23 mm Master cylinder?, From a 930? Options

[HalfMoon]

A friend of mine who owns a race shop that does alot of Porsche work said when queried about reducing travel on the 914 brake system that the 23mm master cylinder from a 930 would far better reduce the travel over a 19mm master cylinder.
Has anyone done this mod and if so, what were the performance gains/differences?
Thanks
D

[ChrisFoley]

We just swapped out a 23mm M/C for a 22 mm VW unit in a V8 track car we have here with BIG brakes.
I thought the pedal could be modulated fine, with good toe-heel control for downshifts, but the owner wanted it to be a little easier on his leg.
The other option was to install a vacuum brake booster but that would have been more work.

[mgp4591]

We just swapped out a 23mm M/C for a 22 mm VW unit in a V8 track car we have here with BIG brakes.
I thought the pedal could be modulated fine, with good toe-heel control for downshifts, but the owner wanted it to be a little easier on his leg.
The other option was to install a vacuum brake booster but that would have been more work.

Is that the M/C for the Corrado that you swapped for? I've heard good things about those being a workable compromise between the 19mm and the 23mm. I'm running the 944T calipers all around and was thinking it would be a good option for occasional track use besides street functions.

[ChrisFoley]

Is that the M/C for the Corrado that you swapped for?

Yes, but I haven't driven the car yet to see how it feels.

[tomeric914]

20.64mm or 22.22mm Master Cylinder Option How-To thread is in my signature. Yes, I've driven it on track and yes it feels much better given my brake setup.

http://www.914world.com/bbs2/index.php?showtopic=312451

Is that the M/C for the Corrado that you swapped for?

Yes, but I haven't driven the car yet to see how it feels.

[HalfMoon]

20.64mm or 22.22mm Master Cylinder Option How-To thread is in my signature. Yes, I've driven it on track and yes it feels much better given my brake setup.

http://www.914world.com/bbs2/index.php?showtopic=312451

Is that the M/C for the Corrado that you swapped for?

Yes, but I haven't driven the car yet to see how it feels.

WOW!
No that is some cool intel.
Thanks a bunch for sharing that.
I know you did the 20.64, but what's your thought on reduction of pedal throw using the 22.22 and other than cost, what would the advantage be over the 930 23mm? Do you think the 22.22 is slightly less stiff than the 23?
David

[tomeric914]

David,

This is basic hydraulics, but somehow, too many internet "car gurus" screw this up. Bigger isn't better.

A larger diameter master cylinder alone will move more fluid but require more foot pressure to affect brake force value X.

A smaller diameter master cylinder will move less fluid but require less foot pressure to affect the same brake force value X.

It is important to get the ratio correct which requires some calculation involving the lever arm of the pedal assembly to the master cylinder as well as all of the piston sizes in the system being acted upon.

914s came from the factory with a 17mm master cylinder which some perceive as squishy. The next step up is a 19mm from a 911 which makes the pedal firmer but harder to push. Some like that feeling because the brakes are "right there". Jumping to a 23.8mm master cylinder in a stock 914 (for example) would result in an extremely hard pedal that would be very difficult to modulate. You would also need some beefy calf muscles to operate the brakes.

My write up opens the door for even more master cylinder / caliper combinations so that we don't have to jump from a 19mm to a 23.8mm in our little P cars.

For a little help on what combinations work, reference Bill Verberg's brake page to see the combinations that the Porsche factory used.

https://members.rennlist.com/1976c38/brakes.htm

WOW!
No that is some cool intel.
Thanks a bunch for sharing that.
I know you did the 20.64, but what's your thought on reduction of pedal throw using the 22.22 and other than cost, what would the advantage be over the 930 23mm? Do you think the 22.22 is slightly less stiff than the 23?
David

[HalfMoon]

David,

This is basic hydraulics, but somehow, too many internet "car gurus" screw this up. Bigger isn't better.

A larger diameter master cylinder alone will move more fluid but require more foot pressure to affect brake force value X.

A smaller diameter master cylinder will move less fluid but require less foot pressure to affect the same brake force value X.

It is important to get the ratio correct which requires some calculation involving the lever arm of the pedal assembly to the master cylinder as well as all of the piston sizes in the system being acted upon.

914s came from the factory with a 17mm master cylinder which some perceive as squishy. The next step up is a 19mm from a 911 which makes the pedal firmer but harder to push. Some like that feeling because the brakes are "right there". Jumping to a 23.8mm master cylinder in a stock 914 (for example) would result in an extremely hard pedal that would be very difficult to modulate. You would also need some beefy calf muscles to operate the brakes.

My write up opens the door for even more master cylinder / caliper combinations so that we don't have to jump from a 19mm to a 23.8mm in our little P cars.

For a little help on what combinations work, reference Bill Verberg's brake page to see the combinations that the Porsche factory used.

https://members.rennlist.com/1976c38/brakes.htm

WOW!
No that is some cool intel.
Thanks a bunch for sharing that.
I know you did the 20.64, but what's your thought on reduction of pedal throw using the 22.22 and other than cost, what would the advantage be over the 930 23mm? Do you think the 22.22 is slightly less stiff than the 23?
David

Well I can certainly agree with the idea that bigger doesn't necessarily mean better. I think many poster have missed the point of the original post. Perhaps if I put it in caps?
THIS THREAD IS ABOUT REDUCING BRAKE THROW.
Yes, I understand and have understood what this entails (more leg power). It's about thinking out of the box and finding a balance. My brakes are not stock and they do a fine job. I'm trying to find a master cylinder size that can reduce throw yet still have some modulation ability. I currently run a 19mm and find that their's still way to much throw for my taste. I had heard the 23mm out of a 930 would reduce the throw considerably, but some real world reporting is indicating difficulty modulating. Seems like the 22.22 might be a good compromise.

[tomeric914]

THIS IS AN EXAMPLE ONLY!!! You need to do the math for your vehicle.

You could calculate it and determine which is best.

Here are the exact sizes available:



You'll need to determine the stroke that you have by simply measuring how far the piston is pushed into the current master cylinder. Then determine how much reduction in stroke you want. THIS IS STROKE ONLY NOT PEDAL TRAVEL. You'll need to calculate your pedal ratio to determine travel.

We're basically calculating the volume of fluid moved. So, for example:

19mm M/C D = .750"
Current stroke h = .625"

V = pi * (D/2)^2 * h
V = 3.14 * (.750" / 2)^2 * .625"

V = .2759766 in^3

So if we want to reduce the stroke by .125", but move the same volume, we solve for the diameter:

D = 2 * ((V / pi / h)^0.5)
D = 2 * (((.275966 / 3.14 / (.625" - .125"))^0.5)

D = .8385"

This result falls closer to the 20.64mm master cylinder


For pedal travel:

A = Distance from pivot point to middle of push / pull point

B = Distance from pivot to point of push on master cylinder

P = Pivot point

Pedal Ratio is determined by dividing length "A" by length "B"



Pedal travel = M/C stroke * Pedal Ratio

(I recall the 914 brake pedal ratio being something like 5.4 to 1)

THIS THREAD IS ABOUT REDUCING BRAKE THROW.
...I currently run a 19mm and find that their's still way to much throw for my taste. I had heard the 23mm out of a 930 would reduce the throw considerably, but some real world reporting is indicating difficulty modulating. Seems like the 22.22 might be a good compromise.