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Old 10-08-2002, 09:40 PM   #1
Legacy777
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Default Master Cylinder sizing, brake pedal dead area, brake math

second edit: have added info for the sti 4-pot & 2-pot calipers

Here is the explanation for my assumption for the Sti calipers. On my original calcs I assumed the pistons would move .125" When doing the 4-pots I assumed each piston would only move half that amount because you have opposing pistons which will hopefully oppose each other's forces and reduce the amount of piston movement.

In the real world it will probably be a little more then half, but not quite the whole amount


Ok......here's a little info/background on my problem and current setup.

There are two distinct and different issues I have gotten, due to the front WRX brake swap & rear turbo legacy brake swap.

I also have SS lines & Mintex 1155 pads.

The first is that the pedal stroke is longer. It takes more travel of the brake pedal before the brakes start to engage. It does not give a smooth transition from light braking to heavy braking. Most of the modulation has to come from my foot.

I just got a set of FSM's for 92 & 93 legacy MY's. I did find out something new. My legacy, stock came with a Master cylinder with 1" effective piston diameter, front calipers with a 2.252" effective piston diameter, and rear calipers with a 1.374" effective piston diameter.

In swapping to the current setup of WRX fronts & turbo legacy rears I now have same master cylinder, front calipers with 1.685" * 2 effective piston diameter and rear calipers with a 1.5" effective piston diameter. So I have increased the volume of fluid that must be displaced front and rear. I had thought I only increased the fronts, but the turbo legacy's were larger.

The turbo legacy has a master cylinder with a 1.059" effective diameter. So it's a tad bigger then my stock one. The WRX has a master cylinder with a 1.0625" effective diameter, which again is slightly bigger still.

I basically have a WRX brake setup. The piston areas are exactly the same as the WRX.

What I am thinking is either swapping over to a turbo legacy master cylinder or WRX master cylinder. I don't know if the WRX one will be too much of an overkill because I don't have ABS. With a larger bore on the master cylinder the more mushy the pedal feels.

I have also heard you can adjust the brake pedal and it's travel, but I have not really checked it out on the car yet.

A possible ideal setup might be turbo legacy master cylinder with single stage booster, as it won't provide as much braking assistance so the braking force will be more linear......I'm not sure yet. I have to do some calculations and such.

The next issue I have gotten is the braking bias is weighted too much front, due to the increase in piston area in the front and such. I have to redo the calculations to see how much it increased over the stock setup now that I know the rears also increased.

I have looked. There are no good aftermarket solutions to adjusting brake bias on a subaru, without re-plumbing the entire car. So what I'm possibly going to look into is getting a proportioning valve from a wagon. The split point for the sedan is 533 psi. The split point for the wagon is 640 psi. It's about 100 pounds greater on the wagon. So it's possible in putting the wagon prop. valve on, I will gain more rear brake bias. BUT.....again, I need to do some calcs and see that it won't be too much rear bias.

I've got some whiteline springs to install, which will prob help nose diving and such a dramatic weight transfer when compared to the stock setup, so that too may help.

So....those are my ideas. All the numbers and info I gave you, that refers to specific diameters and such came directly from the factory service manuals, so I know they are correct.

Here's some general calculations I have done in reference to the master cylinder sizing issue and pedal travel for the stock setup, current setup, and possible solutions.

**List of Assumptions for my calcs.**

1. Assume 0.125" of caliper piston travel on all sliding type calipers, front/rear & left/right.

2. Assume 0.0625" of caliper piston travel on all opposing piston type calipers, front/rear & left/right.

2. Pedal travel due to unmeasureable issues, or small amounts have been omitted. i.e. some of the stuff romanom mentioned here

Calculations:

Legacy MC diameter 1"
Turbo Legacy MC diameter 1.0625"
WRX MC diameter 1.0625"

Legacy MC radius 0.5"
Turbo Legacy MC radius 0.53125"
WRX MC radius 0.53125"


A=Pi*r^2
Legacy MC area 0.785398163 in^2
Turbo Legacy MC area 0.886640895 in^2
WRX MC area 0.886640895 in^2


Legacy Front caliper diameter 2.252"
Turbo Legacy Front caliper diameter x 2 1.685"
WRX Front caliper diameter x 2 1.685"
Sti 4-pot Front caliper diameter x 4 1.60"

