brake bias fitting change on 87
 

Where is the darn thing. i thought you guys said it was on the side of the ABS pump. is it on the master cylinder? stock is 11bar and there are 33bar versions available? what cars did they come stock on?
thanks

Mark

on all the cars i have seen its on the ABS unit

It is on the front face (pointing forward) of the ABS hydraulic pump. It threads directly into the pump, the rear brake line then threads into the bias valve. All cars with ABS has them, 33bar was pre-85(?) and 18bar after that. 45bar and 55bar bias valves are available from Roger and others. I think non-ABS cars have them on the master cylinder.

Picture of ABS pump and bias valve circled.

Cheers!
Carl

^^^^^
I see you make Creme Brûlée, yum yum.

One has to reward oneself on occasion :)

awesome. thats what i thougth. i think i see an "11" on mine, but its hard to tell.

That should be a 18

My 85 Euro has one of those sticking down from the front fitting of the master cylinder.

Mark, yours is always an 11.

For track use, instead of playing with the bias valves (two on my car) I got less aggressive pads for the rear. Hawk Blues in front and Blacks in back. Solved having the rears lock-up in hard braking. There's no ABS on my car. Matching pads all around for the street.

I'm 99% sure the OE on all '87+ cars was 18 bar. They were made in higher pressures - 33 bar, 45 bar, and 55 bar. For serious track, the 55 bar I found was too high, and the 45 bar (OE on 964's) was just right. I like the 33 bar for street.

The 45 bar are no longer available new, but I picked up a couple over the years by asking the 964 guys here on Rennlist if they had one they'd removed (they went to 55 bar for tracking). ALSO, for more rear braking, the OE 993 rear calipers have pistons that are 20% larger than those on the S4/GT/GTS's. Thus, and additional 'push' of the pads with those calipers (not the 993 turbo calipers).

if you are really threshold braking, i can see if you are getting into fade zone, wouldnt that autmatically put up to 2x the force on the rear pads? i mean before fade the proportions are what they are, but at the end of a racing straight, and you have fade and you are reallly pressing hard to fight it, the rears would still be cool and be subjective to the higher predal pressure. something to think about . also, 911s are so much lighter up front, they need more rear bias, due to the completely different end result of weight transfer. something like 2x the force is needed in the rear , to equal the stopping power of our cars with the weight biased more in the front.

Silly me for not proofing my post. Of course it's an 18 bar (corrected previous post) Thank you Gary.

Here are the part numbers for all the Bias valves with their respective values:

Brake Bias Valve 60 bar 965 355 305 01
Brake Bias Valve 55 bar 964 355 305 10
Brake Bias Valve 45 bar 964 355 305 00
Brake Bias Valve 33 bar 928 355 305 02
Brake Bias Valve 18 bar 951 355 305 01

The heavier the braking, and slicks give you that, the less brake you need in the rear. You need more rear bias than stock? Or less?

Maybe try one of those:

http://www.wilwood.com/MasterCylinde...derValves.aspx

It's not exactly a limit, not exactly a proportional, but for a fixed condition (like straight line, dry) it would not matter. Put it on, pull ABS relay, set it up the way you like, then (at the workshop) attach a manometer to the rear circuit and hit the brakes the same way you would do on the track. You will see a number there, which will hint you which limit valve to use.

I will not keep this valve on for more than strictly necessary to do the test, and not with ABS engaged. To attempt that I would do it the right way, data logging and all.

"the less brake you need in the rear"
---
You need as much braking in the rear as you 'need' depending on the combination of tyre and brake pads you have.

The OEM limits for rear pressures, are extremely conservative to prevent wrapping cars around things by n00bs.

You can shorten braking distances greatly this way by 'dialing in' what you CAN use, given the combinations of brake/tyre available to you.

Thing is..when you greatly upgrade your front brake coefficient, and do nothing in the rear, you become far more front biased, and have to compensate.

You can add more force to the rear for more pressure, or upgrade the rear pad to complliment the front's temperature/friction coefficient curve to match.

This is where Mark's at right now. He was 'fine' at point A, added another .15mu up front..but against advice did nothing in the rear and was almost an entirely front braking car..which badly burned up the front pads to the point they performed in wear and usefulness far beyond their operating range. The fix for that isnt "more cooling/more pad" it's re-balancing the car.

He's going the right direction with this change he should have considered many many moons ago. :)