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Discussion Starter #1
I know some people here are very knowledgeable when it comes to selection of MC's vs the calipers actually used.

Here's my setup.

Rear: Typical 11.65" Cobra setup 38mm calipers

Front: Typical 13" Cobra setup w/ A.) 94-98 38mm calipers or B.) 03-04 Mach 1 40mm calipers


For a master cylinder i have a 94-95 Gt/V6 15/16" bore MC


Now, what would be an ideal matching setup? RIght now i want to run the larger Mach 1 calipers but i don't want to run into issues with the pedal being softer than if i ran the 94-98 Cobra calipers?? I have both, so it's just a matter or selecting which to use.

Or, should i ditch my current MC and go with a 93 Cobra MC with either of the two calipers?
 

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Now, what would be an ideal matching setup? RIght now i want to run the larger Mach 1 calipers but i don't want to run into issues with the pedal being softer than if i ran the 94-98 Cobra calipers?? I have both, so it's just a matter or selecting which to use.

Or, should i ditch my current MC and go with a 93 Cobra MC with either of the two calipers?
It all has to do with calculating hydraulic ratios and then plugging in differnt components into the calculations to see how they stack up.

First off, let's get some numbers:

Early PBR Cobra Calipers have two 38mm pistons, giving a total area of 238.8mm^2 (area - pi * diameter). Later PBR Cobra calipers have 40mm pistons, giving a total area of 251.3mm^2. (For comparison, the old SVO calipers had a single 73mm piston with 229.3mm^2).

The 15/16" master cylinder has an area of 74.8mm^2 and the larger 1" master cylinder has 79.8mm^2. The big-daddy is the SVO master cylinder with 89.8mm^2.

When Ford first introduced the PBR Cobra caliper in the '93 Cobra R, it had a 1" bore, giving it a hydraulic ratio of 3.0:1 (caliper area divided by master cylinder ares.) Folks complained about the stiff pedal, so when the '94 Cobra came out, Ford fitted a smaller 15/16" master cylinder, giving it a more conservative 3.2:1 ratio, resulting in a longer, softer pedal.

I SUSPECT (because I can't remember!) that the later Cobras with the 40mm calipers used a 1" bore master cylinder, giving it a firm-ish 3.1:1 ratio. Putting in a 15/16" MC would have given it a less-form 3.4:1 ratio -- still not bad, though.

So if it were me, I'd just mix and match the caliper and master cylinder and try to keep the ratio around 3.2:1 or so. A smaller ratio means a shorter, firmer pedal. A bigger ratio means a longer, lighter pedal.

(Case in point -- for a very short time, I drove my car with early Cobra PBR brakes and an SVO master cylinder, giving me a stupid-hard pedal -- the ratio was 2.7:1.)
 

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Discussion Starter #3 (Edited)
Actually, i may have incorrectly quoted the gt/v6 mc. I beleive the 94-95 GT MC is 1 1/16" and not 15/16" like i previously claimed.

But using your math, that gives it an area of 84.7mm^2

With 38x2 calipers it gives a ratio of 2.8:1

With 40x2 calipers it gives a ratio of 2.96:1


So i guess the question is, is 2.96:1 close enough to be comfortable with? I guess i'll have to try it and see now. If i don't like it, i'll change to a '93 Cobra MC giving me 3.15:1

Thanks
 

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Honestly, I don't know that you'd even notice the difference between the 38mm and 40mm pistons.

Edit: with the same MC.
 

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Discussion Starter #6
Well i'm going to use the 40mm units with the 1 1/16 MC and see how i like it.
 

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Well i'm going to use the 40mm units with the 1 1/16 MC and see how i like it.
I understand the above math and worried that my conversion using the 40mm Cobra calipers with the existing large by huge SVO M/C would yield a very stiff pedal as noted above. However, at the time I was not prepared to switch M/Cs and to my surprise, the SVO unit was not nearly as stiff as I thought it would be given the above. Now, this could be because of my rear set-up which uses the Turbo Coupe/93 Cobra 45mm Varga caliper. I don't know but the pedal feel is nearly identical to the feel of my 01 Cobra. You might be surprised too.
 

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The formula for the area of a circle is A=pi*r^2. Robert posted the formula for the circumference of a circle C=pi*d (usually written as C=2*pi*r). This is a huge difference, since the area varies with the square of the diameter. A 10% difference in piston diameter makes a 21% difference in piston area, which makes a 21% difference in travel and required pedal force.

For anyone that seriously wants to do this, just do it in Excel. Put in all of the master cylinder diameters across the top. Calculate their areas one cell down. Put the different brake caliper piston sizes vertically on the left side. Calculate their areas one cell to the right. At the intersection of each of the caliper/MC combinations, calculate the ratio of areas.
 

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The formula for the area of a circle is A=pi*r^2. Robert posted the formula for the circumference of a circle C=pi*d (usually written as C=2*pi*r).
Holy crap!!! My spreadsheet's been wrong for years!

THAT'S embarassing... :eeek:


For anyone that seriously wants to do this, just do it in Excel. Put in all of the master cylinder diameters across the top. Calculate their areas one cell down. Put the different brake caliper piston sizes vertically on the left side. Calculate their areas one cell to the right. At the intersection of each of the caliper/MC combinations, calculate the ratio of areas.
Yep -- just make sure you use the right formula! :crying:
 

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Discussion Starter #10
Ya know, i was doing it out myself and said to myself "why doesn't this formula for area seem right?"....and yet i still did it out your way ;)
 

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i say go with the 93 cobra 1" bore. the booster will make a big difference too, are you changing it as well? I have the 93 cobra with the stock 91 booster and the pedal is a little firm but not too bad. If i had the 94/95 GT MC and the fox booster you will have a hard pedal.
 

