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The ultimate brakes for 931

 
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kondzi  



Joined: 02 Jul 2018
Posts: 123
Location: MZ/Poland/EU

PostPosted: Sat Oct 10, 2020 9:14 am    Post subject: The ultimate brakes for 931 Reply with quote

So it happened. I have been using this setup for some time now and decided to share here my experience on the conversions.

I will try to describe what it consists of, what modifications needs to be done, what needs to be fabricated. And how it stops

Maybe not in a single post and many edits, as it took me some time to get to the FINAL point and may not remember everything at this stage, but questions welcome. Will try to explain everything. Please correct my wording as I am neither native speaker nor technical expert in English.

DANGER DANGER DANGER
Working on brakes is one of the most dangerous things, as when something goes wrong, you're left with just your gearbox. You're doing it on your own responsibility.

So first - this is my Green 931 with the brakes in place.


And a few words of intro...
Why I was not happy with my stock 931 brakes, which many think of very good brakes? Well, this setup was just to weak for my expectations, that's it

As I like challenges I decided I will try to do something different, not common, not documented, maybe a bit extreme and overrated. As I have been playing around with different brake modifications for some time already in different classic cars I already had some idea what to look for, what to expect and what to never do


To keep it short, the wish list of parts for this mod:
- Porsche 996 Turbo / 997 Carrera 4S front calipers
- Porsche 944 Turbo rear calipers
- Porsche 996 Turbo front rotors
- Early Porsche 944 Turbo rear rotors
- Mercedes G / W430 Master Cylinder (25.4mm diameter)
- Some brake line Tee fittings
- Longer studs to press in into the hub
- Euro / Late 5 stud hubs (if you have 4 stud ones)
- Custom (hard) brake lines
- If you need to refurbish the calipers, then you can buy the parts needed from Brembo / Autofren / Others. In general if diameter of the caliper piston matches, the kits used for ex. VW Tuareg calipers work well (less expensive than OEM Porsche - surprise surprise).

Now the close up:



What you need to know:
- This mod requires 17 inch rims
- This mod may require wheel spacers (depending on the wheels)

I'm using the CUP 2 wheels (7 inch wide all corners) currently and needed to add 10mm wheel spacer at the front, as the wheel arms collided with the caliper other way.

The stock piping was dual circuit with Front Left and Rear Right caliper in one, and Front Right and Rear Left caliper in the second circuit. So called X.

I have changed this setup to the more common Front/Rear circuit. So some piping is needed to achieve that, but not that much.

I am not using any PV, fronts are blocking wheels earlier than rears. For racing maybe a stronger (bigger caliper piston diameter) rear would be preferable with adjustable PV. PV usage would not be possible with the X-piping stock setup.

With wheels on the car:



The works:
- Front hubs turned to allow fitting of the 996 Turbo rotors on the outside of the hub (like in any normal car )
- Custom caliper adapter for the fronts (Radial/Axial conversion)
- Piping
- Bleeding
- Others...

In general it took me a lot of thinking to come up with the mod that has as little works needed as possible. I'm giving my IP here for free. Well I'm nuts, maybe

The custom caliper adapter.
- I've been sharing some progres on my 931 resurrection thread:
http://924board.org/viewtopic.php?t=43790
- In general I did a lot of 3D prints, then CNC prototyping and final adapters from 7075-T6 Aluminium
- To strengthen the threads in the adapter I used helicoil springs.







And this brought me to the 7075-T6 piece CNC machined:





So the evolution was in short like this


So why I chose exactly these parts for the mod:
- 996T/997 C4S Calipers / Rotors - These accommodate 34mm wide rotor rather than the Big Reds that use 32mm rotor - more thermal capacity. This means you will probably never see brake fade with these brakes on 931. Then a bit bigger diameter - 330mm rather than 322mm in Big Reds.
- 944T Calipers - These are direct replacement for the stock 931 rear calipers. I needed to add c.a. 2.5mm washers between the caliper and the trailing arm mount though to center the rotors between the pads.
- 944T Early Rear Rotor - to my surprise these are rather costly compared to the late ones, BUT they have the correct height and are just slip in replacement to the stock 931 and they land almost in the middle of the 944T caliper. So everything works - parking brake, offsets and so on.
- Mercedes 25.4mm Master Cylinder (ATE Standard) - First and most important one it just installs in place of the stock master cylinder, no works needed - just slip in. Second - I have so called medium blacks on my 951S (same as in 928S4) and it uses the 23.81mm Master Cylinder. I don't like the feeling of the pedal. It's soft, it's moving to much. Most probably will switch to Mercedes MC in my 951S as well.
- Brake Line Tee fittings - the Mercedes MC has only 2 outs. Then switching from X to Front/Rear circuits. You'll need to have them to get the piping correct.

