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Brake balance upset — upgrading fronts only?

 
Old 03-14-2019, 06:40 PM
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francisluu
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Default Brake balance upset — upgrading fronts only?

Currently on stock brakes on a 1995 C2. Would brake balance be adversely impacted by upgrading the fronts to (for argument's sake) let's say the Turbo big reds or Brembo GT kit only? I understand people usually do all four corners so call me a cheap bastard all you want

Side note for my own learning and understanding — how do our cars benefit from increased braking from the rear? The RS brake package has larger rears than on the Turbo's.
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Old 03-14-2019, 06:47 PM
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TheOtherEric
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Upgrading just the fronts isn't really the right way to go on a 993 -- you want to do all 4. Doing it "right" on a narrow-body means TT fronts and RS rears. You can do TT front and rear, but the TT rears would call for different proportioning valves;* that's why you want RS rears. By the way, your post incorrectly states the RS is larger than the TT rears. They actually use the same pads. RS pistons are slightly different sized than TT.

* side note: if anyone needs the correct proportioning valves, I have an unused pair for cheap. Bought because I mistakenly thought I had TT rears, when I correctly had RS rears.
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Old 03-14-2019, 06:50 PM
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Originally Posted by TheOtherEric View Post
By the way, your post incorrectly states the RS is larger than the TT rears. They actually use the same pads. RS pistons are slightly different sized than TT.
Learning is fun

Two follow-up questions:
- Can you elaborate on why uprated rear brakes are beneficial on our cars? Is it to control the rear wanting to come forward under heavy braking or something related to stability of the rear?
- The parking brake — is that affected with the larger rear setup or is that just carried over unchanged/not needing an adjustment?
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Old 03-14-2019, 07:15 PM
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If you search on brake upgrade by title there are a fair number of threads on this. You did not mention your intended use of the car. If just for street I would would look at pads, tires, suspension refresh and a proper alignment (you may have come or all of this already).

Here's one thread with some good info although the OP was looking to upgrade for track use. Note post #12 by Bill at the end he talks about rear bias:

https://rennlist.com/forums/showthre...ferrerid=93689
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Old 03-14-2019, 08:33 PM
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Originally Posted by francisluu View Post
Currently on stock brakes on a 1995 C2. Would brake balance be adversely impacted by upgrading the fronts to (for argument's sake) let's say the Turbo big reds or Brembo GT kit only? I understand people usually do all four corners so call me a cheap bastard all you want

Side note for my own learning and understanding — how do our cars benefit from increased braking from the rear? The RS brake package has larger rears than on the Turbo's.
First you need to understand what's important and what's not important

the size of the pads is one of the least important things

the balance and absolute brake torque are the most important, w/ durability following closely

for balance the factory pretty much aced it w/ the balance seen on the 911 through 1983, that balance can be expressed as a ratio, 1.491. Higher is more front and mostly safe, lower is more rear and can be a safety issue when too low. Too low varies w/ road conditions, tire states, what other loads besides braking are being placed on the chassis. The factory and aftermarket vendors always err on the safe side

i've never seen any Porsche that didn't have adequate absolute brake torque #s, some are a bit higher or lower than others but all are more than adequate

993 has some of the best brakes used prior or since

here is a comparison of the stock 993 brake setups, A 993 doesn't need a p/v in the same sense as a 3.2 Carrera w/ a bias of 1.220 does, All have great or at the very least safe bias w/o a p/v The best is the bias seen on 993RS @ 1.426 to p/v limited 2.640(which though it uses the same pads, rotors and caliper size as tt has much larger pistons that generate much more brake torque) the worst is tt @ 2.168 to to a p/v limited 3.697

Note that the actual brake torque at the front is not that different between a normal 993 and a 993RS or tt. the difference is almost all at the rear


Here is a typical bbk from Brembo bias is a very safe 1.960 to 3.564


Here's a comparison of brake stress, brake stress comes for the thermal loads placed on the brakes, the thermal load is a function of the chassis weight and speed potential balance against the brake size. Additional factors that are pertinent but hard to quantify are the air flow through the brakes and the specific traits of the components such as pad design, rotor design brake fluid spec etc.


