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Brake Master Cylinder question!

Old 02-17-13, 03:01 PM
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Lightbulb Brake Master Cylinder question!

Hi!

A couple of years ago i bought some really big NASCAR (Alcon XF) front calipers. This wasn't my most thought out investment ever...

So here i am now, in need of bigger brakes. My OEM brakes cant handle the slick tires im using.

The Alcon calipers have two different size pistons; 44mm and 47mm. To be able to retain the brake bias i need a rear caliper with a single 43mm piston (im looking at Audi RS6...). I have 328/35 front rotors already and im thinking about using Rx7 Type RS 314/20 rotors at the rear. That might not work, but that is not what this thread is about.

NOW TO THE QUESTION!

I want to retain the OEM brake system. I have read about the 929 master cylinder, but if i want the OEM pedal feel and pedal travel i need a 1,25" cylinder (IF my calculations are right). I found one on eBay and it looks like it could fit!

Check it out: 1997 1998 Mazda Protege Brake Master Cylinder | eBay

Am i crazy (read ignorant) or might this work?
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Old 02-17-13, 03:47 PM
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I already see a problem with this. The cylinder i found is non-abs so there is a third port at the bottom. Can i just plug this port? I think they join internally anyway(?).

More investigation needed...
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Old 02-18-13, 01:30 AM
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I replied to an older post like this one, and i modified the stock pedal ratio to 2.85 to 1, and a 929 mc. Also a miata booster raised the pedal back up with stock ratio. 14 in front, brembo gt nascar front. I ran c5 corvette rear calipers on 12.7 supra front rotors in rear. Had to dila out a little rear brake, but could run as is.
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Old 02-18-13, 01:32 AM
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BTW you need 929 with ABS master cyl.
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Old 02-18-13, 04:59 AM
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Great info, thanks! Now i got even more questions for you:

1. How did you modify the ratio?

2. How is the Miata booster different from the FD booster?



I'm afraid bottoming out the 929 cylinder with these brakes, but if you haven't that's great news.

Are you sure about the third port on the master cylinder? I have done some reading and found out that some BMW:s have the third port plugged from factory when using ABS. The non-ABS version uses the same cylinder and all three ports. Isn't the third port the same as the rear port? Two ports for the front brakes and one for the rears (connected to a T-coupling)? Someone must know this!


3. Another question,Is this assumption correct:

I get the same pedal feel with a 1.25" master cylinder and stock (4:1) pedal ratio as with the stock 15/16" master cylinder and a pedal ratio of 2:1.
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Old 02-18-13, 11:35 AM
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the difference between the 929 ABS master and the non ABS master is that the non ABS master comes with a Y fitting, to make the front port have two ports. err well it doesn't come with it, but its in the picture when you look it up in the parts catalog.

so i would think plugging the extra port would be ok too , or you could put your brake pressure sensor in there.
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Old 02-19-13, 03:02 AM
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I modified the ratio by drilling a hole further down the brake pedal and building a very strong bracket . But thing the miata booster with stock ratio is better . The pedal is firmer and the miata booster is smaller in diameter.

I used a % formula to determine what ratio to go with.

1. my calipers square inches of volume were 24% bigger than stock

2. % increase of mc: .803 to .996 is 20%

3. force: stock mc 4:1 = 85 lbs force ( estimated based on common force)
" " 3:1 = 113 lbs (25% increase okay)
1.1/8 3:1 = 140 lbs (60% too high)


Also this works

Ideal pedal force should be 70-100 lbs.
square inches of stock rx7 front caliper = 32mm x32mm x3.14= 3215.36 x2 ( 2 calipers) = 6430.72
34/40 mm caliper =8540.8. Now determine the % difference.

85 pounds pedal force x ratio (4:1) divided by .803 (square inches of master cyl) = 448 psi ( stock rx7 line pressure

[/B][/B])[/B]and at 3:1 would be 423 psi
and with .996 = 341 psi

To get the psi back up takes a lot of pedal force.

so with a 3:1 ratio , 423.41 = (f) x (3:1) divided by .803 , you get 3f=339.99, so F equals 113 pounds which is high but okay.

there are many miata boosters , i think you need year 2000 sport w abs. i would have to check on that
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Old 02-20-13, 01:32 PM
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j9fd3s: Yeah, that's what i thought. Thanks!

racingdriver: Thanks for sharing! I don't know if you have a FD or not, but i get these numbers:

Stock calipers:

Front 36mm pistons.

(18 x 18) x 3,14 = 1017mm2

1017 x 8 pistons = 8136mm2 total front area.

Rear 35mm pistons.

(17,5 x 17,5) x 3,14 = 962mm2

962 x 4 pistons = 3847mm2 total rear area.

That is a total piston area of 11983mm2

Front piston bias: 8136 / 11983 = 67,9%

Stock FD master cylinder is 15/16" or 0,938"


Now for the Alcon XF calipers:

44mm and 47mm pistons.

(22 x 22) x 3,14 = 1520mm2

(23,5 x 23,5) x 3,14 = 1734mm2

(1520 x 4) + (1734 x 4) = 13016mm2 toatal front area (lol)

Now if i go with the C5 calipers (44mm piston) you recommend i get a total rear area of 6079mm2.

That is a total of 19095mm2 and 68,2% front piston bias. Thats about perfect!

The total area is now 37% greater than stock. That means i need a 37% bigger master cylinder.

0,938" x 1,37 = 1,29"

^Is this correct?


The protege cylinder i found is not 1,25", i must have been drunk or something because i cant see how i thought it was that big...

What are my options? Fit a master cylinder from another car or build a dual MC setup? How big MC:s do i need if i go dual?


If i keep the pedal ratio stock, is the Miata booster enough?



PS. These calculations are written for me to keep track of what the hell i am doing! Im the student here
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Old 02-22-13, 01:58 AM
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with the 929 master your stroke will increase but you won't bottom out. get the 2000 miata booster and try it. Both parts are relatively cheap and readily available on e bay. If somehow your pedal is still low then modify the pedal ratio but its a lot of fabrication you probably don't need to do.
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