23 mm Master cylinder?, From a 930? Options

[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.

[mepstein]

To clarify-

I run a 19mm too (as well as a variety of other big brake mods). The primary question I'm pursuing isn't brake "power" (performance gains/differences is/was ancillary), rather it's about "less pedal travel".
That's what the inquiry is really about as well as the advice I had been given.
So I guess the question is for folks who have ACTUALLY done the modification to a 23mm and if they have obtained less pedal travel as a result.
Not asking about theory or opinion, asking about actual experience. Many of us have done the 19mm change and their's reams of information on that mod but not much about the 23mm and pretty much nothing in terms of actual experience.
The main thrust of the question is "brake travel".

The change in pressure and travel when you go to a larger MC isn't theory or opinion. Its a relationship/ratio between the size of the MC and size of the piston calipers. If you keep the calipers the same but increase the size of the MC, the MC will move more fluid with less pedal travel but at the same time, you will need more pressure on the pedal to achieve the same braking force at the caliper. Just like sitting on a see saw, shorten the lever on your side and it gets harder to lift the other end but the travel on your end is shorter. Go too short and at some point, you don't have the force available to move the lever. You also have to make sure the ratio front to back is correct. Its not just about going big, it's about the correct balance.

You won't see much information on World about 23mm MC's because most of the cars simply can't use a MC that large unless they've also switched to very large calipers.

Yep, already knew all that thanks.
So, have you done the conversion and have actual road experience with same and if so did it reduce your brake travel?

Yes, yes, yes, a bigger MC will reduce brake pedal travel. Lots of 911's go through our shop with all different combos. Some of the best working brakes are stock setups that are maintained well. Porsche does their homework when it comes to braking systems. Just like DB Cooper, I raced mountain and road bikes and like a softer, longer travel pedal for less effort and better feel and modulation.

[porschetub]

To clarify-

I run a 19mm too (as well as a variety of other big brake mods). The primary question I'm pursuing isn't brake "power" (performance gains/differences is/was ancillary), rather it's about "less pedal travel".
That's what the inquiry is really about as well as the advice I had been given.
So I guess the question is for folks who have ACTUALLY done the modification to a 23mm and if they have obtained less pedal travel as a result.
Not asking about theory or opinion, asking about actual experience. Many of us have done the 19mm change and their's reams of information on that mod but not much about the 23mm and pretty much nothing in terms of actual experience.
The main thrust of the question is "brake travel".

The change in pressure and travel when you go to a larger MC isn't theory or opinion. Its a relationship/ratio between the size of the MC and size of the piston calipers. If you keep the calipers the same but increase the size of the MC, the MC will move more fluid with less pedal travel but at the same time, you will need more pressure on the pedal to achieve the same braking force at the caliper. Just like sitting on a see saw, shorten the lever on your side and it gets harder to lift the other end but the travel on your end is shorter. Go too short and at some point, you don't have the force available to move the lever. You also have to make sure the ratio front to back is correct. Its not just about going big, it's about the correct balance.

You won't see much information on World about 23mm MC's because most of the cars simply can't use a MC that large unless they've also switched to very large calipers.

Yep, already knew all that thanks.
So, have you done the conversion and have actual road experience with same and if so did it reduce your brake travel?

Yes, yes, yes, a bigger MC will reduce brake pedal travel. Lots of 911's go through our shop with all different combos. Some of the best working brakes are stock setups that are maintained well. Porsche does their homework when it comes to braking systems. Just like DB Cooper, I raced mountain and road bikes and like a softer, longer travel pedal for less effort and better feel and modulation.

(IMG:style_emoticons/default/agree.gif) with all the above,really about overkill,as mentioned a well set up stock system will do the job easily,brake pads for instance have come a long way since the 70's.
We need to remember we have an un-assisted system in these cars,pedal travel may be reduced,is that really needed ???,19mm is more than enough even with bigger calipers...cause I know it works.

[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.

[Dave_Darling]

When you change the diameter of the master cylinder, you are changing the hydraulic advantage over the brake pads. The effect is almost exactly the same as shortening the arm on the pedal. You get less travel but you have to push harder for the same clamping force.

