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944 brake master / booster or 931 ?

 
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Kenodog  



Joined: 25 Jun 2003
Posts: 2579
Location: Vancouver,B.C.

PostPosted: Wed Feb 27, 2019 6:27 am    Post subject: 944 brake master / booster or 931 ? Reply with quote

So does anyone here know off-hand if I could use a 931 brake master cylinder and booster with the 951 big black brakes ? I remember reading somewhere that you need the 944 master cylinder to do this because they push more fluid or some such...



Lee
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Dutch924-racer  



Joined: 23 Jul 2007
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PostPosted: Wed Feb 27, 2019 8:05 am    Post subject: Reply with quote

A 944 turbo (>1986) uses a proportioning valve for the rear brakes. The master cilinder is slightly bigger 23-20mm instead of 23-19mm

Exact size are 23.81, 20.64 and 19.05mm
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Kenodog  



Joined: 25 Jun 2003
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PostPosted: Wed Feb 27, 2019 8:38 am    Post subject: Reply with quote

So the 931 master will drive the 951 big calipers no problem though correct ?





Lee
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morghen  



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PostPosted: Wed Feb 27, 2019 9:15 am    Post subject: Reply with quote

Mine did
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Kenodog  



Joined: 25 Jun 2003
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PostPosted: Wed Feb 27, 2019 9:17 am    Post subject: Reply with quote

Perfect. Saves me buying ANOTHER part for this thing !



Moi
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924-76  



Joined: 05 Jun 2006
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PostPosted: Wed Feb 27, 2019 6:24 pm    Post subject: Reply with quote

I'm running 4 pots wildwood in front and 951 rear caliper with 931 master cylinder; braking power is incredible.
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Fasteddie313  



Joined: 29 Sep 2013
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PostPosted: Fri Mar 01, 2019 8:02 am    Post subject: Reply with quote

I have found that the different MCs do not matter because they don't put out different ratios front to rear even though they have a different "rear piston"..

The front piston is the only piston you push on and the "rear" piston is pushed on by the fluid pressure of the front piston so pressure/volume is a wash between the two sides.. It applies equal hydraulic force out of all ports..

Fasteddie313 wrote:
I have just been informed that the later stepped masters have nothing to do with how much F/R bias the master cylinder puts out and they act just like both circuits off the same master piston..



[QUOTE=mikey_audiogeek;12495337]The stepped tandem MC does not affect the bias. Stepped tandem MC's generate the same pressure in both circuits.

The stepped MC is to match the VOLUME requirements of the different circuits.

This diagram shows the pressure relationship in more detail. Note that the diameter where the rear piston is acted on by the front circuit pressure and the diameter where the rear piston generates pressure in its own circuit are identical.



So you have exactly the same bias as if you were running a 944 or 951 MC.

Cheers,
Mike

[/quote]


The ONLY real difference is they way they are tuned for failsafe mode if/when the front side runs dry of fluid the front piston will phisically contact the rear piston to push it, OR the rear piston runs out of fluid and bottoms out allowing you to still compress the front piston.

So pretty much your brakes should work great from any of the MCs but if you drastically change what calipers you are running the failsafe might not work great..

The MCs will not affect your F/R proportioning but your caliper choices will.

The way I have figured to solve proportioning is to run too-big rear calipers and then turn the rears down with an adjustable prop valve.


Some data I have aquired that may help others -----

Volvo 4 pot front calipers = 4X 38mm pistons = 2267 mm2 surface area per side of caliper

931 rear calipers = 36mm = 1017.4 mm2

931 front calipers = 54mm = 2289 mm2

924 4 lug fronts = 48mm = 1808 mm2 (can be fitted to 931 rear)

951 brembos front = 2X 36mm + 2X 40mm = 2273.4 mm2 surface area total per side of caliper/slave (near as makes no difference the same effective size as volvo calipers )

951 brembos rear = 2X 20mm + 2X 30mm = 1020.5 mm2 surface area total per side of caliper/slave

944 Turbo:
Front Calipers:
'86 Turbo 38mm and 36mm
'87-88 Turbo 40mm and 36mm
'88 Turbo S 44mm and 36mm
'89 944 S2 up to VIN # >> 94KN4 51880 40mm and 36mm

Rear Calipers:
'86-88 Turbo and Turbo S 30mm and 28mm
'89 S2 up to VIN #>>94KN4 51880 30mm and 28mm

For bias calculations
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nickthompson  



Joined: 26 Mar 2013
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PostPosted: Fri Mar 01, 2019 11:06 am    Post subject: Reply with quote

I'm going to steal this thread for a bit. I need a brake booster will a 944 one fit my 924. Good 944 ones are easy to find.
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MikeJinCO  



Joined: 08 Jun 2010
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Location: Maysville, Colorado

PostPosted: Fri Mar 01, 2019 3:39 pm    Post subject: Reply with quote

The 944 booster has a longer push rod and the spacer plate is also longer to make up for it. The space plate also changes the angle from the firewall. It makes the MC point up in front a bit, still fits under the hood. All the bolt holes are the same. I tried the 924 space plate and added a 3/8" or 1/2"(don't remember) plate to make up the length with a 924(7") booster, but rejected the idea. I hope the 9" booster clears the air horns on my 45mm Webers, will find out next week. The 9" booster interferes with the throttle cable location. So changes required for that also. Some of the hybrid 944 guys made up a spacer that didn't use a booster as the it won't fit at all with a SBC in there.

I couldn't find any new 924 MC's or even rebuild kits for early ones so had to go to the 944 or a Tilton or Wilwood setup which is a whole set of other issues.
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Rasta Monsta  



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PostPosted: Sat Mar 02, 2019 12:37 pm    Post subject: Reply with quote

931 cylinder pushes my Brembos beautifully.
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Shurick  



Joined: 15 May 2005
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PostPosted: Sat Mar 02, 2019 9:19 pm    Post subject: Re: 944 brake master / booster or 931 ? Reply with quote

Kenodog wrote:
So does anyone here know off-hand if I could use a 931 brake master cylinder and booster with the 951 big black brakes ? I remember reading somewhere that you need the 944 master cylinder to do this because they push more fluid or some such...



Lee


As Fasteddie313 mentioned above, brake circuits on 944 and 924 are routed in a different way. I used 944 MC on a 931 with either stock and 951tS calipers, but I had to reroute brakes from diagonal to front-rear scheme:

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nickthompson  



Joined: 26 Mar 2013
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Location: Central Georgia

PostPosted: Sun Mar 03, 2019 6:06 am    Post subject: Reply with quote

It has been mentioned on this forum by more than one person that 944 brakes work fine without rerouting the brake lines. I haven't done it myself yet but it has been done by others
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Kenodog  



Joined: 25 Jun 2003
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Location: Vancouver,B.C.

PostPosted: Sun Mar 03, 2019 6:56 am    Post subject: Reply with quote

I bought enough line and fittings to reconstruct the 931 system. I will start bending and fitting them next week when I have time. Then on to the rear suspensuon etc.



Me
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