Legacy rear caliper diameter 1.374"
Turbo Legacy rear caliper diameter 1.5"
WRX rear caliper diameter 1.5"
Sti 2-pot rear caliper diameter x 2 1.5"

Legacy Front caliper radius 1.126"
Turbo Legacy Front caliper radius x2 0.8425"
WRX Front caliper radius x 2 0.8425"
Sti 4-pot Front caliper radius x 4 0.8"

Legacy rear caliper radius 0.687"
Turbo Legacy rear caliper radius 0.75"
WRX rear caliper radius 0.75"
Sti 2-pot rear caliper radius x 2 0.75"


A=Pi*r^2
Legacy Front caliper area 3.983149927 in^2
Turbo Legacy Front caliper area 4.459844201 in^2
WRX Front caliper area 4.459844201 in^2
Sti 4-pot Front caliper area 8.042477193 in^2 (Note: area was calculated for one piston and then multiplied by the number of pistons)

Legacy rear caliper area 1.482734343 in^2
Turbo Legacy rear caliper area 1.767145868 in^2
WRX rear caliper area 1.767145868 in^2
Sti 2-pot rear caliper area 3.534291735 in^2

Assume 0.125" of caliper travel for sliding calipers
Assume 0.0625" of caliper travel for oposing piston calipers

(Calcs for both left & right)
V=(A*0.125")*2 or V=(A*0.0625")*2
Volume for Legacy Front Calipers 0.995787482 in^3
Volume for Turbo Legacy Front Calipers 1.11496105 in^3
Volume for WRX Front Calipers 1.11496105 in^3
Volume for Sti 4-pot Front Calipers 1.005309649 in^3

Volume for Legacy rear Calipers 0.370683586 in^3
Volume for Turbo Legacy rear Calipers 0.441786467 in^3
Volume for WRX rear Calipers 0.441786467 in^3
Volume for Sti 2-pot rear Calipers 0.441786467 in^3

Combined f & r volume for Legacy 1.366471068 in^3
Combined f & r volume for Turbo Legacy 1.556747517 in^3
Combined f & r volume for WRX 1.556747517 in^3
Combined f & r volume for Sti 1.447096116 in^3

Travel=V/A
Pedal travel for Legacy 1.739845"
Pedal travel for Turbo Legacy 1.755781315"
Pedal travel for WRX 1.755781315"

Current Pedal travel 1.9821125"



So.....out of all that math.....

The take home numbers are.

I have close to 2" of pedal travel with my current brake setup due to the small 1" bore on the MC.


If I were to put a WRX MC on, I would have around 1.76" of pedal travel.

Again......these numbers are purely generalistic type numbers, and are not necessarily correct numbers by any means. There are just to judge and compare the different caliper/MC setups against each other.

Like I mentioned before. It looks like the turbo legacy one should provide enough volume of fluid, but be a tad smaller to help against mushy pedal.

With the addition of the single stage booster I would assume pedal feel would improve and brake force would be more linear. Am I correct in thinking that?

Comments/suggestions?

Josh
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Last edited by legacy777; 05-08-2003 at 11:35 PM.
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Old 10-09-2002, 03:45 PM   #2
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Looks like I should've provided cliff notes.......nobody wants to read this thing......to give comments
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Old 10-09-2002, 06:34 PM   #3
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romanom, you out there?

Any suggestions on whether the WRX or Turbo legacy MC would be better.

Also.....whether the single stage would be beneficial or not.
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Old 10-13-2002, 07:18 AM   #4
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I like the way your going about this. Your on the right track.

With the increased volume requirements you will have a longer pedal travel with the 25.4mm (1") TMC (tandem master cylinder) than you had previously. This is just basic commonsense

By going to larger bore TMC you will improve (shorten in this case) the pedal stroke vs. deceleration curve and I don't think you will have a real significant effect on effort vs. deceleration as long as you don't go beyond a 26.99mm (~1.1") TMC.

So your thinking right on the TMC, going to the larger WRX will shorten your stroke, but I doubt you will any difference in effort and feel, if that's the only change you make.

