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Master Cylinder Question

I have added C2 calipers to the front and rear of my 79 SC. I have read that I must change the M/C to the larger size. Why is the master cylinder change needed. Is it due to the smaller M/C requiring so much pedal force that the seals cannot handle it? Is it due to the smaller M/C cannot generate the required force at the caliper to lock up the brakes. Since it is a closed loop system the only thing I can see is the seals are not up to the force required.
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Old 08-22-2011, 11:14 AM
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I have added C2 calipers to the front and rear of my 79 SC. I have read that I must change the M/C to the larger size. Why is the master cylinder change needed. Is it due to the smaller M/C requiring so much pedal force that the seals cannot handle it? Is it due to the smaller M/C cannot generate the required force at the caliper to lock up the brakes. Since it is a closed loop system the only thing I can see is the seals are not up to the force required.
1) You have a lot of front bias, you have gone from ~ 1.6 to ~2.0

2) your slave/master ratio has gone from ~ 35 to ~ 41, at this point it is not necessary to install a bigger m/c, though it is desirable from a performance perspective. A 930 23.8 mm m/c would move the slave/master ratio to a more desirable 31.

In general a m/c is designed to first move enough fluid to service the calipers and then to provide a hydraulic boost to the system. System pressures regardless of the sizes will be the same. All systems are designed for ~100#s of foot pressure and 70 -100bar of line pressures. Slave/master ratios in the 50s and up don't move enough fluid, in the low 40s are used for street use where more boost is desired because anyone has to be able to drive the car, in the low 30s is desirable for performance use. The lower the ratio the higher and harder the pedal and the more control can be exercised
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Old 08-22-2011, 11:44 AM
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What M/C do you recommend for me to use? VCI has a M/C that is listed as a Mercedes part. PP has a few different sizes. I would like to get the bias as close to stock as possible. 17,19,21,23mm are all sizes I have seen, but from your explanation I see that the size is not the only thing that determines the correct M/C. Thanks for the help!
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Old 08-22-2011, 01:22 PM
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What year 930 would be best for my SC?
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Old 08-22-2011, 01:23 PM
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Rick

"Must" is way too strong a word. Yes, with more piston area both front and rear, you need more MC stroke for the same movement of the pistons. That is simple hydraulic mechanical advantage. If you press the same amount with your foot through a longer distance, you will get more clamping power at the disks. The seals, however, have absolutely nothing to do with any of this, so don't worry about that.

Many, including Bill, who has done the numbers on all of these braking combinations, believe that you want to keep close to the same ratios when going to "bigger" brakes. So if you increase slave cylinder size, you make a proportionalte change in master cylinder sixe. This keeps the hydraulic advantage the same. As a general proposition, one cannot quarrel with that.

Racers/track junkies (autocrossers, too, I imagine) like "modulation." I do these things, and I like modulation. This is the ability to have your foot carefully adjust the pressure the MC is creating. Since you have to press harder on a lower hydraulic advantage system to get the same clamping pressures as on a higher advantage one, you are less likely to overbrake.

When I added the power brake booser system to my '77 Euro (which came without it), I found it was a bit harder to just kiss the brakes. Same with my little street SUV - hard to get just a little brake. I think that is somewhat similar, though not exactly the same, as a modulation issue with high hydraulic advantage.

That being said, I have 4 caliper pistons all the way around on my track car. Not Big Reds or similar overkill, but the total area of each piston set is rather larger than the stock S/M calipers the car came to me with. The MCs were differently sized to achieve bias, with the largest being 3/4" (aka 19mm), though it had a bias bar. I did not notice any adverse effects, including an obnoxious longer pedal stroke, after the switch. Later I changed both MCs to 3/4", and dealt with bias with the adjustable bar.

However, I decided I wanted to give my right leg less exercise, so I dropped the MC sizes to 0.70". I did not notice any difference in pedal height, and certainly was not locking up brakes under heavy straight line braking, nor was I smoking a lifted inside front any more than usual when getting the trail braking a bit wrong. Were it not such a PITA to change my floor level mounted MCs (two, with a clutch MC alongside, all squeezed in by the steering rack, with less than optimal reservoir line attachments) and not cheap, I would try 5/8" MCs to see what amount of hydraulic advantage is really too much.

