converting to dual M/C's - size recommendation needed
 

Hi all,

I'm replacing the 23mm Merc Benz master cylinder in my RS rep with adjustable twin M/C's - can anyone recommend suitable sizes?

On the front, they will be squeezing Boxster front calipers on 3.2 discs and, on the rear, 3.2 calipers/discs. The current balance is good (slightly more front bias than stock) in the dry but can be improved in the wet.

You already have a nice high hard pedal. A bigger master piston will only amplify that.

If you want more front bias use a 23.8mm front and 25.5mm rear, this wll require a bit more leg

or

for less leg effort and incresed front bias uses 22.23mm front and 23.8mm rear.

The other adjustment is in the bar, you can do that w/ a dash mounted knob.

I would stay w/ 23.8mm f/ and r and do bias trim w/ a knob .

thanks bill

Bill,

Would you recommend the same twin master cylinder sizes for 930 calipers?

Thx

Martin

A couple questions:
- What dual master setup are you using? (How is it laid out?)
- Does your rear brake circuit have the 3.2 Carrera proportioning valve in it?

If the geometry of the pedal is such that the master cylinders have the same quantity of movement as the stock pedal (like the Fabcar (SmartRacing) unit) then you want the piston AREA of each piston to be 1/2 of the AREA of the single master cylinder that you currently like. The area of the piston is the key to pressure as opposed to the diameter.

A single master takes all of the force coming from the pedal assembly and converts it directly to the output pressure (Force divided by sqr inches equals PSI). In a dual master arrangement with a balance bar each master gets 1/2 of the input force so to get an equivalent line pressure you must cut the area by 1/2.

If your car is using the Carrera proportioning valve you will want to take it out because it reduces the pressure and the bias bar is way better balance the front/rear pressures.

930 calipers have such small pistons that is diffficult to size a twin master for them.

The smallest Tilton is 5/8"(17.78mm) which will require a whale of a leg to operate.

the 996 & 997 RSRs use from 17.8/17.8 to 18.8/18.8 to 20.6/19.05 depending on caliper/course/conditions but their larger caliper pistons can use it w/o spiking pedal effort.

The 930s have great native bias and don't need a twin master setup. I'd just use a 23.8 twin circuit(stock 930) and be done w/ it.

Bill,

Do RSR calipers have similiar piston areas to the 930 calipers and if so what twin master cylinder configuration was used on the 73RSR or later SCRS's?

Thx

Martin

Martin,

RSR caliper piston sizes are VERY close to 930 calipers, if not the same. Next time I go digging into the parts bin, I'll confirm that.

3.0 RSR's used 22mm/17mm dual MC's and the 911SC/RS had a 17.78mm/15.87mm ones.

IMHO, the 911SC/RS setup is better for the 930 calipers, based on personal experience & preference.

AFAIK, yes, but that information has been very difficult to acquire. I have complete documentation from, ~ '95 - '09 and spotty before that.

You can use 5/8/5/8"(15.875/15.875mm)but anything bigger is going to be too big. Why bother as you still have the same line pressure f/r. You would have adjustability via the bar but again stock 930 has great native bias and works great w/ a 23.8mm dual master

Seve please check that, the #s are unreasonable, The SC/RS #s you provided are right on the mark.

Each person will have personal preferences I apparently prefer much more rear than Steve

Steve,

Appreciate the info.

For the RSR set-up are we talking 22mm front and 17mm rear or the reverse?

Thx

Martin

The 23mm master that you are currently using, is it the same as the upgraded ones sold by our host or is it a different model. Do you know what year or part# is on it to try and pick one up here locally

I am not running a 23mm 930 or MB MC, having recently replaced my Bremtek setup (on a 72T) with 930 calipers and dual MC set-up which now needs to be dialled in with the correct cylinder sizes.

Martin

Sorry, my question was to the original poster but I now see this thread is from 2005, although he is probably ready for a new one by now

That is backwards. The smaller the cylinder the less the pedal pressure and the higher the line pressure. A small cylinder will cause a light pedal and a large pedal throw.

The multiplication of force from the master cylinder is a little counter intuitive.

F = pushrod force.
A = piston face area
L = line pressure

The equation is F / A = L
Examples" If you have a piston area of 1" and 100lbs of input force you will get 100 psi.
If you have a piston area of .5" and 100lbs of input force you will get 200 psi.
"Smaller is bigger."


