'75 Duster B body front brake upgrade impressions wanted

If anyone cares, here is the formula I found years ago in a Hot Rod magazine (Jan '82) to calculate brake torque:

Tb = (Ph x Ap x u x 2) x Er

Where:

Tb = Torque (brake)
Ph = Hydraulic system pressure
Ap = Area of piston(s) (one side of the caliper on fixed calipers)
u = co-efficient of friction of the brake pad
Er = Rotor leverage length

Basically it takes the average lever arm of the caliper/pad, hydraulic force and pad friction and calculates a torque.

Based on the above, the following brake torque numbers are generated assuming 1000 psi out of the MC and .4 for the coefficient of friction:

10.87 w/ 2.6" piston - 19524
11.75 w/ 2.6" piston - 21393
10.87 w/ 2.75" piston - 21842
11.75 w/ 2.75" piston - 23933
13" w/ Cobra calipers - 15431
13" w/ C5/6 calipers - 17436

In all the above cases, I used the same pressure and coefficient of friction just to compare them. I used .4 for the pad friction as that is (as I understand) a middle of the road number for an FF pad. And all the to above numbers are in ft/inches.

Based on the above, it would seem that an 11.75" rotor with a 2.75" caliper would be the go to solution. It almost equals the brake torque for the big 6 piston Brembo brakes on the Hellcats which calcs to 24239 ft/inches. But there are two problems; 1. the 15.4" rotors on the HC will hold far more heat than the "big" 11.75" rotors and 2. available pads.

On the street, I doubt the big heat sink advantage of the 15.4" HC rotor will ever show itself, so the 11.75" rotor is probably more than adequate. On the other hand, if you want to more brake performance in the 11.75" setup, it's not as easy as just swapping in better pads since they don't make any. Which is why people have to swap to aftermarket kits at some point (besides just wanting bigger rotors to fill in their wheels).

Being that the single piston calipers take a brake pad that (AFAIK) don't have any aftermarket or performance pads available, it's hard to find something with a good coefficient of friction. As an example, if we drop the 11.75" rotor with a 2.75" piston down to a low end EE pad of about .3, the brake torque changes to 17950 ft/inches. At the same time, if the Cobra caliper on 13" rotors gets a GG pad which averages .5, it's torque numbers change to 19289 ft/inches. So pad selection can have a huge impact on the brake torque.

In the end? I think the 11.75" setup is a great way to go if you don't want to jump up to a 13" setup. But if you ever want more, just know that it will probably require a different brake kit all together.

Note that based on the picture on RockAuto, Bendix does sell an FF pad (SBM84) that would work for the 11.75" setup. That should at least give you a coefficient of friction of .35 and put you close the Cobra setup from Doctor Diff.

Lastly, it should be noted that these numbers ore only theoretical and ignore any design flaws that could negatively impact the actual function of the brakes. Sticking pads, caliper flex, stiction in the caliper? Any one of those and probably others will reduce the actual output of the brakes. Which is partly why I think 72bluNblu saw an improvement when he went to the 13" setup; better pads, better pad support design, more pistons to spread the force consistently and create less caliper flex, etc.

BTW, here a link to the brake coefficient of frictions I have been using - DOT Pad Codes