Bill Verburg

All the 993/964 C4s,RSs and turbos used a hydraulicly boosted 25.4mm m/c

The 964 turbos use a hydraulicly boosted 23.81 mm m/c as does the 964 turbo look

The 993C2 uses a 23.81mm vacuum boosted m/c which can be used to update the 20.64mm 964 C2 vacuum boosted system.

23.81mm is sufficiently sized for any system thus far mentioned in this thread.

Bill Gregory

I mentioned the 993C2 master cylinder as it is the only larger, vacuum-based master cylinder available. I really don't know if you could use the MC from the hydraulic-based systems, but I'd guess not (someone please jump in if I'm mistaken), as they have totally different operating principles.

With the 993 S4's and 993C2 rears, ABS continues to work with no addtional changes. Now, if someone could figure out how to adapt a racing ABS system into our 964's.....

Adrian

Dear Bill V,
I am afraid your data is incorrect. Unless of course that Porsche are lying. I have the brake system specs and the 964 C4 uses the same MC as the Turbo. This data can be found in the service manuals for the C4, Turbo, Turbo 3.6, RS and the factory maintenance manual. All of which sit underneath my computer at home.
In the 964 series only the RS uses the 25 mm MC as standard.
I totally disagree that the 23.81 mm MC is suitable for the 993TT Big reds and would advise anyone contemplating using them for road or track to upgrade to the larger diameter MC.
The larger MC was fitted to the RS for a reason and that reason was that it was for the track and provides a little additional fluid capacity.
I also advise all trackers to consider the brake fluid they use. This is particularily important for the 964s with hydraulic brake boost. Recently there have been a spate of hydraulic pump failures originating in the pressure switch seal failing. The common issues have been "tracked" using "standard brake fluid". I believe that over time the fluid boiling is destroying seals. Please consider using the high temp brake fluid if you are tracking your 964.
Dear Bill G. The original thread started with a C4 owner and a brake upgrade. The C2 with its vacuum system is another issue. I am not sure if the 993TT brake upgrades are really suitable for vacuum boost. I have not researched the available MC upgrades etc that are available for the C2. I personally have a bit of an engineering/design problem installing systems designed for one form of brake system into another without the basics of the original. If you understand what I mean.
Ciao,
Adrian
964C4

PS: It may be possible that the 1994 C4 TL (USA only) was fitted with a larger MC because original stocks had run out. This is just a thought but I know from other issues there are many 993 parts on this special edition model.

Bill Verburg

Adrian, On my brake page it is correctly listed, in my haste I incorrectly included the '89-94 C4 w/ the cars having the larger 25.4mm m/c. They do have a 23.81mm hyd. boosted cylinder. As do the 964t, both 3.3 and 3.6.

I stand by the statement that a 23.81 is adequate for Big Reds, exhibit A is the 964 3.6t which was the first use of the Big Red in these cars w/ 44/36 & 30/34 claipers.

The 25.4 advantage over the 23.81 is only a higher harder pedal. This gives a sportier feel as it is ergonomicly easier to modulate.

Exhibit B is the US 964 T-look and America Roadster using 993/S4 44/36 and 30/34 rears w/ the 23.81 hyd. boosted m/c, the 964 RS had the same calipers but the larger 25.4 and a reduced boost coefficient for better feel.

The 993/S4 is hydraulicly identical to the Big Red, both using 44/36 pistons

The primary concern when selecting a m/c is the slave/master ratio determined by the size of the slave cylinders and master cylinder.

Bill Gregory

Adrian,

I think we're saying the same thing - don't know for sure, however, I'd guess that the hydraulic-based MC's are incompatible to change into a vacuum-based solution. The suggestion for the 993 C2 MC as being an upgrade candidate is because it's from a vacuum-based system - that's responding to the C2 owner. If a C4 owner wanted to consider a MC upgrade, I'd suggest considering the other hydraulic-based MC's. I'd be the first to admit I have no personal experience with hydraulic MC upgrades, and would suggest more research by anyone considering it.

