SC Brake Upgrade ??
 

I am looking for some brake upgrade combinations for my SC that require minimal machining, on a medium budget. I am running 15" wheels. The obvious is 78-79 turbo brakes with full floating set up, but this is not cost effective at this time. I was wondering if anyone has done early 944 turbo calipers on Carrera rotors up front, is this a bolt up?? Also I have heard of 964 4 piston rears and a Carrera rotor on the front of SC's. Lastly would be the 917 style calipers with later non-floating discs?? Will this work ?? Any advise would be greatly appreciated.

One of the simplest upgrades is to install Carrera 84-89 calipers and rotors. The calipers bolt up and they are wider as to fit the wider rotors. The rear calipers have bigger pistons, so a Carrera proportioning valve may be a good idea.

OK, who's got that link. C'mon, you all know the one I'm talking about. Was it Bill V.'s??

xw,
this is what you need to see. It details the options for you in order of cost and effectiveness. Try a search.

Edward

You mean this one?

Well I didn't post the question, but thanks, Bill! I can't tell you how often I've seen posts or references to your excellent write up. You are a real asset to the Porsche community.

Edward

Everyone should send Bill a beer ...

I just did this myself, no trouble at all.

http://forums.pelicanparts.com/porsche-911-technical-forum/275981-size-does-matter-my-sc-carrera-brake-upgrade.html

Installing Boxster calipers and Carrera rotors front and rear is a nice setup. However, some will argue the value of going with the Boxster over the Carrera calipers. I have the Boxster setup on my SC -> 73RS Replica and it's nice and balanced.

There is nothing wrong w/ it, it is an improvement. It's just that you can get the same bang for a lot fewer bucks w/ Carrera front calipers and rotors and leave the rears alone.

I don't know if anyone else has noticed but a lot of the folks here are umm... frugle;)

My dad has the Boxster/Carrera set up and finds it well balanced and loads of breaking power.

Took me a few hours at most and I had Carreras F&R. Works very well and I have no prop valve.

-michael

I was under the impression that being able to see the "Porsche" script on the calipers would add 10 HP :)

In all seriousness, I basically stole the front setup, so it did not hit the pocketbook nearly as bad for the rears.

Would you see any benefit adding a proportioning valve with the Boxster setup? I have not had her on the track yet, but she feels balanced on the street. I fear the rears may prematurally lock on the track, though....

Sorry for barging in like this, but there is one issue that does not seem to be discussed when upgrading brakes. That is brake pad size versus pedal response.

How does this sound? If you add the same amount of force on a big and a small brake pad, the small pad will compress more because the surface pressure (force/pad area) is higher. Since the small pad deforms more than the big pad, the brake pedal will feel more spungy with small pads.

With that in mind e.g. the Boxter caliper upgrade will give a firmer pedal response which gives a better sense of control compared to using a Carrera calipper which has the same brake pads as the SC.

Or am I totally off?

Another factor to consider for track applications is pad cost / life of the pad. Carrera calipers are good, but hard track uses wears out a set of quality pads in a few track days. For this reason on the 3.6 SC, I went with a modified set of 'Outlaw' 4 piston calipers. The plan is to get about 12 to 20 track days on a set of pads.

No, stock Boxsters front and rear are a tad front biased but nothing extreme, you will just go through front pads a little faster than rears.

You are totaly off, while the stiffness of all of the mechanical components can affect brake feel, In 911s it is generally not an issue or at worst a very minor one and pad compression has the least contribution in that regard.

Pedal feel in a Porsche is almost entirely a function of the master to slave ratio.

Is the 17mm master cylinder of the earlier cars good enough or is a 19mm required? I have done the Carrera front/rear upgrade but have not finished the project yet. Thanks to this post I now know I need the p/v! I am pretty sure my 72 has a 17mm master. I have searched, but found no specifics. Any advice greatly appreciated.

Thanks Bill.....and sorry to the original poster for the hijack, hopefully it was helpful :)

Your '72 certainly came w/ a 19.05mm m/c.

For Carrera f/r brakes that will work fine, A slightly better m/c would be a 20.5mm m/c 901.355.012.04(ATE# 3.2120-3901.3).

They are not easy to come by

Hmm, too bad. After having rebuilt the calipers, changed to steel braided brake hoses and being told by the local Porsche workshop that "oh no, an old master cylinder does not make the brakes feel spongy", I was kind of hoping that was the answer to the spongy pedal in my car.

Any suggestions?

Peter,

Spongy brakes. Make sure the bleeders are on top of the calipers and not the bottom. Don't ask how I learned this...

Thanks Jim, a friend I know very well has tried that too, so I will not ask.

Bill,
Thanks for the reply

Jon

Peter,

If your brakes are still spongy, then a couple more things to try.

1. Replace brake lines if they are orginal
2. Try using a pressure bleeder (Pelican has them)
3. Try using brake bleed valves that have a check valve in them (Pelican)
4. If MC is old, replace.

5. tap the calipers dozens of times with a small metal hammer or wrench while bleeding

Thanks for the tips!

