I finally understand how the thing works so I'll share some of my insight into it.
I've been having problems with my 88's brakes so I decided to tear down the brake system and do a numerical analysis of how the 88's brakes, and all Fiero brakes really work. But the one thing that I had the most curiosity about was the Combination Valve. I couldn't find the info I needed to do calculations on how it works, so I spent some time, testing and analyzing how the thing works. I still have yet to connect the pressure gages up to the lines to test the actual effect, but I will get to that at some point in the near future.
For a start, heres what the valve is like in its normal state. I left out the details of the tube mounting cavity and the warning switch, cause im lazy.
As you can see, there is a balance spool near the top, and a proportioning spool near the bottom. Lets look at the operation of the Proportional valve more closely. In the picture, the proportional valve spring is pressing the spool into the cavity. This is causing the seal to unseat itself letting the fluid flow straight through unregulated. The pressure inside the cavity, Plus the pressure downstream (on the outlet side) are both working together to push the spool back against the spring. If the pressure is great enough, the spool will move, causing the seal to seat against the cavity, sealing off the outlet from any more pressure.
However, if more pressure continues to come in from the master cylinder, it now acts against the seal and the spool pushing it into the cavity, which will unseat the seal and let some pressure out to the brakes until it's enough to push the spool against the spring again. In this fashion it only lets some of the pressure through the valve. If you think of it as a balance, You have outlet pressure on one side, working against the cavity diameter trying to push the spool out, and on the other side, you have the spring, and the area between the cavity and the spool pushing back. The math puts these two forces at a ratio, which is the Proportional Valves Ratio. The springs only function is to cause the proportional valve to stay open until a certain amount of inlet and outlet pressure is reached, then it proportions the outlet pressure by that ratio beyond that point.
The factory proportioning ratio is factory modified by the diameter of the PV Spool. Which probably explains why my 88GT's spool is red, and 1/4" in diameter, while my 84 Indy has a Gold spool that is 0.286". Also, the cap must match the spool, and this would explain why it has a plastic insert. So it could be changed out. (change the whole cap with the spool) I tried playing with the math a bit to see what a small change in the spool size would do to the proportioning ratio and cracking pressure and got this:
So if you wanted more rear brake bias, you would need a narrower spool, and vice versa. The spring's cracking pressure also changes greatly with the different spool sizes, so I listed them as well, assuming the spring delivers about 15 lbs of force.
For my 88 GT, this graph represents what the brakes are actually doing, based on the math.
Thats the proportioning section in a nutshell.
There are also a lot of misconceptions on what the Combination valve will protect you against in the event of a brake failure. It doesn't really do much, except tell you that something has gone wrong.
If the rear brakes were to fail: The Balance spool shifts towards the rear section, it doesn't cut anything off from the rear brakes, it just turns on the warning switch. The area that shifts the spool is nearest the front right outlet, that surface works to push the spool away.
If the Front Left were to fail: Again, the spool shifts, this time it pushes the collar with it. It does hit and block the right brakes outlet, but that doesn't matter. The inlet and the outlet aren't cut off at all from one another by anything. So you loose the front brakes completely. Also, notice that the rear proportioning valve is bypassed in the event of any front brake failure.
If the Front Right were to fail: The spool would shift and slam into the front right's outlet, cutting it off. The Front left would retain pressure, The area of the collar might cause the spool to not quite block the outlet completely. So weather or not you would loose both brakes immediately, I am not 100% sure which way that will go.
The Brake system protection is done on a Front or Rear basis. The front and rear brakes are NOT tied into each other at any point in the system. When you press your brake pedal, the master cylinder uses half of your press to send fluid to the fronts, and half to the rears, The master cylinder can bottom out either of the two circuits. If the front brakes fail, your pedal will drop half way down, and the rear brakes work with the remaining pedal stroke. The exact same protection is provided for the rear. So in reality, The Master cylinder does all the brake protection for the whole system. This is also why there is a divider in the reservoir. Because a failure would drain that half of it. Also, the master cylinder has one bore size to activate the brakes. This means that the front and rear brake circuits receive the same pressure from the master.
[This message has been edited by Fierobsessed (edited 06-02-2010).]
Yes, Balance bar is not a valve to cut off either circuit. Rear brake get full pressure when front fails. Also Lawrence Mazza did article years ago with combi valve covered but He had to drop it because article had a 88 upgrade he doesn't want to make it. had good pictures, like this...
