Brake questions, to the sages of 914World.... Options

[Dr. Roger]

Hi guys and gals,

I've got some questions regarding my current setup and possible changes.

I've got the 911 front end w/3.5" spaced front calipers (what are those, E's or S's?) and spaced 914 rear calipers on 911 rotors.
Big V8, 8X18's up front and 11X18's out back.
Also using the stock 17mm master cylinder and having no problems whatsoever with brake pedal spongyness. Stops hard and fast. (IMG:style_emoticons/default/happy11.gif)

Observations:
When my rear calipers were seized, and all of the braking was coming from just the fronts, the stopping power felt about the same as it does now with good rear calipers. Before 100% of the braking was coming from the front brakes. Now it feels like 90-95% of the braking is still coming from the front brakes. (IMG:style_emoticons/default/blink.gif)

My questions:
Does the stock proportioning valve keep that much pressure from going out back?

Will a "T" make that much braking difference? better/worse?

If I go the T route, will I be pushing additional fluid to warrant a 19mm master cylinder or will the 17mm still be fine?
(IMG:style_emoticons/default/idea.gif)
I dunno.

Your thoughts and experience are greatly appreciated as always.

Roger

[jimkelly]

this is what degreeoff replaced his porp valve with.
http://www.914world.com/bbs2/uploads/post-...49632_thumb.jpg
http://www.914world.com/bbs2/index.php?sho...72941&st=20

dr. - i have a few extra t's laying around - need one ? - send me your address - no charge.

jim

[degreeoff]

well I am not 100% here but am 99.9% sure you DON'T want a 'T' without a proportioning valve (manual) as you would be then distributing your braking force 50/50....where the rear only needs 20% or so. The resulting rear tires locking up would cause spins at all the wrong times.

I got all the parts from Jwest engineering (good stuff) he is here on the board venders

[davep]

It is a bit tricky to know the front/rear split with a T in place of the proportioning valve. The problem arises due to the different piston sizes and pad sizes, and possibly due to different pad compounds. However the front brakes are supposed to do most of the braking. Weight transfer under braking also contributes to that. The only way to correctly balance the brakes is to have an adjustable proportioning valve and do a lot of testing.

Your front calipers sound like an 'A' type.

[rick 918-S]

I have 4 piston BMW 2002 calipers up front and stock rears with a 19mm master and the "T" fitting. The brakes still feel like the fronts bit first. I'm sure it's because of the 4 piston calipers up front and the performance pads. If you don't like the "T" fitting you could always change it back.

[Eric_Shea]

If they're "steel" they are A-Calipers up front.

Under normal braking conditions your fronts will more than likely do most if not all of the load.

Given the tire and wheel combo that I "think" you have I would say you are severly underbraked out back but for normal street driving you can probably get by.

I would recommend keeping the factory P-Valve as it is adjustable and it works exactly the way it's supposed to. You really don't want to experience the rears locking up... especially on a car that can potentially go as fast as yours. Break the old epoxy loose, back off the locking nut (13mm) and adjust until you get a proper ratio. Because you have 48mm pistons up front now (vs. 43mm that the system was originally balanced for) and you still have the 17mm MC I'd say you're accurate in saying you're front heavy on braking. Adjust using a video camera and a friend in a vacant parking lot. Tape the lock in full power braking.

Make sure the venting clearance is set to .004"

Finally (the part you probably won't want to hear); I'd look for either a set of 914-6 calipers, Ferarri Calipers (mod the bleeders) or 911 rear calipers and work on a parking brake solution. You have WAY too much tire for a 914-4 caliper, vented or not.

(IMG:style_emoticons/default/wink.gif)

[Dr. Roger]

If they're "steel" they are A-Calipers up front.

Under normal braking conditions your fronts will more than likely do most if not all of the load.

Given the tire and wheel combo that I "think" you have I would say you are severly underbraked out back but for normal street driving you can probably get by.

I would recommend keeping the factory P-Valve as it is adjustable and it works exactly the way it's supposed to. You really don't want to experience the rears locking up... especially on a car that can potentially go as fast as yours. Break the old epoxy loose, back off the locking nut (13mm) and adjust until you get a proper ratio. Because you have 48mm pistons up front now (vs. 43mm that the system was originally balanced for) and you still have the 17mm MC I'd say you're accurate in saying you're front heavy on braking. Adjust using a video camera and a friend in a vacant parking lot. Tape the lock in full power braking.

Make sure the venting clearance is set to .004"

Finally (the part you probably won't want to hear); I'd look for either a set of 914-6 calipers, Ferarri Calipers (mod the bleeders) or 911 rear calipers and work on a parking brake solution. You have WAY too much tire for a 914-4 caliper, vented or not.

(IMG:style_emoticons/default/wink.gif)

Eric,
I'm kinda' confused about not enough brake caliper in the rear.
As they are, the rears are barely being used. If I increase my bias to the point where the fronts lock-up right before the rears do, won't that be where I need to be?
Or are you saying that the rear wheels will never lock-up because there is so much rubber on the road, in spite of how hard the fronts bring the car to a stop? It just seems that I will never need very much caliper out back.... and you're saying I will need more? (IMG:style_emoticons/default/huh.gif) (IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/biggrin.gif)

[Eric_Shea]

Your fronts are currently biased heavy.

Correct, your rears will probably never be able to catch up due to the 3-4x contact patch you now have over stock. They "could" in an emergency situation with the weight shifted to the front and, that's what you would want to avoid by keeping a P-Valve.

Or... you could take the rears off and plug the lines. (IMG:style_emoticons/default/huh.gif) (IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/biggrin.gif)

[Dr. Roger]

Your fronts are currently biased heavy.

Correct, your rears will probably never be able to catch up due to the 3-4x contact patch you now have over stock. They "could" in an emergency situation with the weight shifted to the front and, that's what you would want to avoid by keeping a P-Valve.

Or... you could take the rears off and plug the lines. (IMG:style_emoticons/default/huh.gif) (IMG:style_emoticons/default/popcorn[1].gif) (IMG:style_emoticons/default/biggrin.gif)

So, for my given situation, it sounds like increasing the rear bias by even 5% might be the ticket if I want to keep the current rear calipers?

[Eric_Shea]

10-12% at least but there are smart people who can calculate fluid volume much better that I.

I think Mueller's done this before. He's not around much lately... (IMG:style_emoticons/default/confused24.gif)

[SirAndy]

when you went big in the front and stayed stock in the rear, you changed the balance ...

that's why your front is now doing all the work. by the time the rear brakes kick in, you're already standing.

when i upgraded my brakes, i replaced the prop valve with a "T" and i couldn't be happier.
now the rears actually work instead of being just dead weight.

you can also play with different brake pads front/rear to get better results.

one thing you do NOT want is the rear locking up before the fronts!!! (SPIN!)
so, balance is crucial, but right now, your rears aren't doing any work, so i'm pretty sure a "T" will greatly improve your braking ...
(IMG:style_emoticons/default/bye1.gif) Andy

[PeeGreen 914]

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[Dr. Roger]

That's what i thought Andy and Phoenix.... i can still see some places on the rear rotor where the pads haven't cleaned up some scuffing from pre-install. they're hardly being used.

I tested the brakes yesterday just to see exactly how hard i had to press to get something to lock up and it was easily the fronts by a huge margin.
The rears weren't engaging even at full braking.

I took up jim's offer of a free T. Brake performance report to follow. =)

Thanks all.

[PeeGreen 914]

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