I'm considering upgrading my master cylinder. I have installed the bigger BMW 320 Front Calipers and have Stock Calipers on the rear. I see from other threads that the 19mm master cylinder is a common upgrade, but what about 23mm. Automotion sells a 23mm as an upgrade, but it seem to me that it may to much for my application.

Do a search, plenty of arguments posts about it.

With the BMW calipers, the piston diameter is 48 mm. Here is a summary of differences between 17mm, 19mm, and 23 mm. (provided you have stock rears)

first is the 17mm (stock)
With 30 pounds of force on the pedal (at the rod for the brake master cylinder, not taking into account the increased force applied by the lever aka pedal arm) the system pressure is 28psi, while the pistons in the front calipers are getting 84pounds of force and the rears are getting 58pounds of force.
19mm:
With 30 pounds of force on the pedal (at the rod for the brake master cylinder, not taking into account the increased force applied by the lever aka pedal arm) the system pressure is 25psi, while the pistons in the front calipers are getting 75pounds of force and the rears are getting 52pounds of force.
for the 23mm:
With 30 pounds of force on the pedal (at the rod for the brake master cylinder, not taking into account the increased force applied by the lever aka pedal arm) the system pressure is 21psi, while the pistons in the front calipers are getting 62pounds of force and the rears are getting 43pounds of force.

I've been working on a Flash app (with the assistance of Eric at PMB) to show the differences in how much pressure is required to get the same amount of force at the pad, and how much pedal movement is needed for each option as well. But you can see from the numbers here that it's quite a force drop (which means more leg strength is required to get the same force at the pad) by going with the 23 mm m/c.

It would appear I'm better off staying with the 17mm Master Cylinder.

Not necessarily,

The formula I am still working on for this particular application is the one that shows how much fluid is required to make the pads go from a state of rest to a state of applying force.

More math done...

the surface area of the 17mm mastercylinder is 1.05 square inches.
The surface area of the 19mm mastercylinder is 1.18 square inches.
The surface area of the 23mm mastercylinder is 1.42 square inches.

The total surface area of the pistons in your setup is 20 square inches. In order to move the 8 calipers the .004 to .008 inches necessary to stop your car, you would need to move the piston in the m/c the following distances for each of the m/cs:

17mm: .076 - .152 inches
19mm: .068 - .136 inches
23mm: .056 - .113 inches

edit: Take note, this is the piston movement required. The pedal movement increases according to the lever formula... (note to self: measure pedal arm lengths and include in app)
My personal setup has the 48mm pistons up front as well, and I really like the feel of the 19mm m/c that I have.

The fluid volume is a key element as is the pad area in contact with your rotors. I have a 23mm cylinder because I have big brakes on the front and Carrera calipers on the rear and even the 19mm cylinder had too much pedal travel.

I'm very happy with my current pedal resistance and am close to installing big aluminum brakes on the rear as well. That'll place my pedal a bit closer to the floor, but actually in a better position for heel-n-toe braking/shifting.

A 19mm cylinder was on the car when I bought it with big brakes already on the front and stock 4-cylinder rear calipers. It was excellent.

Read this classic thread.

RIP Lapuwali. Gone too soon.

I better recheck my formulas... there are some inconsistencies between his math and mine. (then again, his is all metric; except the pressure ratios are different than what I came up with) and I should check my ability to formulate complete sentences.

More importantly - stick with ATE or another OEM supplier, not the Chinese junk!

The 19 mm 914-6/911 MC is significantly less than the 17 mm 914/4 only MC from ATE, so I went to the former for slightly better system performance overall at a better price for OEM parts, but according to Eric Shea at PMB, just as important or more so are those great & expensive Porterfield P4S pads!

BTW - if you haven't done so yet, send your calipers to Eric to resto for your resto. Joe, they look like new & include a re-Dad plating too!

I ran the stock front calipers on the rears and 320 calipers on the fronts. Ran a Brass T and a 19mm master cylinder. Brakes felt awesome, did not have weird tendencies and would lock down some sticky 225-50/15 tires reliably.

Now I just did complete new calipers, pads and lines on a local car. After bleeding and adjusting, the brakes stop the car just fine, they still feel a little mushy compared to other cars I drive, but that is also a factor of power vs. non-power.

Pads make a huge difference. But remember this, Brakes don't stop the car. Tires do. Want to stop faster, get stickier tires.

I am running a 19MM Master cylinder on my car with 320i front and stock rear calipers and am very happy with my brakes. I can lock up all 4 tires no problem. There is a lot of controversy about how effective this setup is. I think Eric's math says the improvement is about 6% with this set up over stock. The one thing I would NOT do is take out the rear proportioning valve and put a "T" fitting in rear. Doing this can cause your rear brakes to lock up sooner than the fronts causing you to spin out. It's very dangerous.

Anyways, I'm not an engineer, but I am very happy with the 320i 19mm setup you are looking at.

Slight correction: Pressure in a closed system is the same at all points. 30 PSI at the master cylinder is 30 PSI at the brake caliper. Pressure x surface area will give you the (total) force on the piston. The inverse will give you the travel of the piston.

I found this interesting article a while ago. It is the best explanation of the caliper question I have seen. Thought it might be helpful, probably linked somewhere before.

914 brake upgrade comparisons

ugh.

slight correction: Todd needs to figure in the dispersement of the pressure from the outside of the system into the system. 30 pounds on the cylinder shaft does not = 30 psi on the inside of the system.

One of more often overlooked issues surrounding a change in the master, is the issue of pedal travel. Given no change in the piston diameter in the caliper, installing a larger master will allow for less pedal travel when exerting the same amount of downstream force. The brakes will definitely -feel- harder. However, the trade off is that with less pedal travel per unit of force applied, it will be that much harder to modulate braking force with your foot.

But, as Jake says about engines... "It's all about the combo". It's master/caliper/pad/tire all working in unison. Crappy tires (as Brett so wisely commented), will result in crappy braking. Same can be said about any other component in that path.

-Josh2

Correction of what? As far as I can tell no one's posted erroneous info WRT distribution of pressure inside the hydraulic brake system.

Not since Jim edited his post and changed "psi" to "pounds of force".

I didn't mean to start a pissing contest, but before you edited your post, you were wrong. Note that I didn't say 30 pounds = 30 PSI. I just happened to use one of the numbers that you used. What you said, originally, was that "the system pressure is 28psi, while the pistons in the front calipers are getting 84psi and the rears are getting 58psi", which is not correct. By editing your post, you corrected it. Not sure why you got so defensive... obviously my post made you realize your mistake since you corrected it.

blah blah blah nit pick blah blah blah.
I did the math and helped someone out. so what if I was half asleep and accidentally put 'psi' instead of 'pounds of force'. simple fluid dynamics are probably over my head now, too.

I guess I was late to the party again, lol.

I have the 320I brakes and took off the 23mm MC and replaced it with 19. Best thing I ever did. Bigger is not always better, don't get the 23mm.