Dual master cylinder help - thread size?
 

I've got a 1988 Carrera that I'm converting to a dual master cyclinder set-up with the Fabcar/Smart Tilton set-up. I'm missing some lines and fittings from the kit. I've tried to call Smart Racing and order the lines complete but they haven't returned my call yet.

I need to purchase a T-fitting with to go in in-line with the brake line near the master cylinder. I'm going to use the T-fitting to attach the brake light pressure sensor. Does anyone know the size of the pressure sensor thread? Also can anyone verify that the thread on the lines going into the master cylinder is 10x1.0? The Tilton stuff is AN3 so I have to buy adapters.

Here is a picture of the set-up from the catalog. I don't have the lines and banjo bolts and was just planning on hardlining it straight to the master cylinder without using the soft lines or banjo bolts.


http://forums.pelicanparts.com/uploa...1320683364.jpg

I'll see if I can measure it at lunch without opening the line.

CB Performance carries the adapters and fittings. CB Performance - Online Catalog
Appletree carries the T fittings and brake light switch. VW Brakes – Dune Buggy Braking Systems | AppleTreeAuto.com
Yes, 10x1.0

Reread your post and figured out what you are doing.

The fittings at the master cylinders are standard for the Tilton 74 series:
http://www.tiltonracing.com/content.php?page=list2&id=22&m=b
The important bit is the thread is SAE 3/8"-24 for a 3/16" inverted flare.

Think about this as an option:
http://forums.pelicanparts.com/uploa...1320691308.jpg
Instead of retaining the Tilton's stock bleeder and using a T-fitting use one port for the switches and the other for the brake line. Both ports on the master cylinder are identical. I used a Ford pressure switch that was easier to adapt as both threads are SAE, plus it was cheap and easily available.

What calipers are you using and what size of master cylinders do you have?
The stock MC sizes that they supplied were way too big for my setup.

Thanks for all the info and ideas guys. I just ordered the fittings and parts I need from Pegasus Racing. I'm going to use the bleed port for the pressure switch - good idea Quicksilver.

I have 993tt "big reds". Right now the master cylinders are both 0.7". I have a 5/8" one in a box if I need it too...

The reason I didn't use the bleed port for the switches is there is no fluid flow there and it is vertical. I suspected it would trap a little bit of air there. Using that for the lines also makes it easy to bleed the masters by disconnecting them from the hardlines and putting the ends in the reservoir.

If the dual .7"s give too soft a pedal with the big reds you might want to try a .75" MC for the rear. I am running 5/8" on the front and .7" on the rear but I have the stock calipers. (I prefer a higher pedal effort to give a very hard pedal and to aid in modulation.)

I REALLY recommend getting the remote adjuster. You can grow old and die before getting the balance correctly dialed in without it.

If you want to see what is is like on the car and are up in the OC area get hold of me.

I've got the remote adjuster already, and it is easily reachable while strapped in.

Interesting set up you have, probably due to the difference in 993tt and stock Carrera brake bias. From what I've read and the numbers I've crunched, I am thinking the .7" front and 5/8" rear master cylinders would be more balanced for my car.

I found a good local source for brake line fittings and hose parts, they have a great selection and are WAY cheaper than race shops:

Techna-Fit | Stainless Steel Brake and Clutch Lines

I am thinking the .7" front and 5/8" rear master cylinders would be more balanced for my car.

I run this set up with 930 front & rear calipers.

w/ a dual master set up you put the smaller m/c where you want to raise bias

w/ 993tt's you'll very likely want to raise the rear bias as the rear pistons are very small

you also want to watch the slave/master ratio, The setup on Gumba's car is the same as was used on the SC/RS, which also had 930 brakes and has a nice pedal ratio of 33 w/ 993tts it gives a still nice 33.8. Anywhere in the lower 30s is a good place to be for track use

Thanks for the info.

Bill- I was hoping you might drop in. I learned a lot from your previous brake posts and got my 993tt caliper specs from your Rennlist page. I put together a spreadsheet that optimizes the master cylinder sizes based on brake torque calculated from several vehicle and brake specs (weight transfer, tire & brake friction coefficients, caliper & rotor dimensions, etc) .

How are you calculating the pedal ratio (33.8)? I have a brake pedal leverage ratio of 5.8:1 in my spreadsheet but I'm sure we're talking about different things...

that's not the pedal ratio, the slave/master ration is just the total caliper piston area divided by the total master cylinder piston area. It gives you an indication of 3 things
1) how hard the pedal will be
2) how much effort will be needed
3) the hydraulic amplification in the system

for 993tt the front pistons are 10153.63mm^2 the rears are 4926.02mm^2 the front ratio is 40.9 the rear is 24.9 and the composite is 33.8.

if you compare the front and rear the hyd bias will be 1.64 just about where you want it to be, though if you set the car up right you can go down to ~1.5, of course the bias bar can be used to trim to the back too

if you are using stock 993tt 322mm rotors front and rear the effective rotor diameters are 260mm front and 268mm rear which does help move the bias a little more to the back of the car.

great info, thanks!

You need to multiply the ratio of the piston area ratio by the pedal ratio. That will give full system ratio. There is an additional calculation based on the ratio of the rotor diameter to the tire diameter but not sure if Bill's calc includes that. You really don't need to get that detailed anyway because there isn't that much in choices in master cylinder size.

With Bill's info on the small piston's on rears I would just start with the .7" on both front and rear. You can calculate all of this out three ways to Sunday but at some point you have to try it out and see what is your preference.
- For example: I like a harder, higher effort pedal because I believe you can modulate the pedal faster if it is working more on changes of force as opposed to changes in position.
- Also I feel it gives greater greater granularity in modulation. The difference between 95lbs and 100lbs is pretty easy for most people to distinguish in pedal effort. That is the same percentage difference between 9.5lbs and 10lbs which would be pretty hard to tell.

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In general in a staggered setup the smaller will go on the front brake circuit because the smaller MC produces higher PSI giving a front bias.

The mechanical pedal ratio is certainly needed to calculate the actual brake torque but in this case it is the same no matter what combination of m/c are used so can be ignored as all you are interested in is the relative performance of various m/c.

the mechanical pedal ratio for stock 911s
thru '73 5.4
'74-76 5.8
'77-89 5.2

any one have the ratios for 964 and 993? or any of the newer cars?