996 Turbo Brake Information - DIY and Sorting truth from Fiction!
#121
10-15-2008, 12:57 PM
My track car has an all manual system and it will be a "hoot" to dial in at full song, sanse ABS! 
Can't wait to read your write-up! I'm all ears!
Mike
Can't wait to read your write-up! I'm all ears!
Mike
#122
10-15-2008, 01:15 PM
I wanted to post some additional information I've found as far as pads. I was using the EBC Yellowstuff pads on the turbo during DE's. I have since melted the crap out of a set at Sebring, and worked tirelessly to remove the grime it baked on my new 350mm disks using a grinder and a heavy-duty brillo pad. I have since gone to Pagid Yellow's and am extremely impressed by the difference. I guess that's the reason they cost so much more. They handled Sebring flawlessly this past weekend at a NASA event. I can't even begin to explain the difference in performance and better braking ability. I can't believe I didn't try them sooner! Needless to say, I will be using Pagid's on the Turbo for every track event.
#123
10-15-2008, 02:19 PM
Wow, good information. There are many other vendors selling these 997TT/GT3 rear calipers and recommending them as a safe and effective addition to the 6 piston Porsche fronts... I spoke in length to 2-3 before making my purchase... I would love to hear other opinions on this topic from some of the other brake experts out there.
Mark-your input and data is much appreciated! Thank you for chiming in here!
Mark-what are the specs on the 996GT3 rear calipers? How different are they than the 996TT rears? Other vendors and tuners sell the complete 996GT3 setup, front and rear, for use on the 996TT's... I have been told that the front calipers are the same for the 996 GT3's and the 997TT and GT3's so it would be interesting to see what the 996GT3 rears are set like? Are there other differences in the 996GT3 master cylinders, brake proportioning setup, etc...?
Mark-your input and data is much appreciated! Thank you for chiming in here!
Mark-what are the specs on the 996GT3 rear calipers? How different are they than the 996TT rears? Other vendors and tuners sell the complete 996GT3 setup, front and rear, for use on the 996TT's... I have been told that the front calipers are the same for the 996 GT3's and the 997TT and GT3's so it would be interesting to see what the 996GT3 rears are set like? Are there other differences in the 996GT3 master cylinders, brake proportioning setup, etc...?
I do not believe there are any changes to the master cylinders between the 996TT and GT3 models. There would be no good reason for there to be, as the piston areas are the same. I would wager on differences in the ABS tuning of these cars though.
Last edited by msv; 10-15-2008 at 02:26 PM.
#124
10-15-2008, 02:21 PM
This examines the factors that go into the braking force and front/rear brake distribution. Beyond this, there is the thermal energy side of things. We will save that topic for another time.
The braking force needed at the tire contact patch (where the tire meets the road) is simply based on the vehicle weight and the deceleration rate. For maximum performance, there is an ideal distribution of braking force between the front and rear wheels. This distribution, however, is not constant. It changes based on the deceleration rate. This is due to weight transfer. The amount of weight transfer that occurs is based on the deceleration rate, the wheelbase, and the center of gravity height (we will neglect the effects of aerodynamic downforce or lift for the purposes of this discussion). Of course, the static front/rear weight distribution contributes to the overall values. Once we factor in all these values for a given vehicle, we can determine the ideal front/rear brake force distribution over the entire range of deceleration, i.e. from zero “g” to the maximum possible deceleration rate (which depends on the traction available from the tires).
From this point, we can then approach the needed braking torque that the system needs to achieve. Another parameter that comes into play at this point is the rolling radius of the tires. So with all of this data, we now have calculated the ideal front and rear braking torque over the entire range of deceleration. It is now a matter of determining how to achieve this. This is a combination of the fluid pressure, the piston area of the calipers, the effective radius that the caliper acts upon, and the friction coefficient of the brake pad. These components need to be sized so that a huge number of other factors can be achieved as well, including but not limited to: brake pedal force, workable maximum line pressure, thermal requirements, component life, packaging requirements, and of critical importance to the balance, method of manipulation of the front vs. rear line pressure. This last factor and the sizing of the calipers and discs determine how closely we can approximate the ideal values that we calculated earlier.