Front are not, not intentional anyway, in keep the left side working with right side failure. A very hard to pull to left. Very hard... Not many people can keep the car from going straight. Check nut or balance for something small like small notch in either or how they fit is loose, etc. The nut/balance is not valve by design. (Remember... Brake using high pressure to work... Small opening makes all the different.)
PV is "Simple" regulator. Like Fuel Regulator on steroid. Block PSI to back brakes. Does nothing until PSI is high, like panic stop, then back brakes get about 800-900 PSI max. Bellow cut off PSI, equal PSI to front and rear. Try using two pressure gages (Gage = 0-1500 psi or more) and see.
And has "Natural" Bias in Hydro... See cave, brake upgrade. This Bias works full time, doesn't need PV do to anything. While article is 84-87 models, 88 has it too... see excel sheet and plug in 88 front and rear piston size.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Am I wrong in thinking one of these might be a good upgrade? If I'm correct it should be practically plug and play right down to the brake warning switch. The benefit is you will be able to easily adjust your brake bias. Opinions?
[This message has been edited by Fieroseverywhere (edited 06-02-2010).]
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
And has "Natural" Bias in Hydro... See cave, brake upgrade. This Bias works full time, doesn't need PV do to anything. While article is 84-87 models, 88 has it too... see excel sheet and plug in 88 front and rear piston size.
88 Pistons are all the same diameter, use the same pads on all fours, and same rotors too. So the only bias it gets is what the proportioning valve gives it. I was a bit surprised when I got out the mic on them. The 88's end up with a bit more rear bias then 84-87's
I don't have 84-87 calipers handy, but I had read that they were 1.930" Front, 1.874" Rear. The 88's are all 1.888"
I blew my right front line today when I was testing the brakes (in my driveway sitting still) The spool did not block its outlet. I was unsure what it would do, but it was obvious once tested. And messy. I will get a gage set on the system when my new brake parts arrive. The theory and stuff is all good, but is no match for actual instrumentation and data.
Hopefully I will soon have my corvette upgrade so that the car might actually stop for a change. I also want to do the booster upgrade while I'm at it. I've been very unhappy with my 88's brakes for quite a while. But, as far as I can tell, the fault may lie in the brake pads. I've done a hard stop from a relativley high speed, like 80 or so, the brakes totally give in by the time I'm down to 20 MPH. They were still slowing me down, but only just. I was standing on the pedal with everything I had. They stunk really bad afterwards.
good stuff! I've always wondered how the rear proportioning worked. it just never made much sense to me. so - what happens when you remove the spring?
Thats simple, the proportioning's sections cracking point becomes zero, so no matter what the pressure is on the front brakes, the rear brakes will be exactly 75% or 67% of that pressure, depending on which spool you have. The springs purpose is to disable the PV untill a certain pressure is obtained. This helps the brakes to work much more evenly, rather then running the fronts really hard all the time. Think about it, if you are in stop and go traffic you do a lot of light braking. There is no weight transfer and traction issues to speak of, so the brakes are much better off working evenly.
Then, in the braking and deceleration ranges where the weight transfer and rear traction become an issue, the pressures start to diverge offering more braking force where there is the most traction, at the front.
Thanks for the link to the technical info. So the switch on it is a replacmement for the brake light switch on the pedal. It still has 3 outlets so you can run a single line for the rear and dual lines for the front. The rear only is proportioned. Still trying to work out the whole balance spool part. http://www.wilwood.com/PDF/DataSheets/ds715.pdf
[This message has been edited by Fieroseverywhere (edited 06-03-2010).]
ok, I wasn't sure about 88... Maybe it has a real PV. Very rare uses same piston etc. Older Fiero uses virtually same mechanicals (Front is thiner rotor and rear has Ebrake...) but different piston front/rear.
84-87 pistons... Again, see cave, brake upgrade. Diam is in there...
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Hopefully I will soon have my corvette upgrade so that the car might actually stop for a change. I also want to do the booster upgrade while I'm at it. I've been very unhappy with my 88's brakes for quite a while. But, as far as I can tell, the fault may lie in the brake pads. I've done a hard stop from a relativley high speed, like 80 or so, the brakes totally give in by the time I'm down to 20 MPH. They were still slowing me down, but only just. I was standing on the pedal with everything I had. They stunk really bad afterwards.
