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Discussion Starter · #1 ·
I want to talk some theory and possible resigning of the Bias/Proportioning Valve on my 2002 GT turbo car. As a prerequisite I do not want to change the calipers, master cylinder, etc but I need more line pressure to rear brakes for more holding pressure when power braking - I am looking into better brake pads but I haven’t gone that far just yet.

My car has an ABS Delete Block which is now only plumbed with the lines for the front brakes – One line IN and two lines OUT so the rear brake lines are not plumbed through this block.

For the rear brakes I installed a Line Lock - One line IN and two lines OUT. A line lock acts like a check valve when activated so when it’s activated I can pump up the brake pressure to the rear brakes to add a bit more holding pressure to the rear brakes while power braking the car into boost. Once I pump up the brakes I then stand on the brake pedal with all my strength while power braking and building boost. Pumping up the brake pedal definitely works but it’s still not producing enough holding pressure to keep the rear tires from turning so I want to figure out a way to get more line pressure to the rear brakes.

When I pump up the brakes I can feel the pedal get harder but only up to a point. It seems that once the rear brake lines develop a certain amount of pressure the rest of the fluid gets directed to the front brakes. This feeling in the pedal seems to match what the proportioning valve is made for, I think… Could this be correct?

I’ve read about adjustable proportioning valves plumbed into the rear brake lines but I’m not convinced that this actually will allow more pressure directed to the rear brakes b/c I THINK these adj valves tune the fluid flow only during a panic stop, not just normal pressing the brakes.

Besides an adjustable prop-valve I’ve also been thinking/researching using a stiffer spring/shimming the stock prop-valve and I feel this has more merit.

With the different tire sizes on the front/rear more brake pressure to the rear would help anyway so I’m not too concerned with this change.

Any thoughts on how to accomplish what I’m trying to do?

Thnx
ks
 

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I believe this is the reason for transbrakes Kevin. It is very difficult to get adequate clamping on the rear brake system (as you are discovering) to hold the more powerful engines hence the mechanical trans brake.
 

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Discussion Starter · #3 ·
Yep, I agree and if this was a race car I would have ordered one but no need for one for a street car that will make 1 maybe 2 runs down the track next year. But last night I was thinking the same thing so I looked on Dan Gilsdorfs website to see what a VB cost with a t-brake = $1k (4R70) => way more than trying to figure something else out for a few times down the track. But I am having Dan quote me modifying my current VB so maybe it will be a lot less $$..

Last year while messing around in the driveway using the E-brake I was able to get 7psi @ 3000 rpm which would be perfect and last nights trial with the line lock got me to 4psi @ 2800rpm so it's close..just need a bit more.

ks
 

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Discussion Starter · #4 · (Edited)
No T-brake available with full auto shifting so a t-brake is not an option - still thinkin' & drinkin'.

Before winter I will toss on the slicks and log some short foot-brake launches to see how it runs which will give me something to ponder over the winter.

dumb thought removed...

ks
 

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As you increase M/C bore, you decrease the line pressure the calipers see. To increase the line pressure the rear calipers see, you want to go down in M/C bore diameter. Another alternative is to go to a larger diameter rotor and a more aggressive caliper (bigger piston diameter). If you pursue this route, take the time to make certain the M/C has adequate volume to actuate the brakes when the pads and rotors are worn to their operational limits.

Rear brakes can be tricky items to work with. To gain adequate stopping power to hold the engine at launch, you might discover you have made the brakes too "grabby" for everyday driving. You want to protect against the rear brakes locking up before the front brakes in a panic stop. When the rear brakes lock up first, the car will spin around, and you will find yourself facing backward. Detroit and Race Shops use balance bars and proportioning valves to reduce rear brake aggressiveness.

This is a precarious balancing act you are faced with here. Proceed cautiously.
 

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Discussion Starter · #6 · (Edited)
I removed my thought on the larger MC bore mainly because no matter what bore size (more volume being pushed) I think I'd still be fighting the spring pressure in the prop-valve redirecting the fluid to the fronts vs the rears. I think this is what needs to change but I'm not sure. This is some of the research I'm doing. When simply pressing on the brake pedal does the spring in the prop-valve redirect any fluid or not. Maybe the spring is taken out of the equation until a panic stop occurs and the pedal is hit hard and fast..

Over the winter I may buy a junk prop-valve, dissect it and measure the spring and the pieces to see if I can do some calculations and/or come up with something adjustable or ??..

I have more playroom with the rear brake pressure b/c of the different tire sizes and types of tires used on front/rear : 25" tall street tires on the front vs 28.5" tall DR's on the rear. I know for a fact that I need more pressure at the rear b/c the fronts lock up way too easy.

ks
 

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Proportioning valves balance pressure from front to back in a braking system, but I am uncertain of their internal workings on the OEM units that produce this effect. The aftermarket has adjustable pressure valves available from firms like Wilwood. If I am not mistaken, the aftermarket valves maintain total isolation between front and rear fluid circuits. They can usually be used in conjunction with most tandem outlet or dual-mount master cylinder assemblies. The aftermarket valves also allow you to modulate the pressure differential the front and rear calipers see. On balance, they tend to be reasonably inexpensive, i.e., $40 to $100 at Summit of JEGS — making it hardly worth the effort to fiddle with the OEM part.
 

