Break bias
 

Im in the process of installing a break bias adjuster into my racecar and I was wondering if it was advisable to bypass the stock bias valve for the rear breakes before plumbing into the new adjuster, or should I just leave it inline? Will the stock valve work against the new valve?

Do not use both.....The adjustable bias valves will give full range and having both is not recommended.

Bypass the stocker.

Bypass the stock one. We tried leaving one in on a friends car and you end up with almost nothing from the rears. Example: stock one will cut as much as 70% IIRC from looking at the FSM, an inline adjuster will knock 50% off of that. In the end you have 15% of the line pressure to the rears with that combination of settings. I think the Tiltons will cut up to 70%, so cutting 70% of the 30% the stock one lets through gets you into single digits in the rear and fried brakes up front.

There's no reason you can't do both. I've been contemplating doing this for some time now and all the math work out so that it seems to be a good thing to leave both in place, as the actual brake bias for any given amount of maximum deceleration more closely matches the ideal. That way you'll have better bias in the rain when grip is lower, so you won't be needing to adjust it nearly as much as you would if you removed the stock valve. You can use the aftermarket one to simply take care of the excessive rear bias at really high deceleration rates and keep the stock bias for everything below that.

When talking about bias, percentage numbers are worthless, because all the valves have a "knee" point, below which they do not reduce pressure, so if you simply pick a point and calculate the %, its quite misleading. To adaquately see what a valve will do, you need to plot it on a chart, but even that's not that great because that doesn't show what ideal is and its not referenced to deceleration rates.

My experience didn't come from just calculations. This was actual problems on the track and then in the shop with a brake pressure guage.

Braking with slicks gets passed that knee point pretty quickly. I can picture the graph in the FSM in my head but I don't remember the numbers.

The car is fine now with the stock one bypassed.

you're right!...i agree to you!...

you mean brake bias? Is this a FB, FC, or FD?

thank guys for the input!

Now the next question is how should I plumb it? Should I leave the front going through the valve and put two bolts into the rear in and out lines of the valve housing? Should I pull the front directly from the master Cylinder and remove the stock valve housing altogether?

My car is an FC but this should pretty much be relevant to any of the generations I would think.

Totally remove it.

I'm not doubting your experience, I just don't see why it couldn't work with both and I'd love to hear an explanation of why it won't work well (I note that StopTech also says to not use 2 inline, but fails to give an explanation). Yes it will act differently than with it alone, as it should have 2 knee points and a much more aggressive bias reduction after both, but I don't see that as reason enough for it not to work.

The stock valve's knee point is at ~427psi and then goes at about a 1:2 slope (they give ranges of +/- 50psi on the numbers), whereas the numbers Tilton publishes say that it's got a knee point of between 150-1000psi with a 1:3 slope after the knee point. With it after the stock valve, when it's fully "open" you shouldn't notice it at all.

No problem on the experience we've both been here a while, we were thinking the same way you are when we put it in the car. At first we blamed the Tilton unit, took it apart, all that stuff, because the car was fine before. Then we took the stock one apart, nothing wrong there either.

We finally hooked up a brake pressure gauge to the rear circuit(only have one so we couldn't do both circuits at once). The gauge barely moved with both in line and moved a lot with either one. I think it has to do with the amount of pedal force we use racing that leads to the slopes combining to something too steep to get useful pressure to the rear brakes.
I have not given it much thought after we saw that either one worked solo.

Some year if I have time I may try pulling the stock one out but I have been running it with the Tilton, like many of my CENDIV friends/competitors have and it has been working fine. With the tilton in the least restrictive mode I can lock the rears.