I just noticed that the 996 GT3RS was using 36/44 front and 28/36 rear calipers though w/ 380 & 3555 mm rotors and twin masters

 

Assuming 2wd 993TT with a bit of weight shaved off:

Would the 993TT to 993RS rear setup change make more sense if done together with for example a Guards LSD (50/80 version)?

For the uneducated, (including myself) the 50/80 means that when accelerating the car locks the wheels 50% from side to side and on deceleration it locks them 80%. The deceleration is very important to keep the rear end from wandering under hard braking. With 2wd a stout LSD is a must to put the power down controllably.


Thanks again Bill and others.

 

Well, technically that is not necessarily correct. The 993 ABS system is designed to operate with tires of the same diameter, in about 645mm in diameter. Through the evolution of the series, they began to run taller rear diameter tires in about 680mm in diameter with some of the cars with the widebody work running as much as 710mm in diameter. This can present a problem with the ABS control unit recognizing wheel slip when there is none. Since the rear ABS teeth are part of the stub axle and cannot be changed, Porsche offered a set of 45 teeth rings from the 964 for the front of the car. which returns back to normal the operation of the ABS control unit. So, a car with 645/645 would run with 48/48 and a car with 645/680 or 650/710 would run with 45/48. This is outlined in both the 993RSR and 993GT2 racing manuals.

On my car, I run 964 axles obviously which have 45 teeth on the stub axle, and I run 640/680 tires. I cannot easily change the number of teeth on the rear so I made a set of 43 teeth front ABS rings and run 43/45 to restore the ABS control unit operation. Since my speedometer is through the MoTeC unit, I just calibrate the speedometer using 43 and 45 teeth so all 4 wheel speeds show correctly in the MoTeC data acqusition which is also sent to the MoTeC ECU for traction control and pit speed limiting. The ABS functions normally and without issue on my car.

 

Remember that the TT uses a higher proportioning valve than C2 or RS. So installing RS brakes on the rear of a TT would probably cause rear lockup. People have reported rear lockup on non-turbos using tt front / RS rears, so going to an even higher p.v. (on a turbo) seems like a bad idea.

I think Bill is saying that installing RS rear calipers on a non-turbo will increase the propensity for rear-lockup, in which case you'll definitely want a working LSD to keep the car stable. RS calipers shift bias rearward. If you work out the maths, here are the ratios of front-to-rear brake torque:

993 stock ....1.6:1
993 TT ........2.0:1
993 TT/RS ...1.4:1
993 TT/C2 ...1.6:1

You can see that 993TT shifts bias forward, but they get away with it by using higher (55 bar) proportioning valves to get more pressure to the rear. Looking at the numbers, you'd think that running 993TT calipers on a non-turbo would cause terrible front brake bias, but my pad wear indicates it's not that bad.

 

Although often discussed as such, that is not necessarily the case. 50/80 speaks to the ramp angles on the pressure rings inside the differential. The ramp angles along with the preload set on the plates will determine the amount of torque required to spin one tire but not the other.

 

There is no proportioning valve in the master cylinder of the 993 setups. The TT and C4s have a master cylinder where the pistons for both the front and rear are 25mm and when brake pressures are logged, they are the same front to rear.

The 55bar PRESSURE LIMITING valve will shift the brake bias forward once 55bar is reach in the rear brake circuit thereby limiting the pressure on the rear, but allowing the front to increase beyond 55bar. It does not shift it rearward as you suggest.

 

I should have known that you would have gone through that .

the 993 GT2 Evo w/ the racing ABS5 controller used the 45/48 abs rings w/ 265/645 x18 & 325/680 x18 Pirelli slicks as the standard tire

 

Don't forget that I've been installing GT3 Cup ABS controllers in old E30 M3 race cars for several years. There are similar challenges with teeth count to get them to work properly.

 

993 stock ....1.572 w/ 40bar knee
993 TT ........2.061 w/ 55 bar knee
993 TT/RS ...1.472 w/ 40 bar knee
'07 GT3RSR...1.504 w/ dual master 50/49 bar base
'02 GT3RS.....1.554 w/ dual master 50/55 bar base
below the knee f/r pressure is the same, above the knee pressure to the rear is limited

 

Bill, the brake bias in the R, RS, and RSR both 996 and 997 are done through the sizing of the master cylinders which are different sizes front to rear. In addition, they have a balance bar that can be adjusted via a **** on the dash to change the bias front to rear or vise versa. A pressure trace will show the differences front to rear.

 

Huh? I never said boo about master cylinders. My comment was correct.

Again, I stand by my comment. 55 bar p.v. will allow up to 55 bar to the rear, which is greater than 45 bar. Hence it shifts bias rearward when compared to the C2's configuration.

 

No, your termonology was incorrect. THERE IS NO PROPROTIONING VALVE in the braking system. There is a pressure limiting valve only.

Agreed, compared to a car with a 45bar pressure limiting valve, a car with a 55bar pressure limiting valve will change the bias towards the rear above 45bar ASSUMING ALL ELSE EQUAL like piston diameter, pad sizes, rotor size, master cylinder piston size, etc. which are different between the TT/C4S and the standard C2.

 

The R, RS, RSR all come w/ equal size f/r masters and adjust the balance bars, that's why I specified the neutral or base settings

I won't look them all up but here are a few examples
'07 RSR uses 18.8/18/8
'03 RS uses 17.8/17.8

and additioanlly bias is affected by the rotor diameter which I also mentioned somewhere in the above

 

Yeah, I agree "pressure limiting valve" is more descriptive, but p.v. is just the term everyone here uses ...

 

We've gone seriously far astray here, what is done on the pure race cars has only a passing relevance to our street and dual use cars

If you are going to use the Big Red RS/tt front calipers and rotors then you have to make an informed decision as to what to use in back.

you have 2 choices for the calipers tto be used on the 322x28 RS/tt rear rotors
1) RS 30/36
2) tt 28/28

RS will move bias to the back compared to the tt(1.472 w/ 40 bar knee vs 2.061 w/ 55 bar knee vs historical 1.596 w/o any p/v or abs) and oem wants the stock 40bar p/v where the tt oem wants 55bar

If you move bias to the back you can use it profitably if the car is lowered and stiffened and has an effective lsd(4 channel abs helps). If not perhaps the tt is a better choice for you, even though you are leaving a lot of potential on the table.

Of course you can also do as many track guys do and juggle f/r pads w/ different friction characterisics, the most popular for guys w/ the smaller piston rears is to use a higher friction pad in back.