In earlier days the manipulation of the front vs. rear brake pressure curve was quite limited. This was achieved through the use of a mechanical proportioning valve. The function of this valve was to reduce the pressure to the rear brakes once a certain line pressure was achieved. This is referred to as the “knee point”. Up until this pressure is achieved, the front and rear brakes receive equal pressure. Once the knee point pressure is reached, the rear line pressure rises more slowly as compared to the front. Even with such a simple system, it was quite possible to obtain a good correlation between the ideal and the actual. With the advent of electronic brake distribution and computer control, it has become easier to more closely approximate the ideal pressure curve. This is because designers are not limited to a simple knee point. It is now possible to manipulate the pressure curve in a much more sophisticated manner.
What one needs to recognize is that this has all been done based on the specific parameters of the stock braking system. Changing these parameters makes this null and void. In practice, the braking torque of the rear needs to be maintained slightly below what would be calculated to be “ideal”. This is for vehicle dynamics reasons and stability. Even a “perfect” balance system is highly unstable and is prone to swapping ends. With a little bit more rear brake, it is a certainty. This is why one must be wary of increasing the rear brake torque. The ABS system can mask this by intervening to prevent rear lock up, but this is not desirable. Additionally, the ABS system itself is highly tuned as well, to an even greater extent. Deviations from the parameters that it was designed around (even changing tires) are detrimental to its performance and operation. In the specific case we have been speaking about, installing 997TT calipers on the rear of a 996TT, the fluid volumes are so vastly different, that the ABS function would be severely compromised. The ABS system is a safety feature; it is not there to provide maximum stopping performance. Its function is to prevent wheel lockup and allow you to still steer the car in a panic situation. Maximum performance will be achieved without the ABS coming into play.
The braking force needed at the tire contact patch (where the tire meets the road) is simply based on the vehicle weight and the deceleration rate. For maximum performance, there is an ideal distribution of braking force between the front and rear wheels. This distribution, however, is not constant. It changes based on the deceleration rate. This is due to weight transfer. The amount of weight transfer that occurs is based on the deceleration rate, the wheelbase, and the center of gravity height (we will neglect the effects of aerodynamic downforce or lift for the purposes of this discussion). Of course, the static front/rear weight distribution contributes to the overall values. Once we factor in all these values for a given vehicle, we can determine the ideal front/rear brake force distribution over the entire range of deceleration, i.e. from zero “g” to the maximum possible deceleration rate (which depends on the traction available from the tires).
From this point, we can then approach the needed braking torque that the system needs to achieve. Another parameter that comes into play at this point is the rolling radius of the tires. So with all of this data, we now have calculated the ideal front and rear braking torque over the entire range of deceleration. It is now a matter of determining how to achieve this. This is a combination of the fluid pressure, the piston area of the calipers, the effective radius that the caliper acts upon, and the friction coefficient of the brake pad. These components need to be sized so that a huge number of other factors can be achieved as well, including but not limited to: brake pedal force, workable maximum line pressure, thermal requirements, component life, packaging requirements, and of critical importance to the balance, method of manipulation of the front vs. rear line pressure. This last factor and the sizing of the calipers and discs determine how closely we can approximate the ideal values that we calculated earlier.
In earlier days the manipulation of the front vs. rear brake pressure curve was quite limited. This was achieved through the use of a mechanical proportioning valve. The function of this valve was to reduce the pressure to the rear brakes once a certain line pressure was achieved. This is referred to as the “knee point”. Up until this pressure is achieved, the front and rear brakes receive equal pressure. Once the knee point pressure is reached, the rear line pressure rises more slowly as compared to the front. Even with such a simple system, it was quite possible to obtain a good correlation between the ideal and the actual. With the advent of electronic brake distribution and computer control, it has become easier to more closely approximate the ideal pressure curve. This is because designers are not limited to a simple knee point. It is now possible to manipulate the pressure curve in a much more sophisticated manner.
