There's a great writeup in the Lupuwali classic thread forum on the maths behind master cylinder vs caliper piston size that specifically talks about this.
brake faqsQUOTE(lapuwali @ Aug 10 2006, 01:50 PM)
What's the detail on bigger calipers? Why don't they help?
The entire idea of the brake system is to provide a mechanical advantage to the force applied by your foot. This is provided in several ways. When you push the pedal, there's a leverage effect based on the distance between the pedal pad and the master cylinder pushrod. The longer this distance, the greater the leverage (and the farther the pedal has to travel to move the pushrod the same distance). The distance between the center of rotation of the rotor and the center of where the brake pad grips provides more leverage, again, the farther the distance, the greater the leverage. Finally, the ratio of the area of the master cylinder piston and the total area of the brake caliper pistons also provides leverage. The greater the difference, the greater the leverage. However, the smaller the master cylinder, the farther the master cylinder piston has to move to push the same volume of fluid. The larger the caliper pistons, the greater the amount of fluid is required to move them the same distance.
So, make the master cylinder smaller, or the caliper pistons larger, and you get more force at the pads for a given foot force, but you have to move your foot farther.
The area of the stock 17mm master cylinder piston is 227sq.mm. The front and rear systems are separate, so you count the front and rear brake areas separately. The stock front caliper piston is 42mm, so the area is 1385sq.mm. So, the ratio of the MC to one caliper piston is 6:1.
The popular 19mm MC has a piston area of 283sq.mm. The ratio with stock front calipers is 4.9:1. We've reduced the ratio, so there's less force at the caliper for a given foot force. However, there's also less travel required by your foot, and the brakes feel "firmer", mostly because you have to push harder to get the car to stop.
So, clearly, ratios of 5-6:1 are reasonable, but the 17mm provides more force for a given effort, where the 19mm requires more pedal force, but not so much that the car is unsafe.
If we moved up to the 23mm MC (415sq.mm) and the ratio drops to 3.3:1. This would require super-human leg strength to lock up the tires.
Now, looking at the BMW 320i caliper "upgrade", or the 911 caliper (same size pistons), both use 48mm pistons, for an area of 1809sq.mm. With a 17mm MC, the ratio is 8:1, with a 19mm MC, 6.4:1, and 4.4:1 with a 23mm MC. A really large ratio means greater force, but the pedal travel is so long that the brakes feel "mushy", even with plenty of bleeding. The 48/19 ratio is better than the stock ratio, but only by 6%. Buying new front calipers and a new MC only made it 6% better than stock. Also, pedal travel will be longer with the 48/19 arrangement than with the stock 42/17 arrangement, which many people find so dissatisfying they insist on switching to the less powerful 42/19 setup just to get better feel.
So, moving to BMW/911 front calipers and a 19mm MC means you spent a fair amount of time, effort, and money to get something really close to stock brakes. This is why this "upgrade" is not a worthwhile idea.
OK, smart guy, what about all of those people who insist it makes a big difference?
It does make a difference, there's no arguing that, the difference is just fairly small. 99% of the time, people saying it makes a big difference are generally switching from tired front calipers that need to be rebuilt. Fitting freshly rebuilt stock front calipers and good pads would very likely make just as big a difference as the one they feel with the "upgrade". The other 1% of the time, well, people often engage in wishful thinking. This is something of a subjective thing, brake feel, so people are going to "feel" something that may not be there. Some people insist that appying green magic marker to the rim of a CD makes it sound better, even though there's absolutely no reason for that to work at all, and no objective test has borne that out.
At some point, even a small difference can make the difference, as it were. If the cost to do the upgrade is worth it to you, then do it. You're not harming anything except your wallet.