91C2wrencher

Thank you both for all the great info. To C4guy, sorry about kinda hijacking your thread. I saved this thread in my favs as well as printed out copy for the garage file. Now, how do convince the little mrs that $2k is well spent on this? ..uhm

robmug

Bill

I've been reading this thread and have to say that I'm really impressed with the level of knowledge and information you have provided.

I'm not sure I understand it all though! All we need now, is for one of the independent workshops to be offering packages based on your info which make it easy for us to buy/upgrade.

This would then give us a Phase 1 upgrade, Phase 2 upgrade etc.

I'm sure there's a market for this...we all spend money on our brakes and it would be nice to think that we could upgrade instead of replace existing systems.

AK

Not sure if it would work as I have not compared the MC on the 964 to that of the BMW but I do know that the 750il uses a vacum assisted 25mm MC with all 10mm brake line fittings.

Cupcar

The RS master cylinder is different than the normal C2 master cylinder since the RS uses a hydraulic booster like a C4, so C4 Guy can use the correct RS 25.4 mm master cylinder. Those with a C2 must use the 23.81 mm 993 master cylinder which is still smaller than the RS but much better than the normal 964's 20.64 mm. Your must have special adapters made to fit the brake lines into the master cylinder to use the 993 master cylinder and you must also use the 993 booster unit so this is a little tricky and expensive but necessary to do the job right.

Another note the 928 GTS front brake caliper is identical to the Big Red except it is cheaper, black color and MOST IMPORTANT has the brake bleeders and crossover pipes installed opposite to the Big Red. This is not a big problem as you can simply unscrew them and switch them like I did.

This allows you to use the 993 rear caliper and have the color and Porsche font written correctly, your competitors will never now the difference!

Adrian

Dear Bill V,
In my discussions with RUF and with their research based on two accident which had occurred with 964s fitted with 993TT brake systems they concluded that the RS master cylinder used with hydraulic boost systems not vacuum systems (they convert C2s to hydraulic normally) was required because it provided that little extra brake fluid holding capacity. When this was demonstrated to me I was quite happy with their recommendations.
I do not have time just now but maybe we need to look at the 928 vacuum systems and see what size MCs they are using. This might help the Carrera 2 and RSA owners with cheaper alternatives. In fact we might need to look at a series of vacuum boosted Porsches to see what is available.
Ciao,
Adrian
964C4

PS: To answer Robs question. The brake upgrade kits, phase kits to suit pockets have all been tried and most times fail. What works for one person does not work for another. RUF, FVD etc etc used to offer such kits. They gave up because they got as many questions and wobbly customers as when they did not have kits. Now they tend to treat each customer as a limited edition and make a package which suits that person. Believe me they tried this in the Audi world as well and it also failed.

BrokeAss

Bill could elaborate a little more on the reasons why you can't move the front rotor & caliper to the rear? I'm not sure I fully understand the reasons/ramifications. If the front caliper/rotor will physically fit, and it's just a matter of bias, well couldn't that be handled with a proportioning valve?

Bill, I'd also like your thoughts on this quote from the Vehicle Craft C2/4 Brake Upgrades page:
P.S. This has been a wonderfully educational thread!

Adrian

Dear Terry,
I can assure you that is you remove the bias valve you will end up with locking the rear wheels. Everyone I know that has done this has this problem. Their statement is a contradiction in terms. First they say fit a biad valve then they say remove one. You have one and you keep it. You may and I always advise this when seriously tracking that you install a variable regulator and play around with it.
Ciao,
Adrian
964C4

Bill Verburg

As far as the m/c goes Exhibit C; all 993 C2 have a 23.81mm m/c and exactly the same 44/36 & 34/30 hydraulic requirements as the Big Red system.