The brakes came back when cool? Then Yes. Hot Pads cause brake fad is a good guess. Brake fluid boil usually need bleeding to get system to work right.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I've never had a fade problem with my stock 88 brakes, and I've been in some pretty tough braking situations (on Run For The Hills, etc.). Fierobsessed, you must have got some bad pads. The 86 GT in front of me on one part of the last run, however.... I was smelling his brakes all the way down the hill.
The Brake system protection is done on a Front or Rear basis. The front and rear brakes are NOT tied into each other at any point in the system. When you press your brake pedal, the master cylinder uses half of your press to send fluid to the fronts, and half to the rears, The master cylinder can bottom out either of the two circuits. If the front brakes fail, your pedal will drop half way down, and the rear brakes work with the remaining pedal stroke. The exact same protection is provided for the rear. So in reality, The Master cylinder does all the brake protection for the whole system. This is also why there is a divider in the reservoir. Because a failure would drain that half of it. Also, the master cylinder has one bore size to activate the brakes. This means that the front and rear brake circuits receive the same pressure from the master.
Question. I could hook up my front and rear brake lines directly to the master cylinder, bypass the combination block and still have braking if either front or rear line blew open? I could drive the car while I hunt down an o-ring rebuilding kit for the combination valve. Anyone have a source for a rebuilding kit?
Originally posted by jetman: Question. I could hook up my front and rear brake lines directly to the master cylinder, bypass the combination block and still have braking if either front or rear line blew open? I could drive the car while I hunt down an o-ring rebuilding kit for the combination valve. Anyone have a source for a rebuilding kit?
When it comes to something like this, I would never attempt to rebuild a combination valve. If yours needs rebuilding, send it to a company like Master Power brakes to be rebuilt. Brakes are simply too important to take a chance on a mistake in the rebuild.
If you tear down your engine to rebuild something and after rebuild it does not work correctly then your car will not go. If you rebuild a necessary brake component and you do not have the practical experience to be doing the rebuild, the result could be that your car will not stop. It is unwise to try to save a couple bucks while taking the chance that you might wreck your vehicle, kill yourself or others.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
84-87 combination valve is in my cave... L.M. says 85 Pontiac likely mean 6000, which uses similar setup to Fiero. Picture shown is 84-87 type w/o venting the proportion section like 88 does.
Some time ago I designed an adapter that screws into the combination valve allowing you to manually adjust 3 to 4 times the pressure before the knee point on the graph. Definitely an improvement to be able to dial in the rears.
How do we get more pressure to the rear brakes. I am collecting parts for a Lebaron brake upgrade but the 2.5 from vs 2 1/8 rear piston gives way to much front bias for me. Was hoping there was a way to offset that by increasing rear or decreasing front pressure relative to the other end of the car.
I have been doing some research in to getting more pressure to the rear brakes on my car .I have gone back to stock brakes on the back of the car so I can have a working parking brake . The brakes on the back of the car (or any car) do not provide any where near the stopping power of the fronts .So why bother with big heavy brakes in the back? Even ultra performance cars have smaller brakes in the back . And when I put the car back on the road this year , it still had about the same stopping power and the same front right lockup problem . I was on a camaro/firebird website and what they do to get more braking power to the back is change the prop valve spring to one from a similar year corvette . They also give a Grainger supply part # for a spring that works as well . Aarron88 no longer sells his adjuster .I am thinking of a simpler solution . Why not take the spring out and just stretch it to raise the knee point up ? It would really be hit or miss , but in my case even if the bias remained at 50/50 (where you would be without a prop valve at all) , my rears would still not lock up because of the much bigger caliper piston area in the front vs rear .(4.12 sq " front 2.75 sq" rear ) Anyone tried this ?
[This message has been edited by wftb (edited 10-16-2016).]
I was on a camaro/firebird website and what they do to get more braking power to the back is change the prop valve spring to one from a similar year corvette . They also give a Grainger supply part # for a spring that works as well.
I am thinking of a simpler solution . Why not take the spring out and just stretch it to raise the knee point up ? It would really be hit or miss...
Exactly... it would unfortunately be "hit or miss".
I like the idea of replacing the valve spring with one of known specifications. At least the results would be repeatable.
I decided to try it any way . stretched it about a half inch .Took a while to figure out how to get it back in , but a nut in a 6 point 3/4 socket allowed me to get it started .Bled the brakes and tried it out .Does not seem to have made a difference at all .
Originally posted by wftb: I am thinking of a simpler solution . Why not take the spring out and just stretch it to raise the knee point up ? It would really be hit or miss...
