Seth Thomas

I believe the issue with using a bias valve on an ABS car has more to do when you get into the ABS the bias valve 1) can be damaged 2) messes with the ABS programming of the computer as it is trying to send more/less/preprogrammed pressure to the wheel that is locked. If this car were a 3 channel car I feel it would be easier and not a big issue as the brake lines form a Y for the rear. With a 4-Channel car it becomes more complicated as each rear brake has its own line and the ABS computer can send different pressures to each individual wheel. You are trying to add the bias adjuster after the master cylinder. This is on a car with a single master cylinder like our street cars.

On a race car, like a cup or dedicated car with a Tilton pedal assembly, the bias adjuster changes the leverage angle between the two master cylinders changing the bias f/r with the amount of initial pressure being applied. This bias is changed before the master cylinder thus ABS can be retained.

Sorry for my simple explanation of the racing pedal setup. 1000 different things going on this AM.

OZ951

Thanks Seth, I'm not sure if you picked up on this but the car already has a bias valve after the MC, I am just looking to replace it with a slightly less aggressive version in order to (further) reduce rear brake pressure as compared to the OEM unit.

Whilst a twin MC & lever arm setup is a good solution but not one I'm expecting to add. I imagine that there must be a few cars out there that are running modified brakes with a standard master cylinder and hence might have installed an adjustable bias valve in an ABS equipped car.

I am not an ABS expert by any means so I am theorising here but I suspect that the larger rear pistons will have a greater impact on the ABS's ability to deliver the effect it is programmed to do because the larger pistons have changed the brake caliper characteristics away from what the ABS was originally programmed with. The reading I have done suggests that the ABS modulates the pressure in the caliper based on the measured wheel dynamics and an expectation of how the caliper should respond to a certain level of change in the brake pressure which the ABS will deliver (and the larger pistons have changed that response characteristic).

The bias valve will have delivered a certain level of pressure at the ABS input and the ABS will respond when that pressure begins to cause the wheel to lock. I suspect it doesn't matter that the bias valve has delivered slightly less pressure than say a standard fit, as the ABS has to respond to the wheel lock irrespective of the input pressure. The reason I thought along those lines is that in a standard car wheel lock can occur at different levels of braking pressure depending on the tyre/road/load conditions.

924RACR

In general, I'd say your interpretation of what the ABS is doing is correct - with regards to the feedback loop, controlling on actual wheel speeds/slip, etc.

The main factor throwing off the ABS performance/calibration here will be the change in piston sizing vs. stock, changing the brake torque generated for a given pressure - we call this specific brake torque (N-m/bar). If you're paying attention, you'll also see that changing the pad type will impact this as well.

The prop valve (proportioning valve) function/selection will be a less significant factor, since once your ABS is actively controlling wheel pressure, it is also controlling the front/rear distribution. Indeed, I've seen/worked with ABS systems configured to run either with or without a factory prop valve... but not to get the discussion side-tracked.

The big issue is just prop valve plumbing... all the race prop valves I've seen are single-circuit, designed to work on a front/rear split 3-channel system. You simply insert them in the single line going to the rear of the car, and use them to further reduce brake pressure to the rear.

As Seth points out, if you have a 4-channel system with 2 individual brake lines going to the rear, you'd effectively have to run two individual prop valves to maintain system isolation. Some cars come with fancy prop valves stock that will accomplish this - 2-circuit prop valves. Of course, they're not adjustable.

Of course, you could just try using a less-aggressive pad in the rear...

onefastviking

Hmmm, ok, Seth and Vaughan already pointed out some of my other concerns regarding 3/4 channel and the valve with ABS.
If it were me, I would get the correct piston GT3 matching rear calipers on the car. Sort the brake bias with the pads as suggested already. The GT3 guys that say they are lacking rear braking could also be dealing with a Diff problem rather than a braking problem.

Veloce Raptor

This is the salient point in this thread....very sound advice.

Matt Romanowski

Even more sound advice above. Use the proper calipers and be done.

OZ951

Thanks for the input gents, I have already fitted less aggressive rear pads as an interim measure since I will be unable to resolve the bias valve issue until after my next event.

Fitting smaller rear calipers may be the final course of action, however I do plan to continue investigating the viability of the current caliper since I am well down that road.

Apex996

OZ,

>> I imagine that there must be a few cars out there that are running modified brakes

Far fewer than you might think. I've been researching this for a while now.

First off I'd like to say that I recently discovered your site and was very happy to see somebody else going nuts on an early C2. Most people stop short of where we are going (See my mods link in my sig... blog is very outdated.. I need to update the changes made over the last 2 years)

Many probably don't realize the issues we face with these early C2 braking systems. they are quite different from GT3 or later C2 systems.