As far as the booster is concerened, going to the single will more than likely firm up the pedal as most single boosters are designed to produce less assist in practice. However, in theory the real difference between and single and double (tandem) booster is the maximum output (assist) available not the ratio. Example; a single and tandem can both have a 7:1 boost ratio where for every 10 lbs of force in the booster produces 70 lbs out. The difference is that the tandem will produce as max assist of say 500lbs, while the single will produce a max of 400lbs. The question becomes how much assist does a vehicle require, a 3500lb or less vehicle does not require all that much max assist to be safe.
And in practice since most singles are used on light vehicles, they tend to be designed to with a smaller boost ratio. In my judgement a 10" (255mm) single booster is perfectly safe to use on any vehicle under 3500lbs. One last thing to keep in mind is that the booster must have the same or greater stroke as the TMC and singles have stokes anywhere from 30mm to 36mm, while tandems go from 34mm to 42mm. And, the new booster HAS TO fit in the vehicle the same way the old one did.

Now for the proportioning valve. Changing the prop-valve is purely and totaly a "pesonal judgement call." Since changes made to a vehicle by it's owner are unique and each person has their own likes and dislikes, the ownere must decide if bias needs to be changed and by how much. I can only advise you remember that the front brakes are doing a minimum of 80% of all the braking.

Now for the pedal, adjustment of the pedal is PURELY for the factory. Don't touch this as you can find yourself in some serious trouble. The only adjustment in pedals that will have a great effect on pedal travel (and effort) is the pedal ratio (the amount of mechanical advantage). This is just the ratio of the distance from the center of the pedal pad to the pin where the booster attaches divided by the distance from the center of the pedal pad to the hinge point at the top of the pedal arm. Decrease the pedal ratio and you will increase effort and decrease stroke. You can do this by simply moving the pedal pad itself up on the pedal arm. How much you move it depends on the size of your foot.

Just keep in mind that in the end the calipers on any brake system are the weak spot. No matter what you do to the rest of the system if the calipers have lots of deflection and the pistons lots of roll-back your system will always feel a bit poor



P.S.-Unlike before when I was on this board a lot, I'm working again, working on my advance degree and have ANG (Air National Gaurd) duties so time is limited.
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Old 10-13-2002, 02:46 PM   #5
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Hey romanon,

Thanks for the comments. Like you said, I think I'm on the right track with the larger MC.

I will most likely try and snag the larger one from a WRX, but will wait until I drive both candidates and do the other calculations for the turbo legacy.

The booster......I'm not going to do anything else until I get the larger MC in, and see how I like the setup. I know the single stage boosters are smaller.......which will move how the brake lines line up with the MC......but I don't know if that would be a big deal....but then again it could cause more problems.

The prop valve, I need to do some research on all that stuff. I will post another thread when I get time to go through all that. However, one of the reasons I feel it is too front biased is in another thread (which I can't seem to find now) you mention if your fronts lock up before rears....you have too much front biased.

I won't mess with the pedal stuff.....don't want to screw things up

Piston roll-back will be my braking setup downfall....yeah well....I didn't feel dumpin thousands of dollars on an older car. It'll be better when I get it all worked out.

No worries about the time......I'll just PM ya if I have any revalations or new info

Thanks again

Josh
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Old 10-13-2002, 03:16 PM   #6
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Forgot about the tubes! Most new cars have flex hoses (SS braided lines) in the first 6 inches of the tubes near the TMC to make assembly easier and they have a lot of.....FLEX. The only problem here bending the tubes, if you can get them on the TMC without this happening you won't have problems. You can by Flex Hoses to install inbetween the TMC and tubes. And as I stated before the input rod of the booster must line up with the pedal pin or you can stress the internals of the booster and cause a failure. The normal standard is no more than +/- 3 degrees of vertical travel of the input rod as the pedal is stroked.


If your fronts lock-up before the rears then you should change the prop-valve, I didn't realize that was happening.

To be honest, the pedal is the easiest to change since all you need to do is cut the pedal pad off and re-weld it farther up on the pedal arm. I've done this dozens of times, it's real easy.
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Old 10-13-2002, 05:02 PM   #7
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Yeah, the lines comin up to my current MC are hard-lines. So I shouldn't have that flex there, that some systems will.

I'm pretty sure the MC's are the same between different models. So for the first round of just swappin the MC, I should be ok....should is key word.

