23mm master cylinder, out of what??? Options

[stock93]

What car did the 23mm master cylinder that will bolt into our cars come out of?

John

[lapuwali]

CODE

914/4 & A rear  320i/M front   RX-7 4piston  Boxster front  M rear                                   42x4                 48x4              36x8               35x8               36x4 stock 914/4       17      24                    32                   36                  34                  18 stock early 911 19       20                    26                  29                  27                  14 mercedes          23       13                    17                  20                  19                 10

If the table above looks all messed up, it's Andy's fault. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
I could attach the spreadsheet, too, I suppose.

Down the left are the three most commonly used MCs. Across the top are the brake calipers I have data for.

The numbers in the table are the ratios of the MC piston area to the total caliper piston area for the front or rear half of the circuit. Note the stock 914/4 ratio of 24:1. With a smaller number, pedal effort increases for a given braking force, but pedal travel decreases. If the number gets too small, you need more pressure than you can physically exert on the pedal to lock the tires. If the number gets too big, the pedal hits the stop before you're generating max braking power (actually, before it gets there, the pedal will usually feel all mushy, like it needs bleeding). So, you're trying to strike a balance between pedal effort and feel. Most people like a harder pedal (smaller number), but most people don't have the leg muscles of a Mr. Universe qualifier (number too small). For non-servo'd brakes, the range 19-26:1 seems to be a good one, with personal choice determining where along that line you want to be.

See that the 19mm with stock 914/4 calipers requires more pedal effort, but less travel. Some people like this setup, with a 20:1 ratio, and I can say it's adequate but not ideal (PO fitted this to my car). I can just barely lock the tires with big effort, but day to day the brakes feel OK.

Note that with with the 320i/M fronts/A fronts (the 48x2 calipers), you get more braking force with slightly longer travel using a 19mm MC. This is why so many people like this setup. Less pedal effort than stock, but not so little you have to practically push the pedal through the floor to get full braking.

The RX-7 or Boxster calipers require a 23mm MC just to get down to the 19mm/stock setup. With a 17mm MC, I'd expect this setup to be very touchy and feel awful, requiring a looong pedal to get braking effort. The Boxster fronts with the 19mm MC will feel a bit more powerful than the 320i/M caliper w/ the 19mm setup. I would think a 21mm MC (if such a beast exists or could be made) would be about ideal for these calipers.

Some rear calipers are also included, and you'll note how small the numbers are. This indicates much less braking force for those calipers, which is how you get front/rear brake balance w/o a prop valve. Also note that adding a bigger MC with bigger front calipers REDUCES rear braking even more, moving the brake bias forward. This is why removing the prop valve is usually helpful in such cases. To keep brake balance, you need to increase the size of the rears as well as the fronts when upgrading.