What one needs to recognize is that this has all been done based on the specific parameters of the stock braking system. Changing these parameters makes this null and void. In practice, the braking torque of the rear needs to be maintained slightly below what would be calculated to be “ideal”. This is for vehicle dynamics reasons and stability. Even a “perfect” balance system is highly unstable and is prone to swapping ends. With a little bit more rear brake, it is a certainty. This is why one must be wary of increasing the rear brake torque. The ABS system can mask this by intervening to prevent rear lock up, but this is not desirable. Additionally, the ABS system itself is highly tuned as well, to an even greater extent. Deviations from the parameters that it was designed around (even changing tires) are detrimental to its performance and operation. In the specific case we have been speaking about, installing 997TT calipers on the rear of a 996TT, the fluid volumes are so vastly different, that the ABS function would be severely compromised. The ABS system is a safety feature; it is not there to provide maximum stopping performance. Its function is to prevent wheel lockup and allow you to still steer the car in a panic situation. Maximum performance will be achieved without the ABS coming into play.
#125
10-15-2008, 03:29 PM
Mark, I apologize, had no idea of your depth of experience. You indeed are a wealth of reliable knowledge concerning our brakes. Hope you will post more often when you see brake threads on this forum.
Last edited by landjet; 10-15-2008 at 03:44 PM.
#126
10-18-2008, 07:57 AM
I'm just about to upgrade my brakes and was going to do 997TT rotors/ GT3 calipers up front. Was planning on doing 997TT rotors and 997TT calipers in the rear, but now it apparent that shouldn't be done. What about just upgrading the front as above, and leaving the rear stock?
#128
10-18-2008, 08:23 AM
Seriously, what is a good option for 5-6 track events per year and some street use? OEM up front is not an option which I learned at my last event.
#129
10-18-2008, 09:50 AM
Looks like I might be switching up my setup!
For the record, I've been running around with the 997TT rear calipers on safely on the streets and in the canyons for awhile. However, I haven't pushed them in an extreme way yet...
Mark-what do you think about going with an aggressive front pad and running stock pads in the rear to help balance it out? Is that a good idea or should I go for a the 996GT3 rear calipers and call it a day? I'm heading to the track soon and would like to get this all sorted out!
#130
10-18-2008, 10:09 AM
996 GT3 rear calipers with 997TT rear 350mm rotors...
Looks like I might be switching up my setup!
For the record, I've been running around with the 997TT rear calipers on safely on the streets and in the canyons for awhile. However, I haven't pushed them in an extreme way yet...
Mark-what do you think about going with an aggressive front pad and running stock pads in the rear to help balance it out? Is that a good idea or should I go for a the 996GT3 rear calipers and call it a day? I'm heading to the track soon and would like to get this all sorted out!
Looks like I might be switching up my setup!
For the record, I've been running around with the 997TT rear calipers on safely on the streets and in the canyons for awhile. However, I haven't pushed them in an extreme way yet...
Mark-what do you think about going with an aggressive front pad and running stock pads in the rear to help balance it out? Is that a good idea or should I go for a the 996GT3 rear calipers and call it a day? I'm heading to the track soon and would like to get this all sorted out!
Mark says 996GT3 rear brakes are the same as 996TT rear brakes (couple posts up). Did you use spacers for the 997TT rear set up?
#131
10-18-2008, 10:17 AM
Adding the larger front brakes without doing anything to the rear is no problem at all.
Staggering brake pad compounds (i.e. a higher coefficient pad in the front than in the rear) is a viable method to change the brake distribution, however, the fluid volume difference for the ABS would still be present.
I can't say for sure if it will significantly affect the ABS performance in this case since I haven't tested it.
