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With non-servo brakes, like on the 914, braking force at the brake rotor depends on:

1. the force applied to the brake pedal
2. the length of the brake pedal (distance between where your foot hits it and where the master cylinder pushrod is pushed)
3. the ratio of the master cylinder piston area to the total area of the caliper pistons
4. the pad material
5. the rotor material
6. the diameter of the brake rotor
7. the amount of heat in the brakes already

Note what's missing here: pad area. The size of the brake pads has no effect on braking force. Friction does not depend on area. Smaller brake pads will not provide any less braking than larger brake pads. Smaller pads will wear faster than larger pads, and smaller pads will be more vulnerable to brake fade from overheating, but in the absence of fade, larger pads gain you nothing.

Pedal length and rotor diameter are both leverage effects. Push a 10ft pedal and you'll have to push much more gently for the same force on the master cylinder pushrod than with a 1ft pedal. You'll have to push the 10ft pedal a lot farther to gain the same amount of movement of the MC pushrod, however. If the caliper grips a 10ft diameter rotor, it has much more leverage in slowing the hub than a caliper gripping a 1ft diameter rotor, so the 10ft caliper doesn't have to grip as hard. Pedal length and rotor diameter are obviously limited by things like cabin size and wheel size.

The MC/caliper piston ratio is another leverage phenomenon. Force on the MC piston is multipled by the ratio between it's area and the area of the caliper piston(s). If the caliper pistons are 10x the size of the MC piston, you get 10x the force applied at the MC applied to the caliper pistons(s). If the pistons are 20x the size, you get 20x the force. However, the MC piston has to move farther in the 20x ratio to move the caliper pistons the same distance. So, the tradeoff here is pedal travel v. pedal force for a given caliper force. A smaller MC will give more braking with less effort than a larger MC, but you'll have to push the pedal farther to get that braking. This is why the common 19mm MC "upgrade" isn't much of an upgrade at all. You're requiring MORE force at the pedal to get the same braking as with the stock 17mm MC. However, the 19mm MC gives a firmer pedal that requires less travel, and some people prefer this.

The pad and rotor materials affect the coefficient of friction, which determines how the caliper force is actually turned into frictional force to slow the rotor. Cast iron has a relatively high coefficient of friction, which is why it's used in preference to, say, stainless steel.

Heat is the wild-card. Hot brakes may work better or worse than cold brakes, depending on the pad and rotor materials. Beyond a certain level of heat, the coefficient of friction will always go down unless exotic materials (carbon-matrix pads and rotors) are used, so as you get the brakes really hot, braking will worsen, and you'll be experiencing brake fade. Fade can get bad enough that you press the pedal and nothing happens at all. How much heat is generated depends largly on the weight of the car and the speed you're trying to get rid of. A heavier car will generate more heat (and this rises linearly, so 2x the weight gives 2x the heat load). A faster car trying to slow to a slower speed will generate more heat (and this rises exponentially, so 2x the speed gives 4x the heat load). Whether or not the braking system can get rid of this heat before brake fade becomes a problem depends on many factors, but rotor area, pad area, and airflow across them all figure prominently. Vented rotors give more than 2x the rotor area for a given rotor diameter, so they shed heat very efficiently.