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

I'm assuming your brake system is nowhere near stock?

I'd not go bigger then 19mm for a stock or near stock braking system.

Zach

That would be correct sir, and I also run a sbc so "mo better" braking is always sought after.

Really to big ,running 19mm with huge brakes ,really too large for what ever brake setup you have,my car stops really well trust me running these on 19mm ATE cylinder;
Click to view attachment
I have these front and rear ,larger M/Cs are just a what folks talk about like the BS BMW caliper upgrade...waste of $$$$.
Cheers go well.

I'm running a 23 but I also have 930 brakes. It is a very stiff pedal. Not easy to modulate but I can put your head through the windshield if I slam on the brakes...

Agree with all the above. I am running 23mm M/C with Carrera brakes; very stiff pedal which what I like; can put you through the windshield as Mike as stated; requires a strong leg pressure; 19mm would work with my setup but I do not like the pedal (had it on prior to 23mm).

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".

Thank you for your report based in actual experience :-)

Again, thank you for your real world experience report.

It’s not a 930 MC is a 23 mm ATE MC for a Mercedes, the 930 23 mm MC is for power assisted brakes. I did the conversion on my early 930, replaced the aluminum s brakes and stock 19 mm MC for turbo brakes and the 23 mm MC. It’s stiff with the 930 brakes, I wouldn’t want it on anything less, as for travel, yes less travel, but more pressure to achieve the same results. I also had to fab a spacer as the rod actuator needs to be longer to work on the 23 mm MC.

I can heel toe on about anything, but I always found it easier when there’s a bit more travel on the brake...

You can also adjust the pedal lower to reduce overall travel...

(Edited again after MC size confirmation)

Sorry had to edit above...

My old 911SC track car had turbo brakes front and rear with a 23mm Mercedes master cylinder. It was a great setup. Never had to worry about having enough brakes or fade. Look at the VCI website. Doug sells a 23mm master cylinder kit as well as lots of other brake candy. He is a member here.

Charlie

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.

One small correction: You need to exert more force on the pedal to have the same brake line pressure due to larger MC area. A 23mm MC requires 1.47x more force for the same line pressure as a 19mm (needed if your calipers are unchanged during the swap). You also have 1.5x less travel.

Right. There's no free ride. The later 911's (at least most of the air cooled cars) with bigger brakes used a vacuum assisted brake booster.

Too big on the MC and you loose the ability to modulate the brakes. They become an on-off switch because you have to use your quads to force the pedal down instead of your smaller foot,ankle and calf muscles.

And, important to note, with just a larger m/c your brakes will NOT have any additional braking force, the car will stop exactly the same, brake fade will be exactly the same, the pedal will simply be harder to push.

I come from sport bikes. Have you seen them do stoppies, a wheelstand on the front wheel while braking? Requires a HUGE amount of control to keep it right on that edge, and the only way that's possible is with modulation, i.e. a long lever (or pedal), which is the exact opposite of what you get with a larger 23mm master on the same calipers. Nothing's free, so with a harder pedal and less modulation what you give up is control. You have the same amount of brake so you can still "go through the windshield" but it's more binary, on-off, and that's not usually desirable. For me, anyway. Tastes differ, as always.

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.

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?

Thanks for the valuable feedback Chris.
Your opinion is appreciated (and further valued in that it's backed up with ACTUAL experience)

OK, when I got my car the pedal was way too hard. It sucked, made the car unpleasant to drive and the braking less accurate, so I changed it back to a 17mm m/c. That's my ACTUAL experience.

irony is that I later changed to Tilton pedals (for a hydraulic clutch), and the brakes are back to too hard. So it sucks again.

One more note, this is my opinion based on my experience. And I think that's what most of the advice you've gotten is, opinion based on experience. Bottom line is that I'm right and anyone who disagrees with me is wrong, so factor that into your thinking.

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.

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

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.

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.

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

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

...on a bone stock brake system

To quote my girlfriend: "How dare you bring logic into this argument!"



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 ...


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.

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

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

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.

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

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.

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

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.

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.

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

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:

Click to view attachment

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"

Click to view attachment

Pedal travel = M/C stroke * Pedal Ratio

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

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?

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.

Is the lack of a proportioning valve on the 911s because of the increased rear weight bias? It always made me wonder why the 914 had one and the 911 (or VW Beatles) didn’t.