Staggering brake pad compounds (i.e. a higher coefficient pad in the front than in the rear) is a viable method to change the brake distribution, however, the fluid volume difference for the ABS would still be present.
I can't say for sure if it will significantly affect the ABS performance in this case since I haven't tested it.
#132
10-18-2008, 10:32 AM
The 997TT rear calipers bolt right up and accept the 997TT rear 350mm rotor which also bolts right up.
The 996TT and 996GT3 rear calipers both have the same piston size and fluid volume but the GT3 calipers accept 350mm rear rotors over the TT's 330mm's... I want the bigger rotors for better durability and heat dissipation at the track...
So I may look for some 996GT3 rear calipers to keep my overall system 'safe-balance and ABS design intact' while still getting the benefits of the bigger rotors...
Does that sound right, Mark? Is that what you would recommend?
#133
10-18-2008, 10:38 AM
Adding the larger front brakes without doing anything to the rear is no problem at all.
Staggering brake pad compounds (i.e. a higher coefficient pad in the front than in the rear) is a viable method to change the brake distribution, however, the fluid volume difference for the ABS would still be present.
I can't say for sure if it will significantly affect the ABS performance in this case since I haven't tested it.
Staggering brake pad compounds (i.e. a higher coefficient pad in the front than in the rear) is a viable method to change the brake distribution, however, the fluid volume difference for the ABS would still be present.
I can't say for sure if it will significantly affect the ABS performance in this case since I haven't tested it.
Others seem to be running the 997GT3 rear calipers at the track (Tom Kerr) successfully while mated to your Brembo 6 pot 355mm front kit (I almost went this route but due to wheel choice changed my mind). Would that indicate that the ABS system may be OK for this application or are we talking apples to oranges here?
I know that you haven't tested this and can't likely answer definitively but I am curious as to your best guess? Would you share your opinion on the matter?
I'm ultimately trying to decide if I've got a 'safe' setup if I can address the balance issue with different pad compounds. I was thinking about Pagid Yellows in front and stock pads in the rear... Thoughts?
#134
10-18-2008, 11:23 AM
Kato, I recently tried a new pad supplier and he got me Pagid yellows for the front, and was supposed to get me Pagid blacks for the rear but got Pagid stock pads by accident and when my tech saw them he immediately made the supplier switch to the race pads.
I defer to Mark in these matters though. Mark, Brembo makes the calipers for Porsche cars so you should be able to access that info from Brmbo and clear these questions, concerning mix and match, up. Thanks.
I defer to Mark in these matters though. Mark, Brembo makes the calipers for Porsche cars so you should be able to access that info from Brmbo and clear these questions, concerning mix and match, up. Thanks.
#135
10-18-2008, 11:32 AM
Scott,
The 997TT rear calipers bolt right up and accept the 997TT rear 350mm rotor which also bolts right up.
The 996TT and 996GT3 rear calipers both have the same piston size and fluid volume but the GT3 calipers accept 350mm rear rotors over the TT's 330mm's... I want the bigger rotors for better durability and heat dissipation at the track...
So I may look for some 996GT3 rear calipers to keep my overall system 'safe-balance and ABS design intact' while still getting the benefits of the bigger rotors...
Does that sound right, Mark? Is that what you would recommend?
The 997TT rear calipers bolt right up and accept the 997TT rear 350mm rotor which also bolts right up.
The 996TT and 996GT3 rear calipers both have the same piston size and fluid volume but the GT3 calipers accept 350mm rear rotors over the TT's 330mm's... I want the bigger rotors for better durability and heat dissipation at the track...
So I may look for some 996GT3 rear calipers to keep my overall system 'safe-balance and ABS design intact' while still getting the benefits of the bigger rotors...
Does that sound right, Mark? Is that what you would recommend?
So is there sense in running 997TT rotors front and back and 996GT3 calipers front and back? And would that maintain a safe system?