links are also different so I know they will not bolt on, but I suspect with some good zip ties and fitting you could make one or both of these rear ducts fit, part numbers:
991.575.431.80
991.575.432.80
991.331.481.80
991.331.482.80
In the rear the GT3 Cup uses an even more complex solution that looks better but is probably too hard to fit for most as it might require modifications to the upright:
991.575.334.8A
991.575.333.8A
Up front my best advice is still the two deflectors I showed previously in the thread (assuming they can be fit). The GT3 Cup and previous generations of GT3 all use a similar arrangement, suggesting to me that it is fairly universally effective.
For the front, the previously shown on-wishbone part numbers are:
991.341.483.80
991.341.484.80
(and I would use these rather than a dremel to retain the top lip).
The chassis mounted pieces are:
991.575.131.81
991.575.132.81
A quick google suggests these last parts are $78 each and the wheel liner looks very similar on the PET. No guarantee they’d fit, but at those prices it might be worth playing guinea pig.
01-18-2018, 03:07 AM
#25
Well I certainly can tell you that the brakes on my 2017 GTS need help at the track. I have no problem keeping up with most stock GT3’s even passing some but they have much larger brakes thus less heat issues and longer lasting brakes. The power and handling the GTS has will over power the Carrera S/GTS brakes at Sebring.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
The GTS/S is well braked for sprints e.g. warm-up lap, 5 hot laps followed by a 80-100kmh cool down lap. If you are doing repeated 25 lap sessions at full pace (and plan to do this often) you may need need to consider an upgrade. The DS1.11 is a pretty decent endurance/GT type pad and maintains friction from around 100 degrees C through to 700 degrees C e.g. they can take a lot of punishment and take a lot of heat without losing their performance characteristics. The DS2500 is a much more benign pad (figuratively and literally) and falls off after about 500 degrees C.
Last edited by RRDnA; 01-18-2018 at 03:32 AM.
01-18-2018, 03:15 AM
#26
Rennlist Member
Well I certainly can tell you that the brakes on my 2017 GTS need help at the track. I have no problem keeping up with most stock GT3’s even passing some but they have much larger brakes thus less heat issues and longer lasting brakes. The power and handling the GTS has will over power the Carrera S/GTS brakes at Sebring.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
I went to Giro Disc 350mm Rotors front and rear, Castrol SRF and tried Ferodo ds2500 pads which didn’t last and caused uneven pad build up and bad vibrations when braking. I then went to Ferodo ds11.1 pads and they did a little better with heat but I wore through more than 50% of the pad in 2 days at Sebring.
I can use GT3 pads and have some choices with PFC but the only choice pads.
Remember when the 996 GT2 had 456 hp?
01-18-2018, 09:51 PM
#27
Race Director
Two days hard driving round a high speed track will easily kill a set of pads (where killed equals more than half the friction material consumed and the residual material has a broken, cracked and plucked look - Questions, (1) what state were the brake rotors in e.g. how much lip on the outer edge, (2) do the rotors feel "grooved" (when cold) (3) are there 1-3mm micro-cracks appearing (if cross drilled). (4) do you do much trail braking and (5) most importantly did your brake pedal go long on you at any stage? (6) how and where did this occur (if at all).
The GTS/S is well braked for sprints e.g. warm-up lap, 5 hot laps followed by a 80-100kmh cool down lap. If you are doing repeated 25 lap sessions at full pace (and plan to do this often) you may need need to consider an upgrade. The DS1.11 is a pretty decent endurance/GT type pad and maintains friction from around 100 degrees C through to 700 degrees C e.g. they can take a lot of punishment and take a lot of heat without losing their performance characteristics. The DS2500 is a much more benign pad (figuratively and literally) and falls off after about 500 degrees C.
The GTS/S is well braked for sprints e.g. warm-up lap, 5 hot laps followed by a 80-100kmh cool down lap. If you are doing repeated 25 lap sessions at full pace (and plan to do this often) you may need need to consider an upgrade. The DS1.11 is a pretty decent endurance/GT type pad and maintains friction from around 100 degrees C through to 700 degrees C e.g. they can take a lot of punishment and take a lot of heat without losing their performance characteristics. The DS2500 is a much more benign pad (figuratively and literally) and falls off after about 500 degrees C.
1- no lip on outer edge. Rotors had 3 track days on them and some street miles. They were used with Ferodo ds2500 street/track day pads. They didn’t really groove the rotor and the rotors didn’t show much if any serious wear. However the ds2500 pads left a lot,of pad build up especially on rear rotors with some slight judder feeling and the brake pedal could get longer towards the end of a sesssion. Plus I wore right through the ds2500 pads.
2- the rotors after using the ds11.1 pads for 2 days are now grooved and the rotors have a lip on them. The pad build up cause the judder vibration from the rear is about 80-90% gone.