Here is my issue... (heavily modifed C2 track car with every GT3 part except the drivetrain. Mk1 Cup type suspension).
While my 1999 car "supposedly" has no TC it certainly has some sort of electronic traction component because with the ABS engaged I will absolutely toast my brakes within 3 hard laps. Smoking, burning calipers & pads. I have fiddled with various compounds front and rear to alter balance but found that the true problem was ABS. Using my data logger (Race Technology DL1) I can see the exit speed from slow corners is consistently less with ABS engaged than when I have is disabled (fuse pulled). The brakes are activating and overheating as a result.

With the ABS fuse pulled, I no longer have the "over active" ABS and have eliminated the overheating issues. This has allowed me to go back to a nice aggressive pad (Carbone Lorraine RC6e) which really works well to haul me down from high speeds. (Awesome initial bite). However with ABS disabled, I now must be extremely careful not to lock up the front brakes at lower speed. Talked to some race shops and learned that when prepping the 996 C2 for Grand-Am cup, teams found that removing ABS fuse it effectively biases more brake to the front. Most top teams went to an alternate race ABS system which is currently out of my budget.

So for a while I had to just be very careful. As the weight transfer shifts rearward at the end of the braking zone I must very significantly lighten up the pressure applied or I will lock the fronts and skate off. (Modulating the brake become nearly ineffective) This is especially critical in bumpy braking zone. Sad thing is that with ABS disabled I often cant capitalize on the 911's rear weight bias to trail brake down the apex. Bummer.

Now, like you I have purchased a Tilton pressure limiting valve. However I'm not exactly sure on how or where I want to plumb it. The more I think about it... the more I'm thinking that simply replacing the factory "Regulator Valve" with the Tilton wont work with the ABS re-enabled. But that it may work with it disabled. BTW, is the factory valve a check valve or a pressure reducer?



Would love to get a technical discussion started about exactly what systems are in place on these early C2 brakes that might cause ABS actuation. Supposedly I have no TC, PSM or ABD .... but exactly what kind of ECU/ABS logic is in place that would cause brake actuation on mid-corner and corner exit ... I don't know. anyone?

924RACR

Question to clarify - are you roasting only the rear brakes?

Apex996

Nope.

With ABS active (as stock) :
I don't run this way much anymore. Not really sure which end was getting it worse. After switching to race compound pads it took only 3-4 hard laps and the peddle went to the floor. A non-scientific walk around the car suggested all 4 calipers were roasting. "PORSCHE" lettering on fronts went from white to brown. (see photo) Wish I had some pressure transducers to place in the lines. (I also changed to rebuilt calipers with no effect)

With ABS deactivated (fuse pulled) :
The brakes now stay cooler but bias goes so much to the front that it is nearly impossible to threshold brake on anything but billiard table smooth tracks.

Edit: Some background info. Years ago I never noticed the problem with street tires, semi-metallics or Hawk HPS. When I started running DOT Race tires with Hawk Blacks it began to slightly occur, although I wasn't aware that it was ABS related and just assumed the calipers were old. So they got rebuilt. (fronts new actually)
Modified car more and got faster... began running full race pads (Hawk DTC, CL RE6, etc) this is when I could no longer keep the brake under me. Tried new cooling ducts, etc. No help. Finally resorted to pulling ABS fuse. This "fixed" one problem only to reveal the bias issue.

Apex996

Getting this back on topic...

Are these pulsations present upstream from the ABS pump? eg: where the OE valve is located? Or is Tilton only concerned with the pressures downstream of the pump?

TheOtherEric

Installing a bias valve is an inappropriate solution to a bias problem when you have ABS. That said, my e36 racecar has both. I installed a Wilwood valve upstream of the ABS pump because the ABS system had been gutted out. But later we re-installed ABS and left the valve. It works fine. YMMV.

car_slave

Why not replace your stock bias valve with one of same type - like the ones found on the 944 and 928? Just find out what the stock value is and work from there.

From your website http://www.oz951.com/public/emmypics/IMG_3543.jpg

928 brake bias valve is 33/5. The 944 turbo valve has a lesser value (uses less rear brake)
http://www.paragon-products.com/Brak..._p/944tblv.htm

Apex996

Why try 4 or 5 fixed valves when we are trying to do exactly what you suggest with a Tilton adjustable.

car_slave

Because the Tilton folks apparently didn't recommend using it with ABS. This turns out to be a moot point because the older units use M10 hardware, which the OP wanted to avoid using.

"Just for completeness I will add that I discovered the hard way that the older OEM bias valves I am more familiar with (5/33 5/18) are M10 valves but the 996 5/55 valve has M12x1 input/output ports."

"He (Tilton guy) was of the view that plumbing the M10 valve into the M12 circuit was OK, but the surprise was that he commented that their valves are not recommended for use in ABS equipped cars due to high pressure pulsations from the ABS possibly damaging the seals in the Tilton valve."