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Old 06-18-2005, 02:26 PM
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The pedal ratio also has a lot to do with it, and caliper displacement.

I wouldn't count on SSBC, since they offer the same master for a range of caliper sizes and pedal ratios, assuming it will work as well for one as another. They've had many problems.

There are formulas to determine both the bore and the stroke needed, based on caliper displacement and pedal ratio. The booster adds another variable, but a master should be chosen that is not so large that one cannot still stop if the booster fails.
There were a number of Carcraft articles which delt with this. I think they are still on the web somewhere. There is too much for me to write.

There are also some thumb rules, based on caliper bore area and pedal ratio. These do not work with low-drag style calipers. (caliper bore area and caliper displacement are different)

Basically, for most power or manual disc brakes, you would choose a master cylinder with between 12-14% of the bore area (in sq"), taking care to get one with sufficient stroke, often needing more stroke with 4wl disc. 12% usually works well with a 5:1 manual pedal, and 14% works well with a 6:1 manual pedal or 3.5:1 power pedal.

With a higher pedal ratio, you can up-size the master for more fluid displacement, using the pedal lever to make up for the loss in pressure. This is often used with 4wl disc. It is also sometimes done with low-drag style calipers instead of the special step-bore master cyl.

The vehicle weight has more to do with what size wheel brakes are chosen, which then can be used to help determine the master. The 12-14% rule is fairly common from an MG to an F350.
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