For street use balance is nowhere near as important as for track use, for street close enough to more front is desirable as rear lockup is the main thing to avoid, hence the abundance of front bias seen in street offerings, here thermal loads are generally not an issue

for track use in order to optimize both performance and utilization of components you want as much rear as you can get
the more rear the more absolute stopping power can be used
the more rear the less thermal and brake load on the front the smaller and or more durable the fronts can be

race cars utilize twin masters w/ driver adjustable bias a 993RSR would have bias adjustable from 1.260 to 1.613 w/ no p/v, the neutral position is ~1.45

The more rear bias the more the car needs to be set up for it
low
stiff
effective lsd from 60% up to 100% but certainly at least 40%
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Old 03-15-2019, 02:57 PM
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A better "Cheap Bastard solution: Paint your stock calipers red and call it a day!
If you are street driving close enough to the edge to tell the difference by a so-called "upgrading" your car won't be around much longer and probably you won't be either.
Andy - The over 60 guy wearing the "Some Fear" T-Shirt.
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Old 03-15-2019, 04:18 PM
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Bill at it again! I was hoping for something like this, thank you. The bit about brake torque vs thermal capacity was very interesting in the other thread that was referenced also.

The car usually sees the track more than street so the idea of better braking performance has crossed my mind.
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Old 03-15-2019, 07:00 PM
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FWIW: The 993 SuperCup cars used Big Red (although black color) size calipers and 322 mm discs in front and stock C2 rear brakes with the proportioning valve removed from the system.
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Old 03-15-2019, 07:32 PM
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Are the black versions of the "big reds" readily available? I actually prefer the black (or silver) caliper over red
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Old 03-15-2019, 07:55 PM
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Originally Posted by Cupcar View Post
FWIW: The 993 SuperCup cars used Big Red (although black color) size calipers and 322 mm discs in front and stock C2 rear brakes with the proportioning valve removed from the system.
That was only for the first introductory year, They transitioned from the 964 Cup setup to the 993Cup only changing from the 964RS front calipers which used the 132x48mm pads to the black 993RS/tt calipers which use the 132x58mm pads. Interestingly the 964RS fronts generate more brake torque because of the larger effective radius, hydraulically the are identical. Aftyer that forst year the 993Cups used the 993RS rears w/ the bigger caliper using the 30/36 pistons and 48x132 pads on 322x28 rotors compared to the 964RS and regular 993 rear using 98x44 pads & 30/34 pistons on 299x24mm rotors.

This early 993Cup setup is shown as the first one in the first chart I posted above
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Old 03-15-2019, 08:05 PM
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Originally Posted by francisluu View Post
Bill at it again! I was hoping for something like this, thank you. The bit about brake torque vs thermal capacity was very interesting in the other thread that was referenced also.

The car usually sees the track more than street so the idea of better braking performance has crossed my mind.
For track use I use the993RS 322x32 rotors, w/ floating hats and slotted rather than drilled. The drilled just don't last very long in hd usage. In back the 993RS 322x28 and RS caliper relieves some of the load from the front by moving bias rearward. You do want to lower and stiffen the car and use an effective lsd. The stock 25/65(if you have the M220 option) is ok if rebuilt w/ GT discs in stock form not so much.

The 993RS setup has never had heat issues except a few times w/ worn pads when coming off a hot track and sitting in the pits w/ foot on the brake pedal, some minor heat soak can be evident. Better fluid would solve that problem

the big blacks were also used on 928GTS, if you get a pair of these the bleeds and feeds will need to be swapped

You could put bigger fronts only but then you are giving up the extra stopping power that is theoretically available in back and the fronts have to work that much harder.
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Old 03-16-2019, 04:32 PM
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I ran "Big Blacks" on my 993C2 with stock rears to keep the original look for my car. Loved the look and the performance of the Big Blacks. You can purchase the calipers new from Sunset for ~$850 each. There is someone on Pelican that is selling a new pair of Big Blacks for $1350, which is a great deal.
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