From what you have said.....about getting the booster and pedal alignment right.....I'm not sure if it's worth swappin the booster, if I run into problems.

I shouldn't have any issues with alignment and such with just swapping the MC.....to my knowledge it is just bolted onto the booster with two bolts.

The pedal......honestly.....I'm happy with where the pedal is......just not how much travel it has.

Josh
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Old 10-13-2002, 05:08 PM   #8
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Forgot to add about the prop. valve.

The difference in split points between the current one....and the one I'm looking to put in is about 100 psi.

I do have some info I'm goin to use to work out some of the pedal forces and brake pressures.

But I'm curious what your initial thoughts are.

Thanks

Josh
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Old 10-13-2002, 06:09 PM   #9
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Quote:
Originally posted by legacy777
Yeah, the lines comin up to my current MC are hard-lines. So I shouldn't have that flex there, that some systems will.

I'm pretty sure the MC's are the same between different models. So for the first round of just swappin the MC, I should be ok....should is key word.

From what you have said.....about getting the booster and pedal alignment right.....I'm not sure if it's worth swappin the booster, if I run into problems.

I shouldn't have any issues with alignment and such with just swapping the MC.....to my knowledge it is just bolted onto the booster with two bolts.

The pedal......honestly.....I'm happy with where the pedal is......just not how much travel it has.

Josh


Chances are that if the TMC's are both from the same supplier you should have no problems, some TMC's have different port positions due to special packaging concerns but this is visually noticable. The only other possible problem is the nuts on the tubes being too small or too large, but you can buy adapters for this. Of course the TMC must actually fit on the booster and seal properly. Another thing you need to look at is the "Z-position", that is the red marked distance on the picture attached.

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Old 10-13-2002, 06:14 PM   #10
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Quote:
Originally posted by legacy777
Forgot to add about the prop. valve.

The difference in split points between the current one....and the one I'm looking to put in is about 100 psi.

I do have some info I'm goin to use to work out some of the pedal forces and brake pressures.

But I'm curious what your initial thoughts are.

Thanks

Josh
Using that 640 in place of the 533 is going to be just about right. Also the WRX prop-valve should work well if it packages with the tubes.
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Old 10-13-2002, 08:27 PM   #11
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Quote:
Originally posted by romanom




Chances are that if the TMC's are both from the same supplier you should have no problems, some TMC's have different port positions due to special packaging concerns but this is visually noticable. The only other possible problem is the nuts on the tubes being too small or too large, but you can buy adapters for this. Of course the TMC must actually fit on the booster and seal properly. Another thing you need to look at is the "Z-position", that is the red marked distance on the picture attached.

Is there any way to measure the Z-position between the two master cylinders?

I'm pretty sure this stuff is similar between subie models. I'll try and do comparing with some friends with WRX's.

I've got FSM's for my 90, 92-93's, and the WRX. So I'll do some parusing between the different models.

Josh
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Old 10-13-2002, 08:35 PM   #12
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Quote:
Originally posted by romanom


Using that 640 in place of the 533 is going to be just about right. Also the WRX prop-valve should work well if it packages with the tubes.

I have to be careful on which prop-valve I use. I have to make sure to snag one from a non-abs wagon. The abs & non-abs models have different types of prop-valves.

The WRX prop-valve has an even lower split point then the legacy, plus it's for ABS......so I prob. won't be using it.

Good to know that the 640 should work better.

Josh
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Old 10-13-2002, 08:53 PM   #13
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Quote:
Originally posted by legacy777


Is there any way to measure the Z-position between the two master cylinders?

I'm pretty sure this stuff is similar between subie models. I'll try and do comparing with some friends with WRX's.

I've got FSM's for my 90, 92-93's, and the WRX. So I'll do some parusing between the different models.

Josh
What I do to take the measurement is measure the depth of interior of the piston (where the black arrow is) then the exterior of the piston to the flange, the difference is the Z-dim (or also called the critical dim).

Good luck
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Old 10-13-2002, 08:53 PM   #14
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Quote:
Originally posted by legacy777



I have to be careful on which prop-valve I use. I have to make sure to snag one from a non-abs wagon. The abs & non-abs models have different types of prop-valves.