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The piston size increase approximates a 0.197" increase in piston diameter. While not insignificant the increase in braking capacity is small. Additionally, the increase in brake fluid required to operate and actuate the caliper at the extremes of pad and rotor wear may exceed the master cylinder's ability to supply. It would be wise to measure and test the braking system at these operating extremes before installing it on a running vehicle.

The best way to increase rear brake stopping power is to increase disc diameter and use a suitably sized caliper for the larger disc. That said, you still need to test the braking system at the extremes of possible component wear to be certain the master cylinder has adequate fluid volume to operate the brakes at the extremes of accepted wear for the individual components.

Even more important the modified braking system should not be able to lock up the rear brakes before the front brakes. When rear brakes lock up first the vehicle will spin around leaving the vehicle facing backward with the driver viewing where the car has been instead of where the car is going. In broad terms, this is not good.
 

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Discussion Starter · #10 ·
Small Update:
I swapped the stock GT calipers (38mm) for 43mm calipers off a Taurus (which is a bolt-in process mechanically) and no difference at all in holding grip. The brakes feel way softer now but I will drive it to see how they work. The car needs more rear bite so if they give me this, even tho they feel soft, I will keep them on but I will test them good to make sure they perform as they should on the street. If they don't perform any different than the stockers or I feel there can be an issue I will put the stock calipers back on.

ks
 

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Be careful as you proceed, Kevin.

In a scenario where the fronts lock first and you want to increase grip on the rear calipers reducing the stopping power of the fronts is not the same as increasing the stopping power of the rears. More to the point lowering the overall braking system performance by reducing the stopping power of the fronts reduces the cars overall stopping performance and could be putting you and others at risk.

You indicated that

... I swapped the stock GT calipers (38mm) for 43mm calipers off a Taurus
The change from a 38mm diameter piston to a 43mm diameter piston should increase your caliper grip on the rotor by a factor equal to ratio of the larger caliper's piston area divided by the smaller caliper's piston area. In the case of 38 and 43 mm caliper pistons that should be a 28% increase in clamping force and a corresponding increase in braking. The brake pedal should feel harder and the braking system should be more responsive to pedal pressure not the other way around — unless I misinterpreted what you were saying.
 

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Discussion Starter · #12 ·
The increase in size of the calipers is not on the front, they are added to the rear. I'm trying to get more clamping force on the rear rotors for holding the rear tires when power braking but it didn't help.
In doing this I am also hoping to add more clamping force to the rears so I can get the rears to help slow the car down more than they do now. Right now the fronts lock up so easily it's scary and the rears are just along for the ride not adding any braking assistance.

ks
 

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I misinterpreted what you were saying, Kevin. The same commentary about the caliper's effect on braking performance but this time applied to the rears. Be cautious as you proceed. The surprise when the car swaps ends, if the rear wheels lock, is breathtaking.😊
 

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Discussion Starter · #14 ·
Update 10/30/22:
Going to the 43mm calipers would not hold the car stationary any more than the stock 38mm calipers. After installing line locks on the front and rear lines I can pump up the brake pressure where it feels as solid as a hydraulic jack and even pressing on the brake pedal in this mode with all my strength I still couldn't prevent the rear tires from pushing through the brakes.
Driving around town the brake feels noticeably softer and after some panic stop testing doesn't perform any better than the stock 38mm calipers so more than likely I will put the stock calipers back on.

ks
 

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Discussion Starter · #16 ·
Kevin, I should have asked this earlier but, what ring and pinion do you have in the car?
Ed,
You should be enjoying the weekend off from the BB's but to answer your question the gears are 3.27's.

FYI, I spoke to Lentech about their automatic shifting t-brake VB's for the 4R70 and they are not automatic shifting while racing only driving on the street. Their t-brakes need the shifter to be in low 1 in order to operate which defeats my purpose of auto shifting...oh well. At this time there are more important things that need to be done before getting it to the track so I'll ponder this some other time..

ks
 

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Well that helps. I thought you might have had 3.55 or 3.73's which would aggravate the situation because of the additional mechanical advantage it would give the engine — but that is not the case so that theory goes in the round file :) ...
 

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Discussion Starter · #18 ·
Yea, I figured that was the direction the mind was going down...(y)

The solution goes back to your initial suggestion of a t-brake. Of course a higher stall would help too but I'm not going down that route...

ks
 

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High-stall converters above 3000 rpm are not a good choice for a street-driven car, as I believe you already know. High among the various dissatisfaction factors is the additional heat they impart to the transmission fluid.
 
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