I understand everything there is to know about sizing brake masters to match the bores in the calipers and wheel cylinders. I understand residual valves and pedal ratio and forces generated, and all that other technical stuff you get out of books. What I don't understand is what the difference is if you have the same Wilwood calipers with different bore sizes. Assuming the masters are sized so that the caliper pistons see the same pressure, whats the difference? I know if you have the same pressure over a larger area, you have more force, but what does that mean to stopping the car. If all is equal and you have a set of calipers with 1.38 pistons and one with 1.75 pistons, does it take more or less pedal effort to stop the car? So whats better? smaller pistons with smaller master or larger pistons with larger master? Should I go with 1.38 or 1.75, or should I just hedge my bet and go right in the middle with 1.62. Sized a bunch of brakes before, but one of the variables was always fixed. This is the first time I've gotten to choose everything from scratch. Main goal is to achieve a braking system where I can modulate the brakes the easiest without excessive pedal travel, or not enough pedal travel. My gut is telling me bigger masters/caliper bores are for heavier cars, and smaller masters/caliper bores are for smaller cars.
Every manual brake car I've paid attention to used 1.75 calipers.
1.38 is the stock bore size for most Japanese 4-piston calipers from the 80s/90s. They had power brakes and needed it.
Basically, you get to pick one. Make your decision on what's available to you - if your chosen type of master only comes in three sizes, pick the middle size so you can adjust up or down. It's cheaper and easier to change the master than the calipers.
The only advantage I can see to running a big master and calipers is that you'd move more fluid every time you hit the brakes, which might protect the fluid slightly from overheating due to the higher volume. But really, it's the ratio between the two that matters. Hydraulic ratios are just like lever ratios, they work the same way.
That's your first question. Then you started asking what happens, all else being equal, if you increase the piston area in the calipers. That's easy - changing to bigger piston sizes in the calipers without changing the size of the master or the pedal ratio will give you more brake torque for a given pedal pressure, but the pedal will move further. It's that lever tradeoff.
The more I think about this the more I'm leaning towards bigger is better and heres why. The force in line pressure is exerted in pounds per square inch. If you have the same line pressure in each sized piston, the one with the larger piston will exert a greater total force on the pads. That means it will take less line pressure to do the same amount of work. Less line pressure means less pressure on the pedals, but that can be messed with depending on MC bore size. But, the larger the MC, the harder you have to push to get that line pressure. My head is spinning. I have a set of 1.62 and a set of 1.38 sitting new here in the shop. I'll pick up a set of 1.75s for the front and use the 1.38 in the rear. If anything doesn't work, I can swap up or down with the set of 1.62s I have, and like Keith said, MCs are cheap. Done, on to the next decision to be made.
You're confusing yourself :) It's the ratio between the various bore sizes that matters. They all be massive or they can all be small, the end result (in terms of pedal pressure and travel) will be the same.
It's simple to put together a spreadsheet that lets you figure out the amount of pressure on the pad for X pounds of pressure on the pedal as well as the amount of pedal travel for 0.0Y" of pad travel. That may help get your head straight. If you don't know the pedal ratio, it'll probably be between 4:1 and 6:1, and it'll be fixed anyhow.
[Getoffmylawn]
pedal FORCE, not pedal PRESSURE!
[/getoffmylawn]
