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What's the deal with pedal boxes?


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#1 leesfer

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Posted 14 January 2015 - 01:12 PM

Been trying to figure out a good brake upgrade and know that our pedal ratio is pretty s*** at ~4:1 for a 15/16" master cylinder so I started looking at brake boxes from Wilwood and Tilton... but those have a little over a ~5:1 ratio but only a 1" master cylinders and no booster?

Can anyone who is smarter than me explain how a 5:1 ratio pedal and a 1" master cylinder with no booster can flow enough for 4 pot calipers?

If I am doing my math correct a four pot caliper with 1.75" pistons has a one-side surface area of 4.82 sq in and 1" master cylinder has 0.7854 sq in surface area. So the MC is ~16% of the piston area. But with no booster and a 5:1 ratio shouldn't this be much higher than 16%?

The largest MC that Wilwood sells is 1 1/8" which would make that 20% of the caliper piston surface area. But a 5:1 ratio pedal still seems low for manual brakes?

What 's the deal?

Edited by leesfer, 14 January 2015 - 01:12 PM.

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#2 markhansenconquest

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Posted 14 January 2015 - 02:02 PM

Try convette on here.. he has some brake for sale on here right now and his car is sick........

#3 leesfer

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Posted 14 January 2015 - 02:07 PM

I've already have the calipers + rotors sorted, just worried about the MC right now

I know Convette is running the MC and booster from a SVT Cobra which I have been thinking about doing, too. Just looking around at options. The Cobra booster looks a lot thicker than the Starion stock one, not sure if the strut tower needs to be modded to make it fit or not. If it does, I'd rather just go all the way and mod for a pedal box and manual brakes
1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#4 speedyquest

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Posted 14 January 2015 - 02:14 PM

I'm interested in what you find on this topic as replacing the brake system entirely is something I plan on doing at some point and a pedal box was definitely included in that plan.

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#5 383chevelle

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Posted 14 January 2015 - 03:18 PM

If you have a true four piston caliper then you need to muiltply the area of one piston by 4 not 2 which would make the area of the pistons 9.61625. The area of the master cylinder being small compared to the caliper area is what gives you some hydraulic advantage. Given the same pedal force and caliper a smaller master cylinder will create higher line pressure, but will require more pedal travel to actually clamp the pistons on the rotor. This is caused by the smaller diameter MC discplacing less fluid for a fixed pedal travel when compared to a lager MC. The higher line pressure will lead to more clamping force at the rotor given the same piston area. So there is a trade off between haveing a large diameter MC that you have to push really hard with very short travel or a small diameter MC that you have to push a long way but with less force. There are calculators online that can help with the MC sizing based on car weight, speed, piston area, and pedal ratio.

If I remember correctly brake boosters multiply the force that you input on the pedal, applying that greater force to the master cylinder. This makes the driver have to input less pedal effort for the same stopping power. If you have never driven a manual brake car they take a little getting used to.

Hope this is all correct been awhile since I sized the brakes for the schools FSAE car.  Hope that helps

Edited by 383chevelle, 14 January 2015 - 03:19 PM.


#6 speedyquest

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Posted 14 January 2015 - 03:34 PM

Also another thing to consider, if I remember correctly, I've seen pedal box setups that use dual cylinders for the brakes.

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#7 383chevelle

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Posted 14 January 2015 - 03:48 PM

Yes that is true you have to take into consideration all the pistons that are actuated by that MC. If your going to do a pedal box make sure it has dual master cylinders. This will give you the ability to use a balance bar in the  pedal box to adjust brake bias!

#8 leesfer

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Posted 14 January 2015 - 04:04 PM

View Post383chevelle, on 14 January 2015 - 03:18 PM, said:

If you have a true four piston caliper then you need to muiltply the area of one piston by 4 not 2

Are you sure? I've always heard that you calculate using only one side of the caliper

Great info though, I will look around for some calculators to find a good MC

Definitely going to use a dual MC setup, looking at this one in particular: http://www.wilwood.c...temno=340-12411
1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#9 leesfer

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Posted 14 January 2015 - 04:17 PM

Okay so after a few more calculations, it looks like a 7/8" MC with a 5.25:1 pedal ratio will require 137.5 lbs of pressure to produce 1200 psi in the lines

137.5 seems like a reasonable amount of foot pressure on the pedal

Edited by leesfer, 14 January 2015 - 04:17 PM.

1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#10 ucw458

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Posted 14 January 2015 - 04:18 PM

You're looking at it the wrong way.  Sure the calipers have more surface area on the pistons than the MC.  But the MC will stroke somewhere around an inch where the calipers pistons only have to move a few thousandths of an inch.  So the extra stroke length of the MC makes up for caliper piston diameter.
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#11 leesfer

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Posted 14 January 2015 - 04:45 PM

View Postucw458, on 14 January 2015 - 04:18 PM, said:

You're looking at it the wrong way.  Sure the calipers have more surface area on the pistons than the MC.  But the MC will stroke somewhere around an inch where the calipers pistons only have to move a few thousandths of an inch.  So the extra stroke length of the MC makes up for caliper piston diameter.

Good point
1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#12 leesfer

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Posted 14 January 2015 - 04:50 PM

My next question would be... what to do with the clutch MC and slave? Replace them with a Wilwood cylinder also? Not sure how to go about calculating that one
1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#13 speedyquest

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Posted 14 January 2015 - 05:21 PM

I would just get a wilwood MC of the same size as our original one and call it good, but that is just me.