I have had the Fabcar setup on my 87 for about 8 years now. The pedal is higher effort then stock which also gives a lot less pedal movement. That lets you modulate faster because it is more "changes in force" as opposed to "movement of foot".
My car is downstairs. I'll run down and get the cylinder sizes off of it. My cylinders are staggered so it will be a bit different. I am compensating for the removal of the proportioning valve. To really compare we would need to calculate the difference in piston area with the boxster calipers and the difference in the diameter of the center of the friction surface of the rotor.
Bill is the expert on those numbers.

First off... 2005??? (geeze...)

On mine:
The front circuit is using a 5/8" cylinder (.3068 sqr")
The rear circuit is using a .7" cylinder (.3848 sqr") (The larger cylinder helps compensate for the Carrera portioning valve)
Which gives a total piston area of .6916 sqr" This gives a noticeably higher level of pedal effort then stock on a Carrera but the wife can drive it.

A 23mm single cylinder gives a piston area of .6440 sqr".
(Remember the larger the piston area the less the braking force.)

For a 930 you are removing the power brake booster so that will get rid of the some of your mechanical advantage. As Bill mentioned the bias is pretty equal on the 930 setup so 2 identical cylinders would be better. You can use 2 unequal cylinders and crank the bias over to equalize the line pressures but that is really breaking the rules.

2ea 5/8" cylinders (total piston area of .6136 sqr") will give you a more stock pedal effort. I would lean towards 2ea .7" cylinders (total piston area of .7697 sqr") which will increase the pedal effort but will make the pedal a lot harder. This will make your brake feel a lot more sensitive and your modulation more accurate.
(Note that the .7" cylinders will give you a 25% increase in pedal effort...)

The way to think about it is: How well can you tell the difference between .95 lbs of pedal force and 1 lbs of pedal force? How much easier is it to tell the difference between 95 lbs of force and 100 lbs of force?
They are both 5% differences but as you get closer to your limits of strength it is much easier to differentiate small percentage differences in force.

Remember that you need a large reserve of leg strength for safety, fatigue, and comfort. Also the cylinders are relatively cheap so don't be afraid to try different combos.

there are 2x 23.8mm m/c available for use on 911, which to use depends on whether you have power brakes or not.

If you have power brakes you use the '78 - 89 930 m/c #930-355-011-03(Wayne sells these)

If you don't have power brakes you use ATE # 03-2123-3402.3(for some reason Wayne doesn't sell these) they are available in a kit(you do want the kit) from Smart Racing)

I understand all that, my point was that the smallest readily available m/c that I know of for the most common twin master setups is the 5/8" Tilton(AP does have 14 & 15mm but I don't know if they are adaptable to your setup), which will still require a lot of leg w/ 930 calipers, any bigger m/c just increases the amount of leg that would be needed. using 7/10 front and 5/8 rear would be right at the edge of what a very healthy 200+ # male will be able to use for more than a few stops.

so JMO use 2x 5/8" Tiltons

Ahhh, misunderstood... (and that has never happened to me before!:p)

The 5/8 should be drivable by anyone who is up for a "sporty ride". The 5/8 and .7 combo is an effort but it is doable by my 135lb wife but it definitely isn't her preference. It is fine with me for road trips and I have no cartilage in the balls of my feet and the knees are totally out of warranty. I suspect the bite of the pads might have something to do with it too. I'm using Porterfield R4S pads. Not sure how their C/F compare to others. Steve should know off the top of his head.
I do agree that I wouldn't split the sizes for a 930.

I thought Girling used to make a smaller cylinder but I can't find it right now. I haven't used Girling for over 20 years and I had a durability problem with them. (probably the kinematics of the pedal setup)

I can't remember what comes with the Fabcar setup but I remember that they were too big. (maybe .7" and 11/16" ???) Probably for people using larger calipers and larger diameter rotors...

A couple important details are:
- A bias bar setup doesn't have the built in safeties if one circuit fails. You must inspect it regularly for both mechanical and hydraulic problems. A failure will be very exciting.
- A remote adjuster is really required otherwise it will take forever to get the bias dialed in. I found putting the knob where the ashtray was works real well. You have to keep peoples hands off it though...

Thanks for all the feedback - there's clearly a little more to this than I originally anticipated when I decided to mate the 930' calipers to a twin MC.

Anyway, I'm still a little confused as to why the 22mm+17m f/r (or is it 17mm+22mm f/r?) combination used by the 73 RSR doesn't work well for a 930 set-up which has (I understand) a similiar caliper configuration - am I missing something?

Martin