Gary C2 UK

Hi all,

I know these questions get re-hashed every so often but I would be greatful if anyone has any experience of the following and possible solutions.

I've just fitted 993TT(big red) front calipers and standard C4 rear calipers to my 1990 C2(couldn't find any larger rears for sale).

Initial verdict is massively improved controlled(no ABS)braking but as with all these things a few snags too.

Firstly the brakes seem to out perform the ABS now with definite locking up of the fronts if I stamp on the pedal hard.

Secondly the rears do not appear to be doing much at all. The brand new 964RS discs hardly look like they've been touched by the pads.

I think the rear braking may be corrected by using a new biasing valve but which size should I replace the standard one with.

The problem with the ABS looks altogether more complicated!

Does anyone have any ideas/comments/experience of this.


For clarification my car had the original C2 4-pot fronts and 2-pot rear calipers before.


Many thanks,

Gary <img src="graemlins/jumper.gif" border="0" alt="[jumper]" /> <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

tonytaylor

My C2 has big reds front and rear and they will overcome the ABS but in my case it`s the rears that lock first.
The problem of locking up only occurs if I stamp full on the brakes. If brake pressure is applied in a more progressive fashion there is no locking up and the ABS activates.I have no problems on the track.
I wonder if your set up is a little unbalanced front to rear?

Cupcar

Brake torque is a function of clamping force applied to the disc, the coefficient of friction (stickiness)of the pad material and the radius of the disc i.e. lever arm that the force is applied across.

The clamping force on the disc is a function of the strength of the leg pushing on the pedal multiplied by the hydraulic and mechanical ratios in the system. Hydraulic ratios are the area of the pistons surfaces in the caliper divided by the area of the master cylinder. Bigger pistons in the caliper increase this force, smaller pistons in the master cylinder increase this force. Mechanical ratios are created by the length of the brake pedal arm, a longer brake pedal arm increases this force. Another mechanical force multiplier is the ratio of the vacuum brake assist which functions to multiply the brake pedal force against the master cylinder.

The tire applies a counter torque which is a function of the weight on the tire (analogous to the clamping force on the disc above), the coefficient of friction of the tire (analogous to the coefficient of friction of the brake pad above), and the diameter of the wheel i.e.the lever arm that the force is applied across (analogous to brake disc radius above).

If the force applied by the brake is greater than the force applied by the tire, the brakes "lock up" if there is no antilock. The antilock system measures the individual speed of the front wheels and the collective speed of the two rear wheels (3 channel ABS). The ABS computer compares the wheel speeds a determines plausiblity of their difference. If it determines that a wheel is turning implausibly slower than the rest when brake line pressure exists then the ABS solenoid controlling pressure to that wheel (or wheel set in the rear) limits the pressure until the wheel speed "catches up" to the other wheels.

When the brakes are applied to the rolling chassis weight is transferred from the rear of the car to the front. This weight transferr is a function of the wheelbase length, center of gravity height and deceleration rate. The higher the deceleration rate the more weight shifted to the front wheels.

The key is to match the braking torque to the wheel torque during braking in order to stop all the wheels equally without locking front or rear wheels prematurely. To do this the engineers match the components so ratios above are accomplished correctly. A proportioning valve is placed in the system to match the front to rear braking bias to the deceleration rate dependent forward weight transferr. The proportioning valve does this by limiting brake line fluid pressure to the rear brakes after a predetermined line pressure i.e. a so called "55 bar" valve limits pressure after 55 bar line pressure is achieved.

The factory spends a lot of time determining these system ratios, compromising some factors, so that safe braking is achieved under a variety of weather conditions, with differing manufacture tires in individual chassis with a variable front to rear static weight bias. A Turbo is different that a C2.

In the case of changing to the "Big Red" front brake system (without it's matching proportioning valve and rear calipers) both the caliper piston size and brake disc radius have been increased and brake torque is increased on one end of the car only. The designed in brake balance has been altered such that the fronts have premature lock up and the rears do no relative braking.

Interesting is that the ABS apparently is unable, given the system ratios with the Big Reds and standard rear brakes to limit line pressure to the extent that anti-lock is achieved. <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

GeoC2cab

High,
I swapped my fronts to the rear and installed "big blacks" on the front, essentially the same as big reds except in color, this left my rear end way over braked and it would lock up before the fronts which results in a total loss of control(happened once), so with the help of rennlisters on this site, I have found that the set up I use now works quite well, and I can do some extreme brakeing w/o locking up.
This is my set up, but I can not take any responsiblity on the results you may achive;
With the big blacks on the front w/330mm cross drilled rotors(i'm pretty sure thats the correct measurment), I used 993 4 piston rear calipers w/ cross drilled rotors(the stock size) and a anti-lock proportion valve from the 964 turbo, Everything else in the system is stock. I haven't lockup since installation of this set up, and I have tryed, only felt abs.

This isnt a sales pitch but I have a connection to a few used sets of the rear 993 rotors, just ask and you shall recieve.

Cupcar

The system above described by GeoC2Cab is basically dimensionally the same as a 964 Euro RS in terms of rotor size (322mm and not 330mm I think), caliper piston diameter and proportioning valve setting. Well thought out, should work very nicely. <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

Gary C2 UK

Thanks all for your comments so far.

