Brakes, please help!

[zakmrk2]

Hi all, I'm building a Porsche 911 race car from an 86 3,2 Carrera and I need to change the brakes. I have on the shelf a pair of 930 turbo front Calipers and a pair of 993 black rear calipers, Would the bias be acceptable on piston size etc once the conversion is done. I understand that I may need to run 930 rotors front and back, or di I. I've need seen this odd combination before, would it work guys??

[Bill Verburg]

Originally Posted by zakmrk2

Hi all, I'm building a Porsche 911 race car from an 86 3,2 Carrera and I need to change the brakes. I have on the shelf a pair of 930 turbo front Calipers and a pair of 993 black rear calipers, Would the bias be acceptable on piston size etc once the conversion is done. I understand that I may need to run 930 rotors front and back, or di I. I've need seen this odd combination before, would it work guys??

930/993 on 930 rotors is a little more than usual rear bias, worse comes to worse put an adjustable pressure valve on the rear circuit

[zakmrk2]

Great, thanks for the quick response Bill, just what I wanted to hear.

[ivangene]

Bill knows his brakes - and knows mine - he has helped many of us

[Porsche84]

Swap out your master cylinder to the turbo one. You'll need to push more volume to operate the bigger calipers. I run big red fronts (993 TT) on the front and 944T front calipers on the rear with 930 rear rotors. The balance is really good. I bought a proportioning valve but have not installed because I like the balance. My car is an 84 911 and weighs 2250.

[Bill Verburg]

certainly you need a 23.8mm m/c, the 930 is a bolt in for you

Jerry certainly has way more rear bias then is normal

here are some #S for comparison brake torque in N @70bar line pressure
stock '86
w/o the stock 33bar p/v 1444/1183 for a ratio of 1.2
w/ the stock 33bar p/v 1444/639 for a ratio of 2.26

'74 2.7RS
1444/969 for a ratio of 1.491

930 front 993 rear all on 930 rotors
w/o the stock 33bar p/v 1961/1242 for a ratio of 1.382
w/ the stock 33bar p/v 1961/766 for a ratio of 2.56

993RS w/abs
w/o the stock 55bar p/v's 2306/1618 for a ratio of 1.426
w/ the stock 55bar p/v's 2306/874 for a ratio of 2.64

Jerry's 993tt/944f
w/o a p/v 2306/2230 for a ratio of 1.034

the lower the ratio the lower/stiffer the car needs to be and the effective the lsd needs to be on decel

[Porsche84]

Wow, good info Bill. Did you calculate my fronts using 993 TT front rotors? Maybe I do need to install the bias valve and see the difference.

[winders]

Originally Posted by Porsche84

Swap out your master cylinder to the turbo one. You'll need to push more volume to operate the bigger calipers. I run big red fronts (993 TT) on the front and 944T front calipers on the rear with 930 rear rotors. The balance is really good. I bought a proportioning valve but have not installed because I like the balance. My car is an 84 911 and weighs 2250.

Hey Jerry, I see you have your race car setup with a near 1:1 bias ratio just like my race car. I kept thinking about getting a pedal cluster and making my own 911SC/RS style dual master cylinder setup. But my brake bias is damn near perfect on the track so I didn't follow through with it. It's good to see that "California physics" work in Florida too. Maybe the state needs to grow a lot of oranges for that bias ratio to work.

Scott

[winders]

Originally Posted by Porsche84

Maybe I do need to install the bias valve and see the difference.

Why? Are you rear brakes locking up on you when your fronts don't? Are you inducing oversteer on braking?

Scott

[Porsche84]

Scott, glad to hear. Come to Florida and we can compare at Sebring

[winders]

Originally Posted by Porsche84

Scott, glad to hear. Come to Florida and we can compare at Sebring

I would love to, but traveling to Sebring just isn't in the race budget. Even if it were, my race is built for a particular Porsche Racing Club (PRC) class. The car would be in PCA GT3 but the engine is way under developed under GT3 rules.

Have fun!

Scott

[zakmrk2]

Ok Bill, great info, a few more answers on this please.
You mentioned a 23.8 mm M/C, Is this a kit to fit the 993 calipers to stock 3.2 trailing arms, If so where do I get it from?
Can the 993 calipers be ran on stock size 3.2 carrera rotors
I understand that 930 rotors will push the wheels out 22mm is there another rotor option for my application to avoid this ?
Thanks for the info so far
Paul.

[Bill Verburg]

Originally Posted by Porsche84

Wow, good info Bill. Did you calculate my fronts using 993 TT front rotors? Maybe I do need to install the bias valve and see the difference.

yes

just for comparison here's a 2007 GT3 RSR w/ twin masters, it's not directly comparable to the ones above because I used 50 bar as the front line pressure

Porsche base setting
4545/2818 ratio 1.613

balance all the way front
4545/2367 ratio 1.920

balance all the way back
4545/3607 ratio 1.260

[Bill Verburg]

Originally Posted by zakmrk2

Ok Bill, great info, a few more answers on this please.
You mentioned a 23.8 mm M/C, Is this a kit to fit the 993 calipers to stock 3.2 trailing arms, If so where do I get it from?
Can the 993 calipers be ran on stock size 3.2 carrera rotors
I understand that 930 rotors will push the wheels out 22mm is there another rotor option for my application to avoid this ?
Thanks for the info so far
Paul.

if you use the late('81up) front hubs and rotor then yes the frot wheels are pushed outboard and you need to use 930 front fenders

if you use '78/79 hubs which are the same as 911 hubs '69-89 and the '78/79 flat hat then no the wheels stay the same as on a 911 w/ stock brakes

do use 930 rear rotors, you will have to discard the stone shield and slightly modify the 993 rear caliper in 2 ways, first the stock mounting holes are filled w/ a plug then new holes are drilled slightly lower to fit the caliper to the rotor. Second mod is to grind the throat area a little wider, if you use 965 rear calipers this second step is omitted. I would still prefer and urge that 930 rears be used. It seems that I see them frequently in the for sale section