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Discussion Starter #12 (Edited)
SN95 booster is what i'm using
 

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Discussion Starter #13 (Edited)
If i had the 94/95 GT MC and the fox booster you will have a hard pedal.

Just an update.

I ended up installing the 94/95 GT MC. 1 1/16" bore. I used the Mach 1 40mm piston calipers up front and the standard SN95 38mm piston calipers out back.

I used the STOCK Fox booster and went for a quick drive around the block just to se how it feels. The pedal was hard as a rock! So i quickly drove it back.

Almost done swapping the SN95 booster in now, but i'm wondering how the feel will be with the SN95 booster and the 94-95 GT MC?? Will the pedal still be a little hard?? SHould i plan on swapping to a 1" '93 Cobra MC now?? I'm going to try it first and see how it feels before deciding on the MC

Reason i didn't swap the booster out initially was because it looked like a huge PITA so i wanted to see how bad it would be with the fox booster. It's bad. Nowhere near safe to drive on. I've almost got the new booster in now anyway.
 

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Discussion Starter #14
Okay, setup is complete.

94-95 SN95 booster, 94-95 GT 1 1/16" MC

40mm Cobra calipers, 38mm Cobra rears


Now, pedal is a little on the firm side of things. I've put about 100 miles on the combo and it's lacking some woah factor. At 50MPH if i jump on the brakes they just feel firm. Laying into them gets the car to stop fast, but i can't lock them up. I have the gutted prop valve mod with the adjustable valve as well. Right now i'm at 2 turns in and still can't get a single wheel to lock. Tires are kinda crappy, so i'd expect them to lose traction.

Question is, should i swap to a 1" '93 Cobra MC? Will the smaller bore give a softer pedal and increase braking force? I bought one, but i don't want to swap MC's if it's not going to help.
 

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Discussion Starter #16
Simple enough. Thanks
 

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Discussion Starter #18
Purchased the '93 Cobra MC, but I have yet to swap it out. Vacations and weddings are inturrupting my weekends.

I'll post back when I get it swapped out
 

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Okay, setup is complete.

94-95 SN95 booster, 94-95 GT 1 1/16" MC

40mm Cobra calipers, 38mm Cobra rears


Now, pedal is a little on the firm side of things. I've put about 100 miles on the combo and it's lacking some woah factor. At 50MPH if i jump on the brakes they just feel firm. Laying into them gets the car to stop fast, but i can't lock them up. I have the gutted prop valve mod with the adjustable valve as well. Right now i'm at 2 turns in and still can't get a single wheel to lock. Tires are kinda crappy, so i'd expect them to lose traction.

Question is, should i swap to a 1" '93 Cobra MC? Will the smaller bore give a softer pedal and increase braking force? I bought one, but i don't want to swap MC's if it's not going to help.
this is my exact setup and its too firm to me.interested in how the 93 cobra master feels when you get it on.
 

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Great conversation on the master cylinder sizing subject. A few points to consider with respect to Ford's history on master cylinder sizing with Cobra brakes (and vacuum boosters). The larger 205mm tandem booster was first used in the '93 Cobra and Cobra-R. It has the unique Fox bolt pattern for the dash of the Fox cars. The '94 through '04 V6 and the '94 / '95 GT, Cobra and Cobra-R all have the same size 205mm tandem booster with the SN95 dash pattern. The earlier '94 (and '93 Cobra-R) through '98 Cobra front calipers had twin 38mm pistons while all later fronts ('99 through '04) had the larger 40.5mm pistons. For the earlier 38mm pistons Ford sized the street oriented '94 / '95 Cobra with the 15/16" master cylinder. This combination assured easy lock up even when the brakes were cold. For the track models ('93 and '95 Cobra-R) the same 38mm front calipers were matched to a 1" master cylinder for that extra bit of reserve in travel when things get worn on the track (tapered pad wear, knock back, etc.). Losing the ability to easily lock up the brakes when cold was the right compromise to make for the "R" cars. Thus, our recommended sizing is the same: If you are running the larger booster and 38mm front piston calipers, the 1" master cylinder is a great track / street combination. If it is for the street only, the 15/16" may suffice. If you are running the larger 40.5mm piston front calipers, the master cylinder can be increased by 1/16" to get the same result. In other words with the 40.5mm pistons, the 1 1/16" size is great for the track / street while a street only car might be better with the 1". In all cases if high friction pads are used you can get away with a larger master cylinder diameter if you choose. Conversely if you are running the smaller Fox booster all of the above sizing can be reduced by about a 1/16" in master cylinder diameter. I hope that helps. Also, the applications for the various size Mustang master cylinders are as follows: 15/16" - '94 / '95 Cobra, 1" - '93 Cobra, 1 1/16" - '94 / '95 V6. Lastly, if you happen to have a '96 - '98 V6 car, your pedal ratio is 3.92:1 while all other vacuum boosted cars were 3.5:1. This means that for you all of the above should be reduced by 1/16" in master cylinder bore diameter.

Ron
 
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