My set up is 322mmx32 front rotors with calipers using 44 & 36mm pistons

My rears are 298mmx24mm rotors with calipers 30 & 28 mm pistons.

My fronts are exactly the same as a 964 3.6 Turbo or Euro RS and the rears the same as a C4.

A Euro RS rear uses 298x24mm rotors but with 34&30mm pistons

In effect I am down sized at the rear by two pistons(per side) ideally 34mm actually being only 28mm

So if I fit the proportioning valve from a Turbo or RS I should be close to a good F&R balance?

I think the fact the ABS in not stopping the front lockup at the moment is due to the standard C2 proportioning valve(thanks Adrian Streather)not letting the rears do any work.Nearly all the braking force is going to the fronts which are just too good for the ABS to react to effectively.


Comments/confirmations on the theory please!!

Many Thanks,

Gary

BTW thanks on the 993 parts offer but I believe 993 rotor bell offsets don't work on a 964 without further mods.

Bill Verburg

The easiest thing for you to do is to get the 30/34 rear calipers(they were used on 94/95 C2s & 964T as well as the 964RS. This will change your current hbr from 1.919 to 1.572 which is what the earlier 911s w/o ABS used. If you do this you can ditch the p/v entirely or at least go to a 60 bar version. The caliper will work fine with your existing 299x24mm rotors. Another issue may be the m/c. The 964 C2 used a 20.64mm unit. The cars w/ bigger piston calipers used 23.81 & 25.4 mm m/c. One of the signs of insufficient m/c volume is premature lockup, usually at the small end though <img src="confused.gif" border="0">

Cupcar

The difference in the Euro RS and C4 caliper's rear piston area is 22%. The Euro RS used a 55 bar valve, 60 bar is the highest Porsche OEM available. I doubt the 22% difference can be compensated for with a 5 bar change in proportioning valve threshold.

Gary C2 UK

Bill thanks for the info.

I have been unable to find any used larger piston calipers.

I thought that the 964 Turbo used 28mm rotors so these calipers would be too "wide" for my rotors. The Euro RS uses a machined down version(I believe) of the turbo caliper to fit the 24mm rotor.

I thought the master cylinder was OK as I have a good firm pedal. I'm sorry I don't understand your at the small end or do you mean the rears?? <img src="graemlins/c.gif" border="0" alt="[ouch]" />

Cupcar. At present my car is using the standard pv which Adrian Steather says is 45 bar so I guess were looking at a 15bar difference. Will this be enough of a change or am I looking at this in the opposite way??

Thanks Gary

Cupcar

Gary-
My thought was that if you have the correct size calipers that Bill recommends you would have basically the same system as the Euro RS. Since the Euro RS uses a 55 bar valve to get things "right", you could at most go to a 60 bar valve to compensate for the differences between your calipers and correct calipers. 5 bar = 72.5 line PSI which I don't think is enough to do the job.

As an aside, my car has the complete Euro RS system and I have removed my proportioning valve altogether and replaced it with the factory straight through line as used on the Carrera Cup race cars. With the valve out of my system the car still locks up the front brakes with the ABS turned off. This indicates to me the inherent bias of the system is way to the front to begin with, making your situation hard to compensate for with just a valve switch. <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Gary C2 UK

Well guys thanks for the input which I understand fully.

As the cost and availability of the correct rear calipers is prohibitive I'll just have to take my chances on the 60bar limiter for now.

I know it may not be the ideal setup but I reckon a 60bar limiter is going to help considerably compared to the 45bar one fitted at the moment.

How much do these valves normally cost? Anyone have a 60bar for sale?

Regards,

Gary

Bill Verburg

I've thrown together a web page showing all of the brake data I could find from all of the 70s thru ? Porsche 911, 930, 964,993,996, models. Most of the data a has been culled from factory publications, spec books etc. Some #s are calculated by myself for personal interests. There are also some gaps where no hard info has yet been obtained. Take a look, see what the factory did. It's interesting to compare data from ABS to non-ABS equipped cars and to see the progression in hbr over the years. Of course other factors such as pad area, rotor diameter suspension etc will affect the feel/performance of any brake setup. Let me know if it's at all useful.
<a href="http://www.pelicanparts.com/MotorCity/wmv/brakes.htm" target="_blank">Brake Info</a>

Cupcar

Gary-

Valves are actually cheap around $30 US. I was trying to say, in an oblique way above, that you might try doing without it. Be careful though. One way to test is by turning off ABS. I think this is possible by pulling the relay. My car has factory switchable ABS kit so I don't really know. Have an outside observer confirm which end of car is locking first with it off. I think you will find, as I did even with the larger rear calipers, the fronts lock before the rear which is the safe condition.

GeoC2cab

High,
listen to Mr. Cupcar, he helped me w/ my set up very much, I wonder if he even knows it, Thank you Cupcar., George. Correct size rotor is 322mm.
I also screwed up the parts connection, I can get my hands on 993 rear calipers(I think i have 2 sets)e-mail me if your interested.

Gary C2 UK

Hello GeoC2Cab,

I'll probably try the P/V 60Bar size first as it's easy. I would be interested in the calipers too.

What are the piston sizes of these calipers? How much would you be wanteing for a pair?

Also Bill(Verbung) thanks for the link to the brakes fit. It was a great help indeed.

Gary