Because I have come to like the power braking - it feels like I am thrown forward against the shoulder harnesses more than on the track only car. Though a friend suggested that without the power brakes I am pushing myself much harder back into the seat, and hence don't feel the body movement as much in the track car.

So, while not disputing the engineering, nor the experiences of others, I'd suggest you try what you have before you make further changes. However, if you do go to a larger MC, you have the vacuum boost to help there. And with the MC up under the hood, changing it isn't so tough.

What would concern me more is the substantially higher front bias. Traditionally, line pressure bias adjusters have been installed for the rear brakes. In the 3.2s, for instance, there is a handy small in-line valve which limits the rear brake pressure, so after it reaches a certain amount it does not increase much as the fronts clamp harder. This is to prevent the rears from locking up first, which tends to make the car spin. It can even be had in different rates.

You have just transplanted a stock street Porsche system from a later (and heavier) car to your SC. But I'd be a bit concerned, if maximum performance and somewhat even brake pad wear were what I was looking for, with the rears doing too little of the work. Perhaps Bill can elaborate on why Porsche did that. Did they think that a 2.0 ratio was safer for street cars, and did away with the biasing valve? Do C2s have more dive/weight shift to the fronts under heavy braking?

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Old 08-22-2011, 01:54 PM
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The power brake cars can interchange 20.6mm or 23.8mm m/c, the 20.6 was used in all the normal cars, the 23.8 in all the turbo & t/l cars. The non power brake cars use either the 19.05mm, very rare 20.5mm or 23.8mm m/c from ATE, the 23.8 was also used on some trucks

why Porsche has gone to more front brake bias in the newest cars than in the older is a puzzle. There is so much going on w/ the nannies that braking is no longer the only job that brakes are called upon to do. When I talk to Cayman drivers they often complain of having to change their rear pads more frequently than their fronts because of the traction and stability control functions, Porsche is also very upfront about these being street cars not race cars.
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Old 08-22-2011, 03:32 PM
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bill in your opinion I should change the M/C to a 930 unit and that will put me in the correct ratio bias wire.
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Old 08-22-2011, 05:11 PM
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Bill can answer the MC side of it, but I can answer the bias question.

No. As long as a master cylinder has a bore of just one size, a stroke of X length will move the same Y fluid out the front brake hole as it will move out of the rear brake hole.

Only dual MC systems, reserved for racing applications, can use different sized MCs. Or adjust the bias with the bar which connects their operating rods, with the pedal being moved (in effect) closer to one or the other depending on how you want to adjust the bias.

While I can see how it might be possible to construct a tandem (both frong and rear pistons lined up and operated by a single rod from the pedal) master cylinder in a single housing (as they are normally made) while having one of the two "masters" being a different diameter than the other, I've not heard of this being done on any of our P cars. For one thing, it would require a rather longer MC, I think, and that takes up room. But otherwise it woud be more costly to make. Normally, the bias adjustment is done on production cars by picking the caliper piston sizes in relation to the MC bore and other factors affecting braking. Then, if needed, doing something like Porsche did with the 3.2s by introducting a pressure limiter for the rear line.
Old 08-22-2011, 08:34 PM
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Bill can answer the MC side of it, but I can answer the bias question.

No. As long as a master cylinder has a bore of just one size, a stroke of X length will move the same Y fluid out the front brake hole as it will move out of the rear brake hole.

Only dual MC systems, reserved for racing applications, can use different sized MCs. Or adjust the bias with the bar which connects their operating rods, with the pedal being moved (in effect) closer to one or the other depending on how you want to adjust the bias.

While I can see how it might be possible to construct a tandem (both frong and rear pistons lined up and operated by a single rod from the pedal) master cylinder in a single housing (as they are normally made) while having one of the two "masters" being a different diameter than the other, I've not heard of this being done on any of our P cars. For one thing, it would require a rather longer MC, I think, and that takes up room. But otherwise it woud be more costly to make. Normally, the bias adjustment is done on production cars by picking the caliper piston sizes in relation to the MC bore and other factors affecting braking. Then, if needed, doing something like Porsche did with the 3.2s by introducting a pressure limiter for the rear line.
Some of the water cooled cars do use staggered m/c set ups. But Walt is correct the 911/930 m/c do not affect bias in any way.

