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964 c4 master cylinder and proportioning with big brake upgrade

I have a 964 c4 and am considering upgrading my brakes. I have done a lot of reading on big reds and brake upgrades on the 964. There are two things that never seem to be addressed in the c4 threads.

1. When changing calipers with increased piston sizes, do you loose brake pedal. I am still getting a handle on how the braking system works with the hydraulic pump. Will the pump run more and maintain the correct pressures? or is it recommend to replace the master cylinder? Is there a maximum caliper piston size the stock master cylinder can handle while still maintains a proper pedal feel.?

2. There is some talk about changing fronts and rears in different disc diameters and calipers with different piston size to maintain proper braking bias. Is there no way that a manual proportioning valve could be used to adjust the brake bias when using different brake setups?


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Old 03-05-2018, 08:30 PM
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964 C4 has the 23.8mm m/c stock, this is sufficiently large for most BBK's, the 25.4mm 964RS m/c boltss in if desired

What happens when more caliper volume is installed w/o changing m/c is the pedal gets softer w/ more travel and reduced effort. No the pump does not compensate all it does is reduce effort more

there is a rance of m/c to caliper sizes that works w/i that range the effort and travel vary. For sporting use the rato wants to be ~ 30 usual street ratio is in the upper 30's to lower 40s when you get to ~45 that's too far. On the low end the further into the 20s the more effort is needed, at some point you run out of leg, that point is different for each driver.

brake torque is a function of caliper size, effective radius of the rotor & Mu
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Old 03-06-2018, 03:30 AM
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Hi bill thanks for the great response.
I am wondering how you calculate that ratio. I assume it is something to do with total caliper piston area compared to master cylinder piston area.


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Old 03-06-2018, 02:55 PM
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My 964 has big red calipers with 993 master cylinder

Hope this helps
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Old 03-06-2018, 05:45 PM
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Quote:
Originally Posted by Maych87 View Post
Hi bill thanks for the great response.
I am wondering how you calculate that ratio. I assume it is something to do with total caliper piston area compared to master cylinder piston area.


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That's correct

pedal ratio = (total caliper piston area)/(master piston area)

It should be between ~30 to 40, lower is better for sportin use but also requires more leg, power assist allows lower but even w/ power ~ 27 is a lower limit and ~45 an upper
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Old 03-07-2018, 05:45 AM
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