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.

[Keyser Sose]

I currently run a 19mm and find that their's still way to much throw for my taste.

When I hear that my first thought is to suggest you try re-bleeding your brakes. That's not a flip comment, the bias valve can make it hard to get them fully bled. But if they're good and you really think 19mm is too small then go all the way to 23mm, what the heck? You want hard brakes? That'll do it. As everyone's said it's subjective, so no one else is going to be able to tell you what you like best. Trying the 23 will at the minimum give you some end points for comparison, to see what YOU really like.

[ChrisFoley]

the bias valve can make it hard to get them fully bled.

That's an old wive's tale which is total BS.

[Keyser Sose]

the bias valve can make it hard to get them fully bled.

That's an old wive's tale which is total BS.

i'll defer to your experience, but that was advice I got here and my consistently spongy pedal improved immediately when I removed the regulator. So do those valves ever go bad?