1987 Flatty Starion -- Widebody Conversion -- V-Mount Setup --  JDL Tubular Manifold  --  MX-6 Intake -- MS3-Pro -- AEM FPR  --  ARP head studs -- 3" hood exit exhaust -- Hahn Racecraft 16g --  Tial F38 Wastegate -- Tial Q Bov -- Short route piping -- 5 lug & LSD conversion -- Tubular Front End -- Manual Steering Rack Conversion -- Manual Brake Conversion -- Stack Digital Dash -- AEM UEGO -- D2 coilovers -- full GNT poly bushings & mounts -- four 8" rears -- car lightened significantly...  More to come!


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#14 leesfer

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Posted 14 January 2015 - 05:32 PM

View Postspeedyquest, on 14 January 2015 - 05:21 PM, said:

I would just get a wilwood MC of the same size as our original one and call it good, but that is just me.

That's what I was thinking, too... looks like the stocker is 5/8"
1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#15 383chevelle

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Posted 14 January 2015 - 05:32 PM

To answer the four piston question. If the pistons are active, i.e. solid mounted caliper with 4 moving pistons, then you need to account for the area of all four. If it is a sliding caliper with four pistons then you only have two active pistons that the fluid acts on and the others are just locked in place. So as an example we can set up a simple brake system with one true four piston caliper, one MC and a pedal ratio of 1:1. If we fix the pedal force at 50lbs and say that the pistons need to move a 1/16" to clamp the rotors we can do some math and find out line pressure, clamping force, and pedal travel.
First lets use a 1" diameter bore.

Line pressure will be=force on pedal/diameter of MC bore=50lbs/.785in^2=63.69psi

Clamping force exerted by the caliper=line pressure*total piston area for caliper=63.69psi*9.61625in^2=612.51 lbs

Volume of fluid needed to move a piston 1/16" to clamp the rotor= Area of single rotor*distance piston needs to travel=2.404*.0625=.15025 in^3

Since we have 4 pistons multiply by 4 and get total volume of .60101 in^3

Pedal travel = total volume of fluid needed/ diameter of the MC= .60101in^3/.785 in^2 = .762 in

So for a 1/16" piston movement the pedal will move .762in and will generate 612.5lbs of clamping force at the rotor.

Now what are the numbers if we use a 1/2" bore MC.

Line pressure=254.777psi

Clamping force= 2450 lbs

pedal travel= 3.0625 in

so with a .5" bore MC it takes a pedal movement of 3.0625in to clamp the rotor and will generate 2450 lbs of clamping force at the rotor.

Well hopefully someone will find this helpfull. If you want to expand this for your car just account for the additional calipers in the piston area, and the extra leverage provided by the pedal ratio.

#16 383chevelle

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Posted 14 January 2015 - 05:37 PM

View Postleesfer, on 14 January 2015 - 04:17 PM, said:

Okay so after a few more calculations, it looks like a 7/8" MC with a 5.25:1 pedal ratio will require 137.5 lbs of pressure to produce 1200 psi in the lines

137.5 seems like a reasonable amount of foot pressure on the pedal

That is a resonable pedal force it might even be a bit light. We tested about 50 poeple when sizing the brakes on the fsae car and 150 to 175 was what we decided on for the max pedal force. This test included 5'3", 110lbs female subjects, so it should be no problem at that level.

Also be carefull just picking the stock size MC because you will have two master cylinders with the pedal box only half the brakes will be operated by one where the stocker runs all four brakes. I guess it depends on what calipers you are using too.

#17 leesfer

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Posted 14 January 2015 - 05:46 PM

View Post383chevelle, on 14 January 2015 - 05:37 PM, said:

That is a resonable pedal force it might even be a bit light. We tested about 50 poeple when sizing the brakes on the fsae car and 150 to 175 was what we decided on for the max pedal force. This test included 5'3", 110lbs female subjects, so it should be no problem at that level.

Also be carefull just picking the stock size MC because you will have two master cylinders with the pedal box only half the brakes will be operated by one where the stocker runs all four brakes. I guess it depends on what calipers you are using too.

It's either 137lbs or 180lbs of force, with the Wilwood MC it jumps from 7/8 to 1" ...can't decide if 180 would be too much

The stock MC I was talking about is for the clutch, not the brakes


I will be running 6 piston fronts and 4 piston rears:

Two 1.62" and four 1.12" pistons on the front calipers, four 1.75" pistons on the rears

Edited by leesfer, 14 January 2015 - 05:48 PM.

1986 Mitsubishi Starion: Holset HX35, BEP .63A/R hot side, Turbonetics T3 manifold, Magna MPI with 65mm TB, Schneider valve springs, roller rockers and Howlin' Banshee cam, MegaSquirt ECU, Turbosmart 38MM wastegate, TurboXS RFL blow off valve, 2.5" hard pipes, 4" *DM intercooler, SR20 aluminum radiator and fan shroud, MSD 6al and tach adapter, MSD Blaster 2 coil, NGK plugs and wires

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#18 leesfer

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Posted 17 January 2015 - 11:21 PM

After some measurements, realized a floor mount pedal box will not fit in our cars

Back to square one
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