strandolph

In a 944 NA race car. We can use any non-adjustable brake bias in the 944 Cup and I'm wondering if this is something I should test.

The car is factory-biased with smaller rear brakes and a stepped master cylinder (23mm front piston, 19mm rear piston). I can also mix and match pad composition and ride height to tune bias. The 951, 944S, etc., with their larger front brakes, came with a proportioning valve that reduces rear brake pressure 50% above a certain pressure. So, under light or initial braking, the front and rear are equal, but under heavy braking the rear is reduced 50% (actually 46%). The anti-lock systems on NAs also had a proportioning valve installed--I assume so the system relied more heavily on the front brakes during an anti-lock event. With Big Reds up front, I'd want them to do most of the braking. But with stock NA calipers, how much harder do I want the front to work?

There are four valves that I would consider:

5/18 #928.355.305.02 (reduced rear brake over 18 bar)
5/33 #951.355.305.01 (reduced rear brake over 33 bar)
5/45 #964.355.305.00 (reduced rear brake over 45 bar)
5/60 #964.355.305.10 (reduced rear brake over 60 bar)

What will this get me? I run 275x15 rears and 225/15 fronts. I assume the pressure under the cut-over remains in the system, so with the 5/60, 65 bar = 62.5 rear bar, not 32.5 all of a sudden. So, what benefit do I get from a reduction in rear brake at some certain brake pressure rather than just the steady-state bias naturally inherent in the system?

Steve

stownsen914

Interesting that there are multiple proportioning valve otpions, but handy in your case for setting the car up for the track. The stock system is a compromise designed to handle various scenarios, and unfortunately for you, ideal threshold brake bias isn't one of them. I would think Porsche focused more on safety (erring toward front brake bias) and perhaps achieving even front/rear pad wear.

I'd think the purpose of having the proportioning valve cut in at a certain pressure the way it does would allow something closer to even F/R brake bias at lower brake pressure (like an easy stop approaching a traffic light), and automatically adjust to lower rear brake pressure (moving bias frontward) for threshold braking, to avoid the possibitity of rear wheel lockup. This would be keeping in mind that the harder you brake, the more weight transfer to the front, and thus it makes sense under such conditions to move bias more toward the front brakes.

As for the advantage of having more brake pressure until some specific cutoff pressure, I guess it would be helpful to know how high the cutoff pressure is relative to the max pressure in your system under hard braking. I have no idea what that would be. If you imagine the scenario of, say 100 bar in your brake system, it seems like it would promote more rear bias to have the cutoff pressure for the rear stabilize at 60 bar while the front gets the full 100, compared to the rear maxing out at some lower number like 30 bar.

As for where the extra pressure goes that the valve removes from the rear circuit, it probably just recirculates back to "upstream" of the bias valve, sort of like a dam in a river.

Especially since you've changed brake calipers and are using non stock tire sizes, the factory bias will certainly be thrown off, and you'll gain some advantage playing with brake bias. Maybe someone else on the board will have played with the specific parts you refer to and can comment on which ones to try. Have fun with the project!

Scott

Joe Weinstein

I'll assume you're not running street stock ties, so the added traction you have from your tires
already compensates for any front-brake bias that may have existed in the stock setup. The
more traction you get, the more your fronts will be loaded. The more braking you get, the more
work your fronts will be doing. Can you also do anything to get extra cooling air to your front
brakes? If you played with bias, and it turned out that you could add front bias and get better
braking, it might be better initially, but midway through an event you could run into new overheating
problems in the front. Just being cautious, but unless you can deal with extra heat, it may be
better to have 75% of the best braking, all through an event, rather than have 100% for the first
half of an event, and 55% for the second half...

Oddjob

Rear prop valves are just to keep the rear end from locking up first or causing rear end instability during heavy braking on street cars.

Double check your part numbers - the 928 part number is the 33 bar valve, the 951 part number is the 18 bar valve.

Also, there is a 55 bar unit, which is a 930 part.

I have tried 18, 33, 45, and 55 units on different 944 track cars.

Because your setup is quite a bit different than a factory street car, you will have to do some testing to determine which works best for you. You want to maximize rear braking w/o any concern of rear lockup, so the highest pressure rated valve that doesn't chirp the rears (or make the rear end squirm). All sorts of factors influence that so only way to figure out which valve is best is by trial and error. And if you are a heavy trail braker, you may want a little less rear bias than a driver that does all straight line braking.

strandolph

I'm running the stock single piston caliper brakes on Hoosier R6 tires. No brake cooling issues...

Oddjob

If you are running stock calipers w/o a prop valve, and not having any rear lockup issues - leave it alone. Only reason to go with a prop valve is if you have too much rear brake bias (locking or wagging the tail).

strandolph

Okay, that's good advice. If I do find that I want to add a valve, do you have any advice for an efficient brake bleed? Can you pre-load the valve with fluid?

I double checked the part numbers, corrected the one you point out (I just assumed the one listed in PET for 944 would be 18 and for Turbo would be 33) and added the 55 bar for posterity:

5/18 #951.355.305.01
5/33 #928.355.305.02
5/45 #964.355.305.00
5/55 #964.355.305.10
5/60 #964.355.305.01