Worse then that... Stretch any spring will often cause it to fail later and without warning. US Military has been trying to kill stretch spring myths for decades yet morons still does this to Magazine and a lot other springs. (Even US Army's PS Magazine, magazine in a "comics book" format, have cover this problem several times over the years. [Wiki link because Army IT have problems w/ PS Magazine online.]) Why? Example: Stretch spring in a magazine can/will breaks and often at the worse time cause ammo feed problems resulting in a jammed gun. The Guy w/ a jam weapon in a fire fight is often dead.
Again, PV is an early attempt of ABS. It's job is to stop from spinning the car cause by locking the back axle. Stretch spring in PV can work fine then fail trying to stop for next red light or for people running out in the road. You very likely hit whatever and often facing wrong way. Dead PV for any reason can make any car to spin easy on wet roads etc.
[This message has been edited by theogre (edited 10-16-2016).]
They have parametric searches that work reasonably well. I have no affiliation with them; just reporting my experience as a customer.
In order to fine-tune the cracking pressure, rather than stretching the spring, which I agree would be hit or miss, I would use small shims to compress the spring further.
If the spring fails completely , the valve is actually a fail safe mechanism and results in the lowest possible available pressure to the rear brakes .That would mean that the rears would always keep rolling while the fronts would lock up like they already do anyway .As long as the fronts only lock up , you can keep the car going straight .As far as why it had no affect at all , my only guess is that once I managed to get the spring back in , the fact that it was ever stretched became irrelevent because it still has the same spring rate . Going to do more research in to the other springs I found and maybe make some shims .Shimming is really all Aarron88's device does when you look at it closely .
[This message has been edited by wftb (edited 10-16-2016).]
I just ordered the willwood adjustable proportion valve mentioned in one of the posts in this thread .Willwood part #260-11179 .Read the spec sheet and what you do is start adjusting from the lowest rear pressure and increase the rear pressure until you have the rears doing what you want .Most of the combi valves that I have looked at only reduced the rear pressure from the stock starting point .This is really what I need .And no it does not have the brake failure indicator switch . My vehicle is highly modified and I pay close attention to it .
I've been debating on whether to try out the shim method, or buy the Wildwood combo valve. My Fiero has LeBaron rotors in front with GM metric calipers, and '88 Fiero brakes in the back. The brake bias is severely front-heavy. So I'll be looking forward to your Wilwood combo valve review with great interest. (hint-hint)
So you have 2.5 " diameter front piston calipers front and 1.88 " diameter rears .That gives you 4.9 sq " piston area front and a 2.77 sq" rear piston area .So you are close to what I am at right now .Pretty sure if this works for me it should work for your setup as well .The only thing I am mildly concerned about is how many little fittings I will have to make up .Would be really nice if the threads match up .
Originally posted by wftb: If the spring fails completely , the valve is actually a fail safe mechanism and results in the lowest possible available pressure to the rear brakes .That would mean that the rears would always keep rolling while the fronts would lock up like they already do anyway .As long as the fronts only lock up , you can keep the car going straight .
Depend how broken spring spring effects the valve.... One could let the valve move more to low pressure. Other is spring breaks and piece(s) binds the valve at max pressure. Could be both if piece(s) move. 1 stop little rear. next high pressure to rear. etc. No-one can exactly say where/when a stretch spring breaks, how many pieces or what size. PV have tight tolerances etc and could jam on small crap.
Got the Wilwood prop valve today and I am in the process of finding all the fittings I will need .Of course the threads on the Wilwood do not match up with any of the fiero lines .I found some adapters that might work but of course half the racks in the brake lines and fittings section at Canadian tire were empty . I want to have all the fittings gathered up before I start the job ,some probably will not be available locally . The feature on this valve that has me convinced that it will work is that it starts at minimum rear brake force(knob all the way up) and goes to maximum rear brake force (knob screwed all the way down) .This will make it easy to adjust and it eliminates the sudden change to front bias that the stock valve causes .You do lose your brake warning light , but if you pay attention to your car you can see the signs of impending failure long before the light comes on .
Originally posted by wftb: You do lose your brake warning light , but if you pay attention to your car you can see the signs of impending failure long before the light comes on .
Nope. Often you have little to no warning when light comes on and the pedal hits the floor because Hard and Soft Lines and other parts can fail w/o warning causing no brakes for front or back.
Example: Rear brakes have a hard line running under the battery and battery leaking at any time can eat the line then line blows out next day or month/years later. The area of line is hidden by frame and fender skirt.
