Brake Master Cylinder, 17mm vs 23mm Options

[LotusJoe]

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.

[RJMII]

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.

[LotusJoe]

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