Interesting subject came up on the 993 forum regarding brake proportioning and the limiter valves installed in a stock system.

Does anyone here have a line pressure data channel on their data logger? I’m curious the max line pressure you see under normal racing conditions.

 

I have brake pressure sensors on my 964 race car with the 993 racing ABS braking system (basically the same as a 993tt or 993c4s). I have sensors on both front and rear with the rear pressure limiting valve removed. The brake pressures are the same front to rear and on extremely heavy braking where the car is compressed (braking into an uphill), I'll see 1800-1900psi. Normal braking on a flat is more like 1200psi.

 

I do but in a non ABS car. Ditto the 1200 in normal conditions. Higher for inital pressures from high speed and when under compression. I only read the front for now. I have an ajustable P valve on the rear and have shave off a bit of rear pressure on some corners. T5 at summit for example down hill hard braking. Am planing a second sensor to read the ajusted rear pressure.

 

Interesting. So a 55 bar limiting valve will hold the back brakes at ~825 psi. Under uphill braking and/or with really sticky tires it seems like one would be giving away a lot of rear stopping power.

 

Yes; that's exactly why electronic brake proportioning can be far superior. Well, until the car's substantially modded.

Of course, add in good yaw rate and steering angle sensors, and now you can start to do some brake proportioning side-to-side as well as front-to-back, and get proper yaw control! Then things really start to get fun...

 

Wow... This is great info... l'll have to check but I belive (at least i the 993) they start kicking in at 40bar (580 psi) that is signifigant.

 

1200+/- PSI (on a 0-2000PSI sensor) IIRC... but that's on an SC.

 

I have never really looked at how the proportioning valves work, but I would suspect that they may not have a hard limit. My guess is that they start to work at the stated pressure and impact the balance but probably allow much greater pressures than the rated value.

Anyone here an expert on how these work. All my experience is with balance bars and 2 master cylinder set-ups.

 

Well, I'm not an expert on the hardware... But usually the two deciding factors for a prop valve are the knee point - which 55 bar would be a proper indication of - and the slope, or gain factor, which doesn't seem to be indicated (but of course will be less than one).

So this won't be quite equivalent to a 2MC/balance bar setup, since that has a knee point of 0 bar, and you adjust the slope.

I'm not sure either how the typical adjustable racer prop valve works, but I believe it's equivalent to a balance bar type setup, with 0 bar knee point adjustable slope function...

 

The porsche bias valves are rated like 5/18, 5/33, 5/45, 5/55, etc. The "5" is for 50% reduction and the "18, 33, 45..." is the set/break point pressure in bar.

So with a 5/18 valve, when the rear line pressure exceeds 18 bar, the pressure increase is reduced by 50%.

For example, if 1200 psi (83 bar) is generated at the master cylinder, the line pressure down stream of the 5/18 bias valve is 18 + (83-18) x 50% = 50.5 bar. Using a 5/55 valve, down stream pressure is 55 + (83-55) x 50% = 69 bar.

 

Thats why the lever is right next to the wheel. Out a click for T5 or other rear locking braking cycle and back in for the rest. Before I was dragging the rears along making a nice light wisp of smoke unless I modulated well below what the fronts could take. Now I can stand on the pedal.

I though I had a fancypants system, my P valve has 7 clicks thats one more than the ones that only have 6.


Yes they have a start point and only taper the increase not stop it. A P valve will do nothing to the front rear balance until the pressure hits a setpoint then it reduces further rise by a set %. A balance bar changes the % across the full range. IICR my Tilton valve reduces the post setpoint rise by 30%. The pressure to the down stream brakes goes up just not as much as the others. you can ajust the onset. Some cars have both balance and P valve. They do not do the same tasks. (in cars other than road racing they are use in all kinds of odd ways)

in my car's case it was simple addition and works great.

 

We do log pressures. I'll have to dig up some examples. You'd be amazed as what one can learn by looking at just F/R brake pressures.

Here are a few plots
First Pressure vs. long. G and another comparing four drivers

 

Reminds me of the Randy Pobst quote re: braking - in like a lion, out like a lamb!