Just pointing out the drawbacks to doing this mod .My battery got moved to the front 8 years ago . I have a new stainless line I will be putting in to replace the one coming from the front to the back but the original is still in great shape . Anything can happen , but I want the best possible brakes I can get .There is really very little original fiero parts on my car . Suspension , engine ,exhaust , transmission , brakes , guage cluster , shifter and all kinds of stuff I have forgotten either came out of a different car or got made by me . Because of that , I pay very close attention to the cars condition and fix things immediately or park it till I can fix it .
[This message has been edited by wftb (edited 10-20-2016).]
I'm not really sure what purpose the brake warning lamp serves... It only lights up when the pedal hits the floor, which is when you become aware of a brake issue.
The Wilwood proportioning valve works the same as the stock valve; there is no jump in brake bias with either setup.
The Wilwood unit seems to have an electrical connector; what does it do if it's not for a pressure imbalance switch/block-off valve?
It is possible to have a warning of brake hydraulic failure.
I proof-test my brakes occasionally in my driveway before getting underway. With my left foot, I stand on the brake pedal. With my right foot, I rev the engine to high RPM, and then I lift the throttle rapidly. This creates the most possible manifold vacuum. The brake booster will increase in effectiveness. At this point, I am pushing as hard as I can on the brake pedal, on the verge of slipping off the brake pedal (sometimes I have slipped).
This procedure is supposed to generate more pressure in the system than there should ever be during normal driving, or emergency braking.
If you burst your brake lines, you want to do it safely in the driveway, not during emergency braking when you need them the most (and when bursting is most probable).
The Wilwood switch is just a convenient way to hook up brake lights .Push down on the brake pedal , pressure closes the switch .There is no warning system .So far I have only been able to get adapters to fit the 3/16 " lines .The only way I can see to make this work is a lot of cutting and fitting lines together with compression fittings . Starting to have second thoughts about using the Wilwood valve .
Originally posted by pmbrunelle: I proof-test my brakes occasionally in my driveway before getting underway. With my left foot, I stand on the brake pedal. With my right foot, I rev the engine to high RPM, and then I lift the throttle rapidly. This creates the most possible manifold vacuum. The brake booster will increase in effectiveness. At this point, I am pushing as hard as I can on the brake pedal, on the verge of slipping off the brake pedal (sometimes I have slipped).
This procedure is supposed to generate more pressure in the system than there should ever be during normal driving, or emergency braking.
If you burst your brake lines, you want to do it safely in the driveway, not during emergency braking when you need them the most (and when bursting is most probable).
Your test may seem good but Lines and other parts can fail anytime and w/o warning.
High RPM gives most vacuum? Nope. Try using a vac gauge. Most engines, idle and cruising (<2500) will get high vacuum, ~13 to 22 in/hg. (Highest vac, >22in/hg, is when you decelerate the car.) When you open the throttle more, you loss engine vacuum and you use whatever reserve vac in the booster. (Booster check valve is there to make sure) Many that auto cross keep RPM high and booster can't pump down and then driver is fighting booster spring and basically have little to no brakes for rest of session. When you have a car w/ "Vacuum Motors" to run AC/heat doors or Vac Servo for Cruise, the Vac tank w/ Check valve is to keep them working when you hit the gas a bit. You can't even reach 50% of throttle before they have problem when tank or valve are bad.
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Originally posted by wftb: The Wilwood switch is just a convenient way to hook up brake lights .Push down on the brake pedal , pressure closes the switch .There is no warning system.
Correct. Wilwood and a few others use this for brake lights. Saves builders time etc for making a switch and bracket for the pedal.
[This message has been edited by theogre (edited 10-20-2016).]
Here's an old datalog of my 2.8 L Fiero while driving:
Pressure is measured in the plenum from a port right next to the port for the brake booster. I tried to cover much of the working range of the engine; different combinations of throttle vs. RPM.
At regular time intervals (don't remember), a point is placed on the graph representing the current RPM-MAP combination.
Manifold pressure goes down to 13 kPa absolute above 3000 RPM. Since I live near sea level, that is about 26 in Hg of vacuum.
Maximum vacuum cannot be attained at idle RPM. Maximum vacuum is attained with closed throttle at high RPM.