The WRX prop-valve has an even lower split point then the legacy, plus it's for ABS......so I prob. won't be using it.

Good to know that the 640 should work better.

Josh
Sorry, I just assumed all the vehicles in question are ABS vehicles.
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Old 10-13-2002, 10:22 PM   #15
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Quote:
Originally posted by romanom


What I do to take the measurement is measure the depth of interior of the piston (where the black arrow is) then the exterior of the piston to the flange, the difference is the Z-dim (or also called the critical dim).

Good luck
Thanks......I'll put this info to good use.



Quote:
Originally posted by romanom


Sorry, I just assumed all the vehicles in question are ABS vehicles.
No worries man

Josh
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Old 10-21-2002, 11:13 PM   #16
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bump for the edit
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Old 10-22-2002, 02:59 AM   #17
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It seems you've taken a tad bit of an armchair approach to this.

From your initial post, it's obvious to me that you just need to adjust the nut/threaded portion on the pedal "plunger" to reduce the pedal play which is where your added and unwanted travel is coming from. Do this, making sure you don't reduce the travel to the point that your brakes drag. Also, bleed your system again.

It'll probably feel fine after this.

After swapping in the Impreza L non-abs booster and MC into my RS, it took a while to get the fine adjustments in travel to where it felt "right".

As to replacing the MC with a WRX MC, I wouldn't suggest replacing it w/o also replacing the booster. That said, I don't see the need at all since you have yet to adjust things to figure out what's really going on.

- Steve
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Old 10-22-2002, 09:07 AM   #18
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This reason below from romanon is my main reasoning for not wanting to touch the pedal. Now I don't know if you two are talking about the same type of adjustment.....or different ones.

Quote:
Now for the pedal, adjustment of the pedal is PURELY for the factory. Don't touch this as you can find yourself in some serious trouble. The only adjustment in pedals that will have a great effect on pedal travel (and effort) is the pedal ratio (the amount of mechanical advantage). This is just the ratio of the distance from the center of the pedal pad to the pin where the booster attaches divided by the distance from the center of the pedal pad to the hinge point at the top of the pedal arm. Decrease the pedal ratio and you will increase effort and decrease stroke. You can do this by simply moving the pedal pad itself up on the pedal arm. How much you move it depends on the size of your foot.

The increased brake fluid volume demand due to the larger caliper pistons is why the problem has occured. This make sense for an engineering background, which I do have......

I'm not saying you're wrong....or anyone else is wrong or right. I'm just saying from an engineering stand point, and just dealing with volumes and the fact that a certain piston will only be able to displace a given amount of fluid in a specified stroke. By changing the calipers, you end up with something having to change......that change is the increase in pedal stroke. I would rather increase the MC piston size then "pre-load" the MC, or keeping the range at what the MC operates very narrow.

I have bled & rebled the brakes several times. Pedal is not going to change because of that.....it's not a spongy feel, the pedal is hard.....the travel is just too much.

When you did the single stage booster swap......do you happen to know what MC piston size was?

Josh
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Old 10-22-2002, 09:16 AM   #19
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Quote:
Originally posted by zzyzx

As to replacing the MC with a WRX MC, I wouldn't suggest replacing it w/o also replacing the booster.
- Steve
I agree with this, it's not an absolute if you understand the system and know what you're doing, but it is best practice.

Quote:
Originally posted by zzyzx

it's obvious to me that you just need to adjust the nut/threaded portion on the pedal "plunger" to reduce the pedal play which is where your added and unwanted travel is coming from. Do this, making sure you don't reduce the travel to the point that your brakes drag.
This is a tricky operation and I would recommend not doing it, it may work out well or it may not and it's hard to tell until it's too late if it did work out well. If it worked out for you, great, I would just keep this as a last resort.

Last edited by romanom; 10-22-2002 at 11:07 AM.
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Old 10-22-2002, 10:42 AM   #20
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Taken from www.spdusa.com:
Quote:
The pre-1999 cars use the booster kit alone. Unfortunately, the 1999 and later cars have a redesigned master cylinder/booster that is not interchangeable. We can still provide the single stage booster kit, but 1999 and later cars will require a new master cylinder at an additional expense.
So be aware that the chances of actually being able to use the WRX MC are fairly slim.