I just reread the original post, I had missed the fact that you have the 964? C2 calipers front and rear, so depending on what rotors you are using the bias is a more reasonable ~1.7, but the slave/master ratio is a more unreasonable 43, still in the acceptable range but more of a granny pedal than a race pedal, I'd change to a 930 m/c if it were my car. W/ the 930 23.8mm m/c and 964 C2 4 piston calipers f/r the slave/master ratio is 32.3, unboosted that requires a lot of leg, boosted it is very reasonable.

The only thing you can do to change bias at this point is to use different pads f/r. Many if not most of the 996/997/Cayman/Boxster track guys run higher friction pads in back to compensate for the stock front bias issues
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Old 08-23-2011, 05:26 AM
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Let's also not forget that for decades Porsche has been afraid of oversteering litigation, including rear brakes locking before the fronts. Hence the pressure valve in the 84+ Carreras. And 951's as well.

Also, the '90+ cars, whether a C2, C2 Turbo, 993, 996TT etc, all had ABS, so a heavily front biased car still wouldn't lock it's front brakes. Rather, the ABS control unit would do the "leg work" for the driver. So putting those brakes on a car that doesn't have ABS will result in potential front brake lockup.
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Old 08-23-2011, 06:09 AM
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Since the front brakes do more of the stopping then the backs is there an advantage to use the situation to my benefit by putting different pad compunds on the front or should I try to shift more stopping power to the back. In my mind I see the car nosing down during braking and lifting the rear up so more braking in the rear does not seem to buy much.
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Old 08-23-2011, 12:17 PM
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There are guys that have written Phd papers on this subject, but I'm just a grunt engineer and if I understand correctly the only difference I will feel in the force applied to the pedal. I have spent many hours making this conversion as correct as possible. I have not driven any other 911's except for mine so I have only my car with the original brake to compare the pedal feel against.
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Old 08-23-2011, 12:40 PM
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Quote:
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Since the front brakes do more of the stopping then the backs is there an advantage to use the situation to my benefit by putting different pad compunds on the front or should I try to shift more stopping power to the back. In my mind I see the car nosing down during braking and lifting the rear up so more braking in the rear does not seem to buy much.
For street use all the manufacturers want front bias as that is safest for the vast majority of drivers most of the time, but for racing you want to relieve as much of the load from the front as possible, this allows longer life and lighter components. To take advantage of more rear bias the car needs to be set up for it. There was a great article in (I think) 'Racecar Engineering" a few months ago about the ideal bias for max braking in sports car, they considered several different engine layouts from front to mid to rear and for our cars they determined that somewhere around 1.5to 1.4 was the ideal ratio of brake torque front to rear with a properly set up car. Most cars are not set up for this and will want a bit more front just to be safe.
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Old 08-23-2011, 02:48 PM
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Originally Posted by speedman View Post
There are guys that have written Phd papers on this subject, but I'm just a grunt engineer and if I understand correctly the only difference I will feel in the force applied to the pedal. I have spent many hours making this conversion as correct as possible. I have not driven any other 911's except for mine so I have only my car with the original brake to compare the pedal feel against.
It's a little more complicated than that, the ergonomics of the human leg enter into the picture, that's why ~100# max leg pressure is a design goal, a healthy 6' 200+ pound male can exert considerably more than this but an 85 yr old grandmother might have difficulty and the human leg is most efficient at modulating pressure at a fixed position rather than over a moving range.

The lower slave/master ratio reduces the hydraulic multiplication but it also reduces the range of motion of the pedal, it is higher and harder and thus easier to modulate, for street use this is rarely critical and the extra boost is generally appreciated but in a race car where you are braking and using the gas w/ your right leg at the same time while trying to do a max g decel into a corner it is very important
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Old 08-23-2011, 02:56 PM
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