More like 25.7%, thats why you want to remove the p/v for the Boxster/Carrera setup;)

if you use Carrera rear the sock m/c is fine but for a better pedat you could also use the 930 m/c, if you use the Boxster rears you will want the 930 23.8mm m/c

I replaced the fronts and rears on my 84' Carrera with Boxster calipers and a 23 mm mc about 4 years ago. Also put on new rotors and brake lines(everything was original and pretty much expired). The conversion has had no problems whatever. The car has the 7 & 8 Fuchs, 16"ers and again, no problems. The brakes have great "feel" and are very direct and positive acting(like Porsche brakes in general).

Bill,

What is the foundation of the 25.7% number (pad area?)

SC's brake bias with 48mm and 38mm pistons have brake bias of 61.5% front and 38.5 % rear.

Boxster brake setup with Carrera 42mm rear brake pistons has a brake bias is 37.9 % to the rear and 62.1% to the front. Thus the ~0.6% increase in brake bias to the front as compared to the SC's setup.

Carrera(stock) vs Boxster/Carrera(in a Carrera chassis)

Since the only difference is the front caliper piston all you need is the ratio of 1 side of a boxster front /1 side of a Carrera front
2274mm²/1809.56
= 1.2566

or 25.6% more front for the Boxster/Carrera vs Carrera/Carrera

You can go through far more comprehensive analysis but the answer is still the same.

I have a huge spread sheet w/ every contributing factor in it. believe me when I tell you this.

Bill,

Yes, the Boxster front 2 pistons on half of the caliper have an area of 2274mm^2 and the Carrera front caliper has a surface area of 1809.6mm^2. Therefore, the Boxster’s front surface area is ~25.6% larger than the surface area of the Carrera’s front caliper piston.

However, the Boxster/Carrera setup has a front bias of 62.1% where as the Carrera/Carrera setup has a front bias of 56.6% (not factoring proportioning valve). Thus the Boxster/Carrera setup has ~9.8% more front braking bias than the Carrera/Carrera setup.

The result of the Boxster/Carrera brake bias ratio is very similar to the stock SC brake bias setup (only 0.6% difference in front brake bias). Since, the SC’s did not have a proportioning valve (AFAIK), the Boxster/Carrera mostly likely will not lock up the rear tires under hard braking.. However, if one wishes to push the limits to reduce braking distance, more rear bias may be necessary.


FWIW, I have run a Carrera/Carrera setup w/o a proportioning valve on the 3.2 914 track beast. The brake bias was great. Only if I did something silly, did the rear tires hint that they were locking up first.

Nope

Boxster/ Carrera is in the ratio 1.642 or 64.2% more front the way you want to think about it
Carrera/Carrera is in the ratio 1.306 or 30.6% more front


a 914's needs are not a Carrera's needs

Bill,

What is the math to determine 64.2%? If ones disagrees with anothers numbers, it is helpful to list the math as to verify the results. We both agree on the areas of the piston. The math to determine front brake bias based on piston size is determined by the equation front piston area/(front piston area + rear piston area[using 42mm per BA's book page 218]). Substituting in the Boxster/Carrera numbers this results in a front bias of 2274/(2274+1385) = 62.1%. So 'Nope' to 64.2% (unless other equations are used).

As far as the % change in brake bias, the two methods of determining brake bias {mine of (front/(front + rear))} and yours of front/rear will result in different values if the percent change equation of ((new-old)/old) is used. However, if one uses your equation then uses the equation of new/old-1, the percent change is the same as seen below:

http://forums.pelicanparts.com/uploa...1147527323.jpg

You have the basic facts right

Carrrera/ Carrera =1.306 ~30% more front than rear
Boxster/Carrera = 1.642 ~64% more front than rear

1.6/1.3 is ~1.26 or 26% change moved to the front

Man, is this getting analytical or what?? Really great info here!