3-Rotors are slotted GiroDisc and there is not really any cracks.
4-I do trail brake but only really turn 7 at Sebring were I’ve been driving these last 5 track days. Turn 1 is a very easy off the brakes turn and going into turn 17 you drag the brakes very lightly until the turn into the 1st bridge going back to power.
5- I’m using Castrol SRF so the fluid isn’t over heating and at a bleed check I got no bubbles but the ds2500 pads got soft a lot. The ds11.1 pads did a lot better with heat but the pedal was harder after the brakes cooled for and hour. Also after the session is over I would drive right off the track and out the gate and 4 miles up the road to the gas station....so lots of driving to gently cool off the brakes.
6- all 5 track days on the pads-rotors were at Sebring
thanks for any insight....
01-18-2018, 09:59 PM
#28
Race Director
This is interesting feedback given your experience at Sebring, Mike. Did you experience similar brake challenges with your 997 (was it 997s)? I have a pet theory that the 991.2 turbo engines make life hard on the brakes due to their off-corner torque and improved acceleration, which is carried from there until the next braking zone—and the 450-hp 991.2 GTS is seriously fast.
Remember when the 996 GT2 had 456 hp?
Remember when the 996 GT2 had 456 hp?
The 991.2 GTS with this turbo engine at the track is very very underbraked. I’m having to back up my braking zones compared the the 997 GTS. The 991.2 GTS really needs to be ordered with PCCB brakes then convert to the large gt3 rotors...or Porsche really should have made this car with gt3 380mm brakes. Of course on the street and twisty road driving the stack brakes are awesome...zero issues...but for track days the 991.2 GTS can keep up mostly with a gt3 but it needs serious help with brakes.
01-19-2018, 04:04 AM
#29
mdrums - This is how your scenario reads to me. The DS2500 pads weren't up to the task at hand, they cooked onto the brake rotors and the rotors became "warped" - hence the pedal judder. The pads wore sufficiently and to such an extent (very thin) that the brake fluid was reaching its temperature limit and thus you experienced the pedal getting soft. After this experience you went to a different pad but did not replace the rotors. The new pads (which are a good pad but quite aggressive) had to deal with warped or partially warped rotors - the brakes still work but are sub par in terms of what you were expecting. These pads chew away at the rotors and at an early stage differential heating takes place due to the build up generated by the DS2500. On cooling the rotors now have a lip and concentric ribs (can be felt by rubbings ones fingers across the rotors).
This is what I would do - (1) get a new set of rotors front and rear (as its virtually impossible to skim slotted or cross drilled rotors to achieve a smooth contact surface). (2) Assuming the DS1.11 are butchered either go for the same pad or consider Pagid RSL29s - in terms of bite the Pagids sit midway between the DS1.11 and DS2500, and retain a decent coefficient of friction right out to ~ 650 degrees C. (3) Consider changing the brake fluid over to AP Racing R4 - this fluid has a dry boiling point of around 350 degrees C versus ~300 degrees C for the Castrol SRF. Just be aware the wet boiling point of the R4 is lower than the SRF, meaning you will need to change the fluid more often. Good to see you do track side bleeds. (4) I don't think you mentioned your tires - if you are running P-Zeros consider switching to MPSC2 N1 - bearing in mind its the tires grip that really pulls you up and your leg that generates the brake torque.
I suspect if you do the above you will have a better braking experience, be able to brake later with confidence and better modulate your trail braking
if that doesn't work you will either have to reduce the cars work load e.g. do shorter stints interspersed with more cool down laps or upgrade to larger rotors.
As after thought did you bed in the DS2500 pads?
This is what I would do - (1) get a new set of rotors front and rear (as its virtually impossible to skim slotted or cross drilled rotors to achieve a smooth contact surface). (2) Assuming the DS1.11 are butchered either go for the same pad or consider Pagid RSL29s - in terms of bite the Pagids sit midway between the DS1.11 and DS2500, and retain a decent coefficient of friction right out to ~ 650 degrees C. (3) Consider changing the brake fluid over to AP Racing R4 - this fluid has a dry boiling point of around 350 degrees C versus ~300 degrees C for the Castrol SRF. Just be aware the wet boiling point of the R4 is lower than the SRF, meaning you will need to change the fluid more often. Good to see you do track side bleeds. (4) I don't think you mentioned your tires - if you are running P-Zeros consider switching to MPSC2 N1 - bearing in mind its the tires grip that really pulls you up and your leg that generates the brake torque.
I suspect if you do the above you will have a better braking experience, be able to brake later with confidence and better modulate your trail braking
if that doesn't work you will either have to reduce the cars work load e.g. do shorter stints interspersed with more cool down laps or upgrade to larger rotors. As after thought did you bed in the DS2500 pads?