Subaru Master cylinder Sizing

So I started working out my brake setup and I am lost on how I should size the master cylinders for my car.
I have dual masters and a hydraulic hand brake

Here are the master cylinders I have:
0.625"
0.70
0.75
0.75

Pedal ratio is 6:1

Front calipers pistons 2x 1.685"

Rear caliper pistons 1x 1.5"

Front rotor 10.8" diameter
Rear Rotor 10.4" diameter

Any recommendation on what cylinders to go where? and can you show me how to come to that conclusion.

Anybody? Where is Cosworth? I read a thread on ss that was back and forth between he and TCE and wow...

Also what I have are girling masters, are they alright or should I stick them on eBay and get something different?

Buy this book. I find it very helpful for figuring out such things. Lot's of useful equations and tables and diagrams and shit.

Or one could go by precedence aka praxis
F .7
R.75
Handbrake .625

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

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Doivi Clarkinen
Buy this book. I find it very helpful for figuring out such things. Lot's of useful equations and tables and diagrams and shit.

Perfect oredered one, albeit not for $40

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john vanlandingham
Or one could go by precedence aka praxis
F .7
R.75
Handbrake .625

That was the conclusion I came up to before I went to bed. It seemed to make more sense than a recommendation I saw for the front to be a .625 with rear 0.75.

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FunctionAuto

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john vanlandingham
Or one could go by precedence aka praxis
F .7
R.75
Handbrake .625

That was the conclusion I came up to before I went to bed. It seemed to make more sense than a recommendation I saw for the front to be a .625 with rear 0.75.

Like so much we need--as you see here---some starting point.
sure there's a million variables but the biggest is straight hydrualics and if a dozen cars all have the more or less same size front pistons and rear pistons then and more or less same size masters then it's worh a try as a starting point..
That's why we scanned all those Ford Motorsport Manuals cause they all have set up sheets worth perusin'

The calipers and masters really don't know what they're bolted to.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Quote
FunctionAuto
0.625, 0.70, 0.75, 0.75
Front calipers pistons 2x 1.685"
Rear caliper pistons 1x 1.5"
Front rotor 10.8" diameter
Rear Rotor 10.4" diameter

Well, I'm going to disagree that just saying, "well this manufacturer ran 0.75 and 0.70 so I should also" isn't a good way to start when you aren't dealing with a similar setup on your car, especially considering the effect of piston area. For example, the Ford books are all 4 pots all around. If your front to rear piston area ratio isn't similar to what Ford WRC cars had you could be WAY off.

But for you, I'd start by making a smart spreadsheet to calculate the following:
Front caliper piston area: 4 * pi r^2 = 8.9197
Rear caliper piston area: 2 * pi r^2 = 3.5343

Brake line pressure.
(pedal force in pounds * pedal ratio) / area in square inches of master

Clamp force
Total piston area * brake line pressure

Brake torque:
(Effective disc radius * clamp force * pad coefficient) / 12

Now, you'll need to figure out the coefficient for the pad you are using, what the effective radius of the rotors are, and what the starting point of front to rear distribution you want when the balance bar is set in middle. Then you can estimate the front and rear torque with different MC sizes and be able to get a decent idea of what size master you need. There's more to it than this, but this can get you to a decent starting point. There's also the question of what you want your balance bar to do and that can vary.

Why aren't you running the 4/2 setup? You will need it eventually and it's not a super expensive upgrade.

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Grant Hughes

There is a spread sheet that an injun-ear made over on a few of the subaru sites that compares all of the brake torques of the factory brake setups. The subaru 4pot has 0.32% LESS torque than the 2 piston sliders that are on my car currently. I know the 4 pots serve other functions, more material to disappate heat etc...But if they are the same as far as torque goes is there a huge advantage to upgrading?

Truth be told I bought some 4pot, then the kid raised the price the next morning and sold them to someones else. The Jerk. Then that guy logic posted something about how because they are steel doesn't handle heat well and causes frequent caliper rebuilds. So then I started to look into wilwood and other semi affordable options with no luck. As far as the rear goes Barrett was saying the 2pot rear wear through very quickly and with no real difference in brake torque, it was cheaper to stick with the rear 1pot vented legacy setup.

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FunctionAuto
There is a spread sheet that an injun-ear made over on a few of the subaru sites that compares all of the brake torques of the factory brake setups. The subaru 4pot has 0.32% LESS torque than the 2 piston sliders that are on my car currently. I know the 4 pots serve other functions, more material to disappate heat etc...But if they are the same as far as torque goes is there a huge advantage to upgrading?

Truth be told I bought some 4pot, then the kid raised the price the next morning and sold them to someones else. The Jerk. Then that guy logic posted something about how because they are steel doesn't handle heat well and causes frequent caliper rebuilds. So then I started to look into wilwood and other semi affordable options with no luck. As far as the rear goes Barrett was saying the 2pot rear wear through very quickly and with no real difference in brake torque, it was cheaper to stick with the rear 1pot vented legacy setup.

Sounds good to me.
4 pot have less torque because of what factor? Less piston area overall or different rotor size or did the factory setup have a different sized master cylinder?

Mark's car still has drum brakes in rear and stock Impreza L in front and he's done fairly well with it by keeping good fluid and Porterfield pads on it. And stock master with no prop valve. Not even braided lines I don't think. He's done fairly well with the car, won Rally Colorado regional day 2 in 2006 but that win was all codriver for sure.

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Grant Hughes

Link to spreadsheet?

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Grant Hughes

http://www.main.experiencetherave.com/subaru/techinfo/brakemath.xls

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FunctionAuto
the 2pot rear wear through very quickly

Wheel scrapers will help emmensly with that.

Just remember, with a slider calper you double the piston area. That may explain the lower torque of the fixed caliper.

Off the top of my head, with all the stuff as delivered from the factory, one of the big things about the 4/2 setup is that the brake distribution is shifted way to the rear compared to the WRX/etc stuff. Front loses a tiny bit of clamp force due to slightly smaller pistons on the 4-pots vs 2-pot sliders, but rear gains a lot of torque from larger rotors; 290x18 vs. 266x18 for the vented Legacy rotor. Cheaper way to get a 290mm rotor is ye infamous H6 upgrade, but that's a non-vented rear rotor.