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Old 10-22-2002, 07:06 PM   #21
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AWD 5MT EJ22T AWIC Swap

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Jon,

Thanks for the info.

I found out some more info Which in leu of Jon's info is good news.

I don't know why I didn't notice this before but in the FSM the 1990 Legacy LS wagon w/ ABS has the same MC size as the WRX, 1 1/16"

So.....what I will probably do is snag a MC from that car.....keep the same booster I have......which should be alright.

Now the question about the booster.

In the FSM it gives an effective diameter for the booster. I'm assuming this is the diameter of the diaphrapm. The larger the diaphragm, the mor brake assist you will get correct?

My current booster has effective diameters of 7.09" + 8.07" The LS wagon has numbers of 8.07" + 9.06" Which also happens to be the same for the WRX.

So......if my logic is correct, in keeping the smaller diameter booster, I would be getting less assist correct?

Here's a scan from the WRX FSM.....has some of the brake numbers & info I've been referring to.



comments

Josh

Last edited by legacy777; 09-24-2003 at 09:18 AM.
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Old 10-22-2002, 07:32 PM   #22
romanom
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Quote:
Originally posted by legacy777

Now the question about the booster.

In the FSM it gives an effective diameter for the booster. I'm assuming this is the diameter of the diaphrapm. The larger the diaphragm, the mor brake assist you will get correct?

My current booster has effective diameters of 7.09" + 8.07" The LS wagon has numbers of 8.07" + 9.06" Which also happens to be the same for the WRX.

So......if my logic is correct, in keeping the smaller diameter booster, I would be getting less assist correct?

Josh
No, size of booster doesn't set the assist ratio, but just the max assist available.

Example:

A 7/8 tandem booster with a 6:1 ratio will give 60 pounds of assist for every 10 pounds applied to a total (run out) of 250 pounds. A 8/9 tandem with 6:1 ratio will also give 60 pounds for every 10 pounds in, but up to a max of 300 pounds.

Then after you reach the run out the boost ratio goes to 1:1 for every 10 pounds extra force applied you only get 10 extra pounds out. Basically the curve of force in vs. force out goes from a slope of 6 to a slope of 1 at the run out point.

This is why larger vehicles have larger boosters to allow for more overall force applied. It is customary however to give a larger vehicle a higher assist ratio, so normally a smaller booster will give less assist but not always.
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Old 10-22-2002, 07:48 PM   #23
romanom
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FYI: Where boost ratio comes from:

Internal to the booster is an input rod with a piston at the end and an output rod with a soft and thick rubber disk. The ratio of the rubber disk and piston sets the boost ratio.

Boost Ratio: Rubber Disk / Piston
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Old 10-22-2002, 08:33 PM   #24
Legacy777
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AWD 5MT EJ22T AWIC Swap

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Quote:
Originally posted by romanom


No, size of booster doesn't set the assist ratio, but just the max assist available.

Example:

A 7/8 tandem booster with a 6:1 ratio will give 60 pounds of assist for every 10 pounds applied to a total (run out) of 250 pounds. A 8/9 tandem with 6:1 ratio will also give 60 pounds for every 10 pounds in, but up to a max of 300 pounds.

Then after you reach the run out the boost ratio goes to 1:1 for every 10 pounds extra force applied you only get 10 extra pounds out. Basically the curve of force in vs. force out goes from a slope of 6 to a slope of 1 at the run out point.

This is why larger vehicles have larger boosters to allow for more overall force applied. It is customary however to give a larger vehicle a higher assist ratio, so normally a smaller booster will give less assist but not always.
When you say 7/8 vs. 8/9, are you talking about the shaft diameter, or something else?

Assuming the info from Mike Shields is correct, I should be able to use the 1 1/16" MC and my current booster.....hopefully...

Josh
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Old 10-22-2002, 08:59 PM   #25
romanom
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Quote:
Originally posted by legacy777


When you say 7/8 vs. 8/9, are you talking about the shaft diameter, or something else?

Assuming the info from Mike Shields is correct, I should be able to use the 1 1/16" MC and my current booster.....hopefully...

Josh
7/8 and 8/9 are technical "slang" for the diameter of the booster diaphram. 7/8 is a 7" / 8" effective diameter tandem booster.
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