Master Cylinder in place:


The 931 weights much much less than 996T or 997 C4S as you can expect. So these brakes in general are much overrated for the stock 931, but... You never now when you'll need them

HOW THESE BRAKE?
Well... I took my so called wife for a ride. Gently asked her to press her arms on the dash, elbows straight. Did a 80-0 MPH brake. I heard I would never never do that again as it made her sick. Test passed

Questions?
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)
'74 Ford Capri 1.3 (factory specs)
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Mike9311  



Joined: 14 Dec 2004
Posts: 899
Location: Chicago-ish

PostPosted: Sun Oct 11, 2020 2:38 am    Post subject: Reply with quote

Very nice!

Its really great to have 3D printing now instead of carving out test pieces in plywood and delrin just to test fit

I can imagine how nice it feels to brake in that car
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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peterld  



Joined: 10 Dec 2006
Posts: 784
Location: Noosa Heads QLD Australia

PostPosted: Mon Oct 12, 2020 1:37 am    Post subject: Reply with quote

When too much is not enough!!!!
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2058
Location: MI

PostPosted: Mon Oct 12, 2020 12:02 pm    Post subject: Reply with quote

Ultimate? Those words have been used before..

Funny they are red, only one master, and no bias control? ....

Very nice though
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2021
Location: Somerset, UK

PostPosted: Mon Oct 12, 2020 9:25 pm    Post subject: Reply with quote

Great info. Much appreciated.

I guess it takes a lot less effort on the pedal to get awesome brake power? How do you find the brake modulation and feel before locking up? I guess its the tyre that makes great brakes work, if the contact patch is too small then lock ups are all too easy? Moderns have monster brakes, they are over servo'd and potentially limited feel, but they do have ABS and electronics to help modulation and avoid the easy lock ups.

My stock turbo callipers will lock up all four wheels all too easily under emergency braking, with or without R tyres. Had a few scary moments on road and track where I stamped on the stop pedal and locked up all four tyres including 225 wide R's. So whilst I'd love to go to Brembo's, I don't need the opportunity to lock up more easily.

Then there is the wheel rim clearance issues.......... 16'' is big enough (too big for some), I wouldn't go to 17's and affect the tyre compliance further on road or track. I believe '85 951 Brembo's will fit under my Fuchs if I did go that route in the future. But it will be down to wanting the look of Brembo as I don't believe I need the additional braking power......even with 250bhp.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252


Last edited by Carrera RSR on Mon Oct 12, 2020 10:20 pm; edited 1 time in total
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morghen  



Joined: 21 Jan 2005
Posts: 8210
Location: Romania

PostPosted: Mon Oct 12, 2020 9:37 pm    Post subject: Reply with quote

Excellent information on the mod. Thanks for sharing.

I had the 951 M030 setup on my GTS wannabe and using premium street sport tires the braking performance was greatly improved when the tires gripped properly. When the grip was less than ideal, the brakes were worse than the factory ones as per Steve's comment...the bigger brakes lock up easier than smaller ones. So for hot weather and higher performance 924s i do recommend big brakes, for standard ones and street & all weather use..not really.
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kondzi  



Joined: 02 Jul 2018
Posts: 123
Location: MZ/Poland/EU

PostPosted: Mon Oct 12, 2020 10:21 pm    Post subject: Reply with quote

To answer the pedal feel/modulation. That is one of the issues I have with my 951S. Although it's equipped with ABS that kicks in very late (compared to modern daily cars) I don't like it. The 951S MC is 23.81mm diameter. That's why I researched options to get it more stiff, which led me to information 964 RS and 993 RS used 25.4mm MC. Then I searched all the catalogues of ATE and similar to find something that would fit the Brake Booster of 931. These were long hours, really long, but found the Mercedes MC that fits nicely, but needs Tee joints. Piece of cake

The feeling of this setup is superior to the 951, yes you need to press it a bit harder on the pedal, but at least I can tell, that I can feel the moment just before the wheels would lock up.

I used to have old (10 YO) tires when doing all the initial tests and these were locked between 30-40 km/h (old rubber). Tires were 215/50. Few weeks ago I bought new street tires 225/45/17 all corners. This was a HUGE change. Obviously I can still lock the wheels, but with much more push on the pedal than with 10YO tires And I can really feel the brakes just before they lock - that's something I really missed and miss in 951 currently.

The other even more important part is I lacked the brakes at highway speeds. In Poland where I live you can go up to 150 km/h (93mph) without any penalties on highways. And you risk only about 150USD / 120EUR when speeding (and some drivers' "points"). In Germany usually there is no limits on highways. And that's mainly what is addressed by these brakes, as stopping the spinning mass at these travel speeds is the hardest thing. No way to lock wheels at 100 mph, even if all corners were in the sky

On the other hand, I'm not driving these cars in winter/wet conditions anyway, so I was targeting warm and sunny conditions only.
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)
'74 Ford Capri 1.3 (factory specs)
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Cédric  



Joined: 27 Aug 2004
Posts: 1998
Location: Sweden

PostPosted: Tue Oct 13, 2020 12:18 am    Post subject: Reply with quote

Carrera RSR wrote:
Great info. Much appreciated.

I guess it takes a lot less effort on the pedal to get awesome brake power? How do you find the brake modulation and feel before locking up? I guess its the tyre that makes great brakes work, if the contact patch is too small then lock ups are all too easy? Moderns have monster brakes, they are over servo'd and potentially limited feel, but they do have ABS and electronics to help modulation and avoid the easy lock ups.

