Marc Shaw

Interesting - but even though a standard 964 does not have the 40/40 LSD of a cupcar, is switching to a 60 bar unit a reasonable step with a 993 brake (Big Red) upgrade? IIRC a switch to the 993 master cylinder is also recommended, yes?

Marc

Jamie Summers

993 M/C would certainly be worth thinking about, but on ordinary 993 calipers (not Big Reds) the pedal doesn't need such a huge shove to get the ABS to kick in that it would make me think that I need more assistance.
Of more concern to me is whether there is sufficient brake fluid reserve in the reservoir to ensure that the brakes will still work properly when the discs and pads are getting low. I know that Adrian's book recomends upgrading to the bigger C4 reservoir.

garrett376

Sorry Peter, I can't even read what you write because that avatar is so distracting!!!!

Marc, with a C4 you are fine if you go to a Big Red front/993-rear brake setup with the stock master cylinder and reservoir. The stock 993 calipers are not "Big" - they a bit smaller than 993TwinTurbo calipers - those are the "Big" ones! I don't know if the 55bar stock C4 valve is that much different than a 60bar turbo - that's why I am researching the "no bar" valve...

RallyDogRacing

I'm curious, would it not be simpler to delete the stock valve (with it's on/off nature), and replace with a Tilton or similar adjustable proportioning valve? It would sure seem to allow a finer adustment of the desired braking characteristics to the driver's pref. I put these in every Audi that ever saw the track, but admittedly they are using relatively horrible brakes in comparison...

garrett376

Yep- that would be the best because then you can adjust for all track conditions, which are always changing; anyone hook one of these up to the 964 brake system? Ideally the control lever would be somewhere in reach of the driver, but that's a heck of a lot of brake line re-routing!

Oddjob

The factory proportioning valve does not shut off pressure to the rear brakes. The numbers on it 5/18, 5/33, 5/45, 5/55 etc. are the reduction ratio in % (the 5 refers to 50%) and the pressure at which the reduction begins (18 bar, 33 bar, etc). So for a 5/45 valve, the rear brakes will receive full pressure from the master cylinder up to 45 bar, then after that, the pressure above 45 bar is reduced by 50%.

So if you are pushing 75 bar out of the master cylinder, the rear brake caliper pistons will see 60 bar [(75-45) x 0.5 + 45].

Jamie Summers

Jim,
Thanks for clearing that up - makes sense to me.

Jamie

Oddjob

Ive done quite a bit of screwing around with proportioning valves on my 944 series track cars. Although somewhat different, the concepts and including some of the parts, are the same as for the 964s.

For example, a plain 944 n/a does not use a proportioning valve. But a 944S, which has the same brakes (master cylinder, calipers, rotors) comes with a 5/33 prop valve. The only difference is that the S is a little more front heavy than a plain 944. I put in a 5/45 bar on the S and it worked, so I then tried a 5/55 and it would lock the rears, so I went back to the 5/45 valve.

For the 944 Turbos and Turbo S cars, they came with the 5/18 prop valve. I replaced it with a 5/33 on a Turbo S (928S4 front calipers – same as 993 fronts), and no problems at all. Of course the car has ABS, but the brake balance is still to the front. On my 87 Turbo (which has the same brakes as the 4 piston C2/C4 calipers), I put on a 5/33 and it is very close to being too much bias to the rear. I can lock either a front or rear depending on track surface, brake pad compounds, steering input, etc.

RallyDogRacing

That sounds MUCH more logical. It also somwhat explains the why the white plastic cap with the sealing dome has notches in it's ridge, presumably for a fluid path...

As far as the tilton, I rarely ever felt the need to adjust mid-lap on the understeering-pig-Audi cars I love so dearly. Generally speaking I left the lever full-open to rears for track, and locked it down 2-5 clicks for daily use. Were you inclined to want to use a rotary valve (finer range of adjustment) & mount it in the cabin, I would estimate you're only adding around 6ft of brake tubing in total. Could be done quite stealthily I believe.

My BTDT with the Tilton was that it did not interfere with the ABS system at all on the Audi cars. I will admit however that when running on the track or driving agressively I always locked the center-diff which provided ideal brake force redistribution, and shutdown ABS in the process.

Brings me to my question: is there an ABS over-ride switch that I simply haven't found yet? Can the ABS function be shutdown whilst retaining the accell characteristics of the PDAS?

garrett376

Thanks for the update, Oddjob - that does make sense!

But, why does the plastic piece fit perfectly over the brass part sealing it off completely which makes those slots on the edge non-functional at that point? That confuses me!!

RallyDogRacing

I am speculating that the spring pressure almost completely closes it. As in when the valve is at working pressure that the springs prevent the cap from closing the gap past 0.00Xths or something... Perhaps a semi-duty cycle opening and closing effect needing the slits to prevent cavitation? (stretching here...)

Oddjob

Im not certain of the particulars of the internal mechanism. I would probably have to take one of my own apart and play with it a little to get a better idea. The pics are great, and I really appreciate you posting the guts for discussion, but I cant tell exactly what is happening with just a quick look.

What I can say is that there are certainly more pieces and more going on in there than a simple pressure regulator (which would hold a predetermined pressure/set point) or a pressure relief valve (which is a blow off valve - opens and releases pressure once the set point pressure is exceeded).

Pressure regulators are great for things like the fuel pump where you want to maintain a known constant pressure so the DME can control fuel flow through the injectors.

Pressure relief valves are used as a safety device, to keep something from overpressurizing and damaging or blowing up.

hjcarlin72

Great information. Thanks for the pictures. If you get the PCA Mag, Panorama, they started a tech series on porsche brakes and are going to specificaly discuss recommended upgrades for the 964 next month. Stay tuned!!!

garrett376

Since Jeff Curtis got me searching this thread again... I thought I'd provide an update. It works tremendously! I have locked the rear brakes up only one occasion - on a downhill portion of a track where I was driving a very choppy lap - the car was heading downhill to the right, I gave the car too sharp of a turn to the left then was quick and hard on the brakes (basically came in too late, too fast, to an off-camber turn that is heading downhill) - this combo locked up the rears momentarily, but with ABS all was fine. When that area is driven smoothly, it works great - never had a rear-lockup since. Conclusion - it works really well; if I had another track car, I'd do it again

And if I didn't mention it before, when reinstalling the valve after removing its innerds, install the lower brake line into the valve so you can fill it with brake fluid before assembly - it's impressive how much fluid you can pour into it - this would take a long time to bleed properly if it was installed empty and bled...

Larry Herman

Sounds like you got it right. If I can chime in here, the common misconception is that the brake proportioning valve provides a "proportioned" output. Like you stated earlier, it does not. It passes through whatever pressure is input to it until it reaches it limit, and clamps the pressure. A better name for it is a pressure limiting valve, as that is what it really does. So below it's setting, your brakes could be unbalanced; i.e. more pressure to the rears than would be desired.

For an unrestricted racecar, a Tilton double master cylinder with balance bar would be the best setup, as it allows for a completely proportional front/rear pressure balance throughout the entire braking range. A more difficult but equally effective setup is to get the proper sizing of front and rear pistons/pads and run without the "proportioning" valve, just like garrett376 has done.