I guess I will get a new master cylinder and a pressure bleeder, since the rest of the brake system is virtually new. Had to change a few meters of hard lines when changing to steel braided brake hoses.

Peter,

Consider upgrading to the 930 MC. It has a larger piston which means less throw but more brake force from your leg. This is a great option for a track beast but may not be desireable for a street car. Depends on what you would like. The installion is the same as the stock unit.

Jim,

thanks, that was the answer to my next question! I have just bought a pair of 993 RS front calipers and a pair of 964 Turbo 3.6 rear calipers in the hunt of the spongy pedal, and I got the impression from reading threads here that the original MC might be a little on the small side. I guess I will feel a little embarrassed when it turnes out that it was "only" the MC.

Peter,

If you have the piston sizes of the calipers, I can go through the math and calculate the front to rear brake bias.

Hi Jim - could you explain how an old MC can lead to spongy brakes? Perhaps this is my problem too...

Thanks,

Jeff

edit - assuming it isn't leaking, which mine isn't.

Jim,

I did some math myself and found them more front biased than the stock, but I would appreciate it if you had a look at the numbers.

I have not measured the items myself, but according to Bill V's table:

Front disc is 322 mm in diameter (993 RS disc)
Front caliper piston diameters are 44 mm and 34 mm

Rear disc is 309 mm in diameter (930 rear disc)
Rear caliper piston diameters are 34 mm and 30 mm

By comparing braking moment at a unit brake system pressure and assuming the brake pad coefficient of friction to be the same for all the pads, I found the new brakes to have a 61/39 front/rear bias compared to the stock which have 54/46 front/rear bias.

To calculate the braking moment I used the following equation:

http://forums.pelicanparts.com/uploa...1145791182.gif

p_sys is brake system pressure, (I chose 1 MPa (1 N / mm2)), A_pistons is the area of the pistons in one caliper, mu_pad is the coefficient of friction (I chose 0.55). Ø_disc is brake disc diameter and h_pad is the height of the pad. The part within the brackets is the average lever arm for the brakes.

Does this look reasonable?

I run the Wilwood Superlite Series on my Carrera rotor for years; I've changed the model a few times, the Forged unit is the latest. With (4) 35 mm pistons and larger pads it works well on the track; I realize the caliper is overkill for the rotor, but, it's an inexpensive switch. The rear is OEM 3.2 Carrera, running with the 55 bar p/v.
j.p.

http://forums.pelicanparts.com/uploa...1145797448.jpg
http://forums.pelicanparts.com/uploa...1145797540.jpg
http://forums.pelicanparts.com/uploa...1145797603.jpg
http://forums.pelicanparts.com/uploa...1145797633.jpg

The pics of the inner side, show a BBS 15" modular race wheel and the custom hard line to the strut.

Peter,

Great equation. Is H_Pad measures just the pad? (i.e. if the pad height is 50mm). Or is it in relative to the position of the pad to the rotor?

Anyway, I went through the math and the brake bias is 60.1% to the front and 39.9 to the rear. The math is based of a 23mm MC and the caliper piston areas on one side of each caliper

Jeff,

Spongy brakes.... If all else is done, (new lines, new/rebuilt calipers, good pads, system bleed, proporting valve OK), then the only thing left is MC. As these cars are 20+years old, sometimes things are placed as preventative maintenance.

Jim,

good to see that my calculations are at least reasonably close to yours. Do you think the bias is to front heavy? I was playing a little with the idea of getting an adjustable brake bias.

H_pad is just the height of the friction material of the pad (not the steel backing plate), and I assume that the outer edge of the friction material is flush with the rotor. That is not exactly true, but quite close to the real thing.

I measured my brake pads with a slide caliper, but the sizes differed a little from the ones in Bill V's table. Since I am going to use the pads that I have, I used my measurements.

My stock pads measured 54 mm and 40 mm in heigth, and the new ones were 60 mm and 49 mm.

Thanks for helping out!

jpahemi,

nice setup you got there. Do the calipers bolt right on?

Peter,

A 60.1/39.9 is split good. Some say about a 38.5 / 61.5 is about ideal for the street. Measure rotor temps after a few hard braking stops. If the rears are not working enough (cold rotors), then rethink the bias thing. For a track car, the rear bias could be more, but then one has to watch the rear brakes locking up.

FWIW, Porsche increased the size of the rear pistons in the Carrera as compared to the SC's. This was to even out the brake pad wear. Of course the Carreras had a proporting valve.

Peter:
The caliper mounting ears need to be enlarged from @ 10.9-11.01 mm to 12mm; the spacing is 3.5" which allows direct bolt on. The hard line from caliper to strut needs to be made; I found an outfit that makes custom stainless hard lines with metric fittings; I ended up sending them a coat hanger wire model of what the line needed look like.
j.p.
Ps. I realize the rear needs to be upgraded as well, for now all I did was replace the 22 year old calipers with new ones. It's nice seeing the new yellow zinc finish show through the wheel.

Freudian slip?