C4 Guy, there is a need for bias in any brake system, the bias comes from several places primarily hydraulic bias and mechanical bias. Early 911 & 930 always used ~1.6:1 hydraulic bias, later cars use up to ~1.9:1 and p/v to further move bias to the front under heavy braking and ABS.

combinations used by Porsche and corresponding bias are as follows

• 911-'83 2x48 & 2x38 1.596
• '78-81 930 4x38 & 4x30 1.604
• '82-89 930 4x38 & 4x30 1.604 w/ .46 p/v
• '89-94 964 C4 2x40,2x36 & 2x30,2x28 1.72 w/ .46 p/v
• '92-93 964RS 2x44,2x36 & 2x34,2x30 1.572 w/.46 p/v
• 993 C2, C2S 2x44, 2x36 & 2x34, 2x30 1.572 w/ .46 p/v
• 993C4, C4S 2x44, 2x36 & 2x30, 2x28 1.919 w/ .46 p/v

A complete breakdown is on my brake page

Besides hydraulic bias there is mechanical bias from the size of the caliper/pad and the size of the rotor, basicly bigger moves bias to that end. One can further juggle bias w/ different front and rear pad compounds(the factory doesn't so we won't go there). Now there are 3 different size of caliper/pad and 3 different front rotors and 3 different rear rotors used oem. So there are quite a few different choices of mechanical bias to choose from. Not counting rotors from other cars besides the 964/993. Unfortunately not all rotors can easily be fitted to all cars.

Now to get to your question, If you move the std small 964 front caliper(40/36, 172cm sq pad) to the rear, it will bolt on but, problem #1 it wants a 28mm rotor not the 24 that is there, problem #2 the front 44/36 in combination w/ the proposed 40/36 rear gives you a hydraulic bias of 1.116. This is not good as the rears are now locking considerably before the fronts. In this example the mechanical bias doesn't change so that doesn't help either.

As to VCI's statement, I find it confusing, if they meant to say the 964 turbo look/RS America/Carrera RS and 964T it makes sense because they all already have the correct small 34/30 rears. The 964C4 uses the small 30/28 rear. 964 turbo look/RS America/Carrera RS and 964T all have 44/36front and 34/30 rear for 1.572 hyd. bias

A C4 w/ Big Red front and the small 30/28 rear has 1.919 bias, mechanical bias has beeen moved further front by the large rotor and pad, a p/v will move that even further front under hard braking. The 993tt and 993C4S have the same hyd. bias but compensate by using the largest 322x28 rotors and the largest 30/28 caliper/pad combination to move the net bias back to the rear balancing out all of the changes.

You will notice right above the statement that VCI has the correct 34/30 small rear caliper listed as an option. They just narrow a 965 one to fit the stock 24mm rotor. An easier option is to use the already narrowed 993 small rear 34/30.

On my car(911 Carrera) I have satisfactorily used small 40/36 on 304 & small 30/28 on 309, medium 44/36 on 304 & small 30/34 on 309, and large 44/36 on 322 & small 34/30 on 309 all w/ a non boosted 23.81mm m/c. Unfortunately the 964 rear rotors aren't as accomodating. I am sure I left something out but that's all I have time for right now.

Cupcar

Hi Bill-

Not to put too fine a point to it, but don't all the '93 and later 964 C2 and C4 cars (including the RS America) have the same caliper dimensions and basic hydraulic proportion front to rear with the same 55 bar proportioning valve? This system is not the same as the 964 RS.

All proportioning valves have the same reducing factor slope of 0.46 listed, but the pressure threshold is different based on application, it was 45 bar on the 2 piston rear caliper C2 and 55 bar on the 93 and later 4 piston rear caliper C2, C4 cars as well as the European Carrera RS.

The Carrera Cup 964's had the same calipers/discs as the European RS and used no proportioning valve at all.

Bill Verburg

Yes that was meant to be America Roadster w/ the 34/30 not RSA which of course uses the same 40/36 & 30/28 as all the other normal 964s.

I didn't want to mention the differing knees for fear of muddying the water even more. It is perfectly clear to me but I can see that with all the different possible combinations of similar sounding parts it can be overwhelming.
here is another summary of the different calipers

t/m-transverse mount
r/r-radial mount
#-rotor thickness