RKD in OKC

A recent post looking for a 30lb brake bias valve got me to thinking.

Have a 5/55 bias valve on my GTS. Went with 55 because a 55 really worked well in my 90 GT. 33 was not quite enough for the GT. I thought it was a good match for the GTS and my driving style. However...

Just before an autocross event, probably when I pulled the mats out, I accidentally unplugged the wire under the dash that goes to the accelerometer under the driver's seat. At the time I didn't realize what had happened except that both my ABS and Differential Controller lit up the dash telling me something was wrong. And my times were much slower with that one rear tire spinning on acceleration, and it was way to easy to lock up the tires braking.

Anyway, at one point on the course there is a long straight that ends in a very tight corner. Braking went from 100mph down to 35mph. On a couple of my laps I noticed my rear tires were locking up and smoking in that brake zone. I am wondering if the 55 bar rear bias valve is causing my ABS to kick in sooner because the rears are locking up than it would with a lower bias valve, decreasing my overall braking capabiity?

Discuss freely

JWise

I would think optimal braking would be achieved with all four locking simultaneously. If the rears are locking prematurely, wouldn't that be indicative of too much rear bias? (previously masked by the ABS)

RKD in OKC

That's kinda what I been thinkin

pcar928fan

Yea, I concur. I think I have the 33 bar in my S4 and that thing stops so fast it will pop your eyeballs out of your head! With FAR less pedal pressure than my GTS's (either of them...both with stock bias bar and 235 front tires vs. 225's on the S4) the car will out stop them easily! It really is amazing how quickly that car comes to a stop and how easy it is to make stop that fast. I guess I need to try and 33 bar bias valve in at least one of the GTS's (probably the '92 since it gets all the good stuff!) and see if I can make them stop as well as the S4. You would think the bigger calipers and rotors and tires would make the beast stop better...not so!

Just goes to show you size does not always matter! (lucky for me!)

JWise

Let me do some research tomorrow night but, IIRC, the factory 928 GT Cup car ran a 45 bar bias valve...

puyi

Hi,

I did not understand how bias valve is behavioring!

When you talk about a 55 bias valve, is it the max pressure for rear brake? As if you use 33 the max pressure to rear brake would be 33 bars?

I wonder it because, I would like to see how I can mount a pressure reductor valve to the rear to my track car. If I choose a 55 bars bias valve, It could make it possible to mount a restrictor to the rear brake with allow to vary the rear brake pressure from 55 to less pressure.

Thanks for the income

Best

Puyi

Vilhuer

AFAIK its more complex system than simple max pressure cut off. Up to 33 bar, 55 bar or whatever set limit is pressure will raise at same rate at front and rear. Above that particular point pressure which gets through valve is not full pressure increase. For example in 33 bar valve when front pressure is 40 bar rear can be 36.5 bar, at 45 bar its 39 bar etc. This means pressure will increase indefinitelly but only half of the front increase. Ratio is not necessarily 1/2 but Porsche valve curves I have seen have been in that region. There is graph of how these valves behave in some factory document(s) and also in certain MY brochures.

Gary Knox

There is also a 45 bar brake bias valve. It's no longer available new, but it was used as OE on the 964 model cars. I've obtained two of them through Rennlist by posting a "wanted" on the 911 (964) Forum. Took a few months to get the first one and a year to get the second one. The 964 guys would probably gladly swap their 45 bar for a 55 bar (that's what the guys who supplied me with their used valves had done).

By the way, if you want to increase the brake pressure to the rear wheels by about 22%, you can replace our OE rear calipers with the OE calipers from a '993 C2 (NON -turbo). Those calipers fit perfectly, and have 30 and 34 mm pistons instead of our 28 and 30 mm pistons.

Gary Knox

WallyP

" If I choose a 55 bars bias valve, It could make it possible to mount a restrictor to the rear brake with allow to vary the rear brake pressure from 55 to less pressure."

Perhaps a translation issue, but a restrictor does not affect the final pressure, only the rate at which the pressure is achieved. In other words, if you had only a pinhole in the flow to the rear brakes, the rear pressure would still eventually rise to the full line pressure.

puyi

Ok, so I don't understand anything of how it really works!

Who can make an exemple of how it really works and how the change of the valve affect the system?

Many thanks

puyi

mark kibort

Ideally...... for a street car. But, as I have said many times, for a 928 racer, with heavy trail braking, you want a lot less rear brakes. try trail braking with a car with "equal" bias for lockup straight. what happens, you dive into a turn, crank on the wheel setting the nose under braking and the inside rear will lock, then the outside rear. then, that will not be good. better to let the LSD do a lot of the braking, along with leaving it in gear longer, leaving it in gear, acts like ABS, but that has to be coupled with shift, brake style that allows for threshold braking, but only when in gear. in otherwords, locking up the rear is hard with it in gear, because you will stall the engine.

anyway as proof, i have video racing both with and without ABS connected or working. no difference in my experience, except the safety net that the ABS provides sometimes, as well as its more subtle growel worning. (grrrrr, means modulate now! )

Vilhuer

From '86 MY service info tech document.

svp928

Thanks Errka!

Steve

JWise

Well, scratch that until I can find proper documentation. All I see is reference to a brake pressure regulator, of unspecified bar.

RKD in OKC

Thanks Jarrod, I'd really like to know.

I think I can do some tuning on the rear lockup with the rear shock rebound adjustment, but getting it as close as a can with the bias valve then dialing it in with rebound makes more sense to me.