My experimenting with the stock proportioning valve continues .I tried shimming the tube to prevent motion of the valve to see if I could get it not to work entirely and lock up still happened at the same amount of pedal pressure .So getting the right amount of shim in there would be a long process .I used pieces of 1/4 " plastic tubing for the shims .So looking at the diagrams , I assumed that getting rid of the seal that is on the right end of the plunger will disable the valves ability to proportion and result in a 50/50 brake distrubution . So I took the seal off , bled the brakes and took it for a ride .Now the fronts still lock up but a lot closer to stopped than they were before .And not much noticeable smoke , you just hear and feel them lockup .I can also pump the brakes to avoid lock up but I probably could do that before just never bothered trying . So this tells me : 1: even at 50/50 brake distrubution front and rear I have too much brake force at the front . 2: the Wilwood valve is not going to do anything because the best it can do is achieve a 50/50 front rear balance if I plumb it the way it is designed to be used . I might be able to plumb it so it controls the front brakes .The prop valve is non functional right now so I can do that without touching the rear brakes .Or I can add to my collection of calipers and try to find some front calipers with a piston area of about 3" to better match the rear calipers .
I saw your rig and your results looked good .But I have already hooked up the wilwood to the front brakes .Not a real clean install , I am one fitting short of not having to splice a line together . So far looks promising . I started with full pressure to the front brakes .Slightly worse than with just the disabled prop valve .I then backed the wilwood off about 3 turns .This made a noticeable improvement .Still lockup at the bottom of travel but going a lot slower when it locks up .Easy to modulate .Then it got dark so I called it a night .Try it again tomorrow hopefully some positive results .
Originally posted by wftb: Or I can add to my collection of calipers and try to find some front calipers with a piston area of about 3" to better match the rear calipers .
At some point, polishing a turd becomes more effort/frustration than starting with the right thing.
But why are you bent on avoiding lockup? The ability to lock the wheels with ease is good; this means that you have sufficient brake torque to threshold brake. Do you want to be limited by MC travel? Or limited by leg strength?
Lockup is only bad when the wheels don't all lock up together.
Maximum braking happens when your tires are operating at around 11% slip .Having enough braking force to lock up your tires , even if both fronts lock up at the same time , is only going to be advantageous if you have an ABS system to prevent lockup and keep the wheels turning at the maximum slip rate . The ability to stomp on the brakes and have no lockup until just about stopped is what I am after .I always own a newer car and I want my Fiero to be able to stop as fast as the 2014 Focus hatchback that I currently drive .Right now , it has a long way to go .I have the Car and Driver test of what my car (86 gt , 4speed , 2.8 L v6 ) performed like when it was new .Braking from 70 MPH took a distance of 200 feet .Fade was listed as none but I find that strange because the original version of the fiero as tested by road and track was found to have a 43% increase in pedal pressure after 6 stops from 60 MPH . And it had identical brakes as the 86 GT . Stock 84-87 brakes would probably disappear at a track day after only about 6 laps I think . The same R&T test listed a stop from 80 MPH at 289 feet .That is almost a football field .You are very right , I am polishing a turd .But I will have the best brakes I can build , one way or another .
[This message has been edited by wftb (edited 10-23-2016).]
Maximum braking happens when your tires are operating at around 11% slip .Having enough braking force to lock up your tires , even if both fronts lock up at the same time , is only going to be advantageous if you have an ABS system to prevent lockup and keep the wheels turning at the maximum slip rate . The ability to stomp on the brakes and have no lockup until just about stopped is what I am after .I always own a newer car and I want my Fiero to be able to stop as fast as the 2014 Focus hatchback that I currently drive .Right now , it has a long way to go .I have the Car and Driver test of what my car (86 gt , 4speed , 2.8 L v6 ) performed like when it was new .Braking from 70 MPH took a distance of 200 feet .Fade was listed as none but I find that strange because the original version of the fiero as tested by road and track was found to have a 43% increase in pedal pressure after 6 stops from 60 MPH . And it had identical brakes as the 86 GT . Stock 84-87 brakes would probably disappear at a track day after only about 6 laps I think . The same R&T test listed a stop from 80 MPH at 289 feet .That is almost a football field .You are very right , I am polishing a turd .But I will have the best brakes I can build , one way or another .
The challenge to your goal is that the available traction from your tires is not constant. Dial it in on a hot summer day with good summer only tires, then stomp the brakes on a 40 degree day and you will have almost instant lockup, because the summer only tires have less traction when cold. Change road surface and the lock-up behavior will change again. Stick with the same tire compound front and rear so at least their traction characteristics will change together, then learn to modulate to limit lockup as conditions vary.