My stock turbo callipers will lock up all four wheels all too easily under emergency braking, with or without R tyres. Had a few scary moments on road and track where I stamped on the stop pedal and locked up all four tyres including 225 wide R's. So whilst I'd love to go to Brembo's, I don't need the opportunity to lock up more easily.

Then there is the wheel rim clearance issues.......... 16'' is big enough (too big for some), I wouldn't go to 17's and affect the tyre compliance further on road or track. I believe '85 951 Brembo's will fit under my Fuchs if I did go that route in the future. But it will be down to wanting the look of Brembo as I don't believe I need the additional braking power......even with 250bhp.



A build like this is for brake feel and the cool factor (and increase fade resistance if you have such issues, which is common on heavy cars on race tracks), it wont cut your braking distance in general by a single meter, it could make it worse if you dont get the brake balance correct. Brake distance is determined by the tyre grip, brake balance, center of gravity more or less, as long as you can lock the brakes (which you can on most cars ). The standard brakes are great on these cars, but of course a brembo caliper will have a more solid pedal feel for sure, and you can use mild brake compounds wihtout overheating. Joakim had around 400hp when he switched to Ferrari brakes from the stockers, and he used his car on track aswell, just a side note.

I dont want the be rude in your thread, the brake setup is really cool and probably works great, just want to get the physics straight here

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Shurick  



Joined: 15 May 2005
Posts: 508
Location: Russia, Moscow.

PostPosted: Tue Oct 13, 2020 1:56 am    Post subject: Reply with quote

morghen wrote:
I had the 951 M030 setup on my GTS wannabe and using premium street sport tires the braking performance was greatly improved when the tires gripped properly. When the grip was less than ideal, the brakes were worse than the factory ones as per Steve's comment...the bigger brakes lock up easier than smaller ones.

Same setup, same problems. Great dry braking and ease of lock up in wet.
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'79 931 -- intercooled K26-3060-6.10 turbo @ 1.2 bar, EFI+EDIS, 951S brakes, stripped interior, 951 look.
'86 924S -- R.I.P.
https://www.instagram.com/ru_pacecar/
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kondzi  



Joined: 02 Jul 2018
Posts: 123
Location: MZ/Poland/EU

PostPosted: Tue Oct 13, 2020 6:11 am    Post subject: Reply with quote

I think as always the truth is somewhere inbetween. I do agree that friction is something that is a key factor to stopping the car, however before it stops, it needs to slow down. And this is my part about highway speeds and spinning mass. From physics the kinetic energy is proportional to the spinning speed to the 2nd power. So both are important. First stopping the wheel spinning fast, then grip between the tire and ground. And everywhere heat dispersion.
I do agree about the cool-factor as well
Anyway performance for the buck factor is quite good with this setup with some eBay search and own works
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)
'74 Ford Capri 1.3 (factory specs)
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Cédric  



Joined: 27 Aug 2004
Posts: 1998
Location: Sweden

PostPosted: Wed Oct 14, 2020 11:45 pm    Post subject: Reply with quote

kondzi wrote:
I think as always the truth is somewhere inbetween. I do agree that friction is something that is a key factor to stopping the car, however before it stops, it needs to slow down. And this is my part about highway speeds and spinning mass. From physics the kinetic energy is proportional to the spinning speed to the 2nd power. So both are important. First stopping the wheel spinning fast, then grip between the tire and ground. And everywhere heat dispersion.
I do agree about the cool-factor as well
Anyway performance for the buck factor is quite good with this setup with some eBay search and own works


naah, it isnt really in between, the rotating wheel intertia is quite small relative to the moving mass. However it doesnt matter much for our brakes since its the tyre friction which will limit the retardation and brake distance.


In the track world, the point where you change brakes is when heat management is reaching its limits. (you tried racing compounds, brake ducting etc but still overheat or goes through brake pads to fast), or want a more solid feel with stiffer caliper designs. Big brakes weighs alot (but so do our iron brakes) so big brakes also have performance drawbacks.

I really want to add that im not bashing your brake upgrade, it is really cool and thought through, i just want to get the physics right here
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kondzi  



Joined: 02 Jul 2018
Posts: 123
Location: MZ/Poland/EU

PostPosted: Thu Oct 15, 2020 5:40 am    Post subject: Reply with quote

Good discussion is priceless I did weight both set of calipers, will add photos later - currently my ISP is down and same my web server for storing photos. Have not touched physics formulas since my studies, but will try to do some calculations to show that the spinning mass matters, and most of the heat you mention is generated right there

EDIT:
I might be also so wrong in my perception, but numbers don’t lie. I’m really curious where that will lead us
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Konrad
'89 951 US
'88 Mustang 5.0 LX Convertible (factory specs)
'84 911 Carrera 3.2 RoW (factory specs)
'81 931 RoW (TBD)
'81 Ford Capri 2.8i (factory specs)
'79 Ford Capri 2.9 (heavily modded)
'74 Ford Capri 1.3 (factory specs)
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