--DD

[tomeric914]

...on a bone stock brake system

When you change the diameter of the master cylinder, you are changing the hydraulic advantage over the brake pads. The effect is almost exactly the same as shortening the arm on the pedal. You get less travel but you have to push harder for the same clamping force.

--DD

[SirAndy]

When you change the diameter of the master cylinder, you are changing the hydraulic advantage over the brake pads. The effect is almost exactly the same as shortening the arm on the pedal. You get less travel but you have to push harder for the same clamping force.

To quote my girlfriend: "How dare you bring logic into this argument!"
(IMG:style_emoticons/default/biggrin.gif)


As for real world experience, i settled for a 19mm MC after replacing the 930 brakes with Boxster brakes.
I prefer the slightly longer travel and less pressure needed as it allows for better brake modulation.

My opinion, based on actual real life experience ...
(IMG:style_emoticons/default/shades.gif)

PS: And i too can put your head through the windshield. Not sure why this is quoted as a positive "must have" but i can easily provide that, if needed.

[Dave_Darling]

...on a bone stock brake system

No, that's on any brake system. If you keep the rest of the system the same in either case, you have to push harder and get less travel. Period. (Up until you have such a massive disproportion between master cylinder and caliper diameters that you cannot move enough fluid to actuate the brakes.)

The same applies in reverse if you just change out the calipers for those with larger piston diameters. You effectively make it the same as a longer lever on the pedal, meaning you do not have to press as hard but you have to move the pedal a greater distance.


The real question is, how is the pedal throw/pedal effort tradeoff to each individual driver? That's a very subjective question...

--DD

[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

[porschetub]

When you change the diameter of the master cylinder, you are changing the hydraulic advantage over the brake pads. The effect is almost exactly the same as shortening the arm on the pedal. You get less travel but you have to push harder for the same clamping force.

To quote my girlfriend: "How dare you bring logic into this argument!"
(IMG:style_emoticons/default/biggrin.gif)


As for real world experience, i settled for a 19mm MC after replacing the 930 brakes with Boxster brakes.
I prefer the slightly longer travel and less pressure needed as it allows for better brake modulation.

My opinion, based on actual real life experience ...
(IMG:style_emoticons/default/shades.gif)

PS: And i too can put your head through the windshield. Not sure why this is quoted as a positive "must have" but i can easily provide that, if needed.

Thats the facts hard to move from that,recent test drive I really overcooked a 80kph corner @ 120kph,quick poke on the pedal and done ..no BS, nothing out of place ,8 pistons pulled me up without fuss all with 19mm ATE m/cylinder.

[ChrisFoley]

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

The 23mm M/C is really 23.8mm in diameter.

[tomeric914]

The reason for going with a larger master cylinder is that something else in the brake system has changed. In my case, I changed out the front and rear calipers to those which have piston areas equal to a 911 which goes to your statement in BOLD below. Please read my write-up.

...on a bone stock brake system

No, that's on any brake system. If you keep the rest of the system the same in either case, you have to push harder and get less travel. Period. (Up until you have such a massive disproportion between master cylinder and caliper diameters that you cannot move enough fluid to actuate the brakes.)

The same applies in reverse if you just change out the calipers for those with larger piston diameters. You effectively make it the same as a longer lever on the pedal, meaning you do not have to press as hard but you have to move the pedal a greater distance.


The real question is, how is the pedal throw/pedal effort tradeoff to each individual driver? That's a very subjective question...

--DD

[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.

[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.

[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?

[ChrisFoley]

So do those valves ever go bad?

They don't go bad but I suspect the spring loses some stiffness. One can feel/hear where the regulator begins reducing the rear braking (while stationary) by depressing the pedal with gradually increasing force and then releasing at the same rate. The spring makes a noticeable sound.
Between higher grip modern tires and aging of the spring, I like to increase the regulator setting by about 1/4" on the set screw for many of my customers.
This isn't easy because the allen set screw is loctited in place. I usually replace the set screw with a longer allen cap screw.

I prefer this mod over removing the regulator altogether.