Goughary

i was going to change the proportioning valve to add brake bias to the rear on my C4...


why wouldn't I just add one of these:


http://tiltonracing.com/product/scre...tioning-valve/


and take out the proportioning valve? this way in winter when the weather sucks and I'm driving in the rain on old snow tires, I can take away rear bias, since I would have too much rear if I were set for summer tires with bigger width in the back...



Thoughts?

Spyerx

I removed mine, well I pulled the innards out of it. Not hard to do and it's fully reversible. Picture taken for "records" should I wish to put back. Mine is (was) the 45 bar variety. (91 with the 2 piston rears). After the car is setup I'll have a proper tube formed to eliminate this part. But I've kept it there "just in case" and if I'm getting any early rear lockup Ill try different valves.

I asked this exact question of the engineer at Brembo Race Technologies who I've been working with. His answer was no, he didn't recommend using a proportion valve for my setup. He said if I want proper bias control, get a dual master braking system. :-)

Anyway, I know you don't have the same braking setup I've put on mine. But he didn't recommend a manual adjuster like this without proper data to adjust it (wheel speed and brake pressure sensors). His feeling is its another unknown variable added to an already complex system.

Vandit

Bust out your TI-89s starting with reply #45
https://rennlist.com/forums/964-foru...-needed-3.html

Goughary

basically - running this tilton unit would be the same as taking out the innards of the stock valve - when its fully open. and then as you dial in the brake bias, you are taking bias away from the back...

So your guy that you were talking to is thinking this is way more complicated than it is...

I'm kinda leaning toward doing it - Bill Verberg suggested that its hard to add too much rear bias to the c4 without changing calipers - i.e. by taking out the innards of the bias valve and running no valve, you are still not over biased in the rear (i assume he meant for dry track weather in a track prepped street stock C4 with sticky tires)

So this could be interesting - and then if it sucks- I still can toss in my 965 proportioning valve that I had been planning to swap out for my stock C4 valve.

alexjc4

A few things to consider.

In the wet you get less grip and therefore develop less braking g's, this means less weight transfer from rear to front when braking. For this reason race cars run more rear bias in the wet, not less.

Adjustable proportioning valves are useful but have limitations. The knee point is what they let you adjust, this is the brake pressure at which the rear pressure stops rising 1:1 with the front. The ratio of the pressure rise f vs r after this knee point is fixed, iirc AP Racing valves are 0.4, Tilton are 0.3. Another consideration is hysteresis (also present with the factory valve); the machanism used in proportioning valves means that when you release the brakes the pressure doesn't fall at the front and rear along the same "lines" as it rises . Instead the front pressure tends to drop until the f/r ratio gets to 1:1, only then does the rear pressure drop at all. So if you are modulating the brakes the bias will change (rearward).

More specifically for 964s , many have reported good results with no proportioning valve at all. This gives significantly more rear bias at high brake pressures, above the knee point (unchanged below the knee point). The static weight distribution combined with the relative size of the front and rear calipers on a 964 mean this not as crazy as it would be on most "coventional" cars. Specicifcally theoretically the increased rear bias would be benificial in the wet.

Don't forget to thoroughly verify that all three channels of you abs as functioning correctly, and if you make changes to your bias, test hard off road, ie at an airfield/track.

Bill Verburg

964 has a lot of front bias, which just increases w/ higher line pressures
C2 or C4 w/ stock 4piston rears is 1.734 below 55bar this goes to 3.21 above 55 bar
C2 w/ 2 piston rears is 1.508 below 45bar and 2.793 above

The factory is extremely sensitive to brake bias issues as a result of the legal environment we live in and errs on the side of front lock up every time for street cars. This was not always the case. 911 up til '83 had a bias of 1.491

For a while('84-89) they went to 1.220/2.260(under/over 33bar) to try to address pad wear issues.

For street use these are all fine and safe, for the ultimate track performance you want to move as much to the rear as possible, icing on the cake is to be able to manually vary bias from the cockpit.

So the goal is to get neutral bias near where you want to be and then ideally be able to move it front or back.

I use 1.426 on both of my cars, one w/ and one w/o a p/v. This is close to where you want to be

factory Cup/RSR bias is widely variable some use twin m/c and some don't

typical is
2007RSR w/ 1.613 neutral, 1.875max/1.26min

1984 SCRS w/ 1.244 neutral max/min I didn't calculate but the range will be similar in magnitude to the above RSR. This is unusually low due to the Rally orientation of the car. W/ tin equal size m/c it goes to a more normal 1.579

An adjustable p/v is certainly less desirable than twin master setup, among other issues there is a response time lag that can be very objectionable at the limit.

If it were my car, i'd gut the p/v as Spyrex has done whether 964 4/4 or 4/2

Goughary

I agree that an adjustable p/v is less desirable than a dual master...but I'm not going to fit a dual master...

What in thinking is that the tilton unit in place of the stock p/v is the same as gutting the p/v at full open, and then if I need or want less rear brake bias, the tilton unit allows me to dial that back toward the stock setup.

Changing to a gutted p/v or changing to a turbo valve leaves me with no further setup ability.

So for example, when I switch in November from pilot sport 255 rear, to pilot alpin 225 rear, is a gutted valve going to be what I want? Not sure. Would be nice to play w the valve and set it up well for the use...

Bill Verburg

Native 964 bias is so far forward that it won't make a difference. The factory had to accommodate everything from 225/50x16 rear to 255/40x17 rear and beyond in their setups w/ a lot of headroom.

Spyerx

Bill is hitting on the feedback i got from the brembo engineer. Fwiw he is a race friction engineer so he's pretty familiar with bias and getting it right.

That said its not hard to try just a little time and a $109 part.

What would be cool is if you can get some g data from a data system like an aim solo. To compare. Unfortunately I'm not sure you can get wheel speeds from the computer on these.

Bill Verburg

For the '96 up OBD2 cars there's a lot of info that can be pulled from the car, very little for the OBD1 cars

Goughary

Well the setup will be c4 front calipers front and rear, since that's what I bought...so that adds rear bias. And then the tilton valve acts like gutting the stock valve. But then I'm going to a 225 front tire for summer, which adds front bias...

Then winter is 205/225

As to data and testing...if anyone has the equipment, I'm happy to loan the car out for a test day. Would be fun. The way we used to adjust brake bias years ago during the sport Renault years was to go screaming down our street in new Canaan, ct and threshold brake to zero and watch for wheel lock with no body panels on the car...

Would be fun to do things in a bit more...uhmmm....professional manner...lol

Anyway, I bought couplers and I have a brake line bending tool, so what I'm going to do is couple where the lines go into and out of the oem p/v and run them to the tilton unit and give it a shot. Basis what you and bill have been saying, I'm sure I'll end up running it full open all the time.

panzerfaust

hello bill would my 84 m491 car have a 1.220 bias? seems like its close to the SCRS 1.244 which ran a dual MC. i had an 86 930 which didnt stop worth a beans compared to my m491 im assuming nothing was change just the M491 car is 350lb lighter

Bill Verburg

No, an M491 has 930 brakes, brake torque bias is 1.579

The SC/RS has the same calipers and rotors but uses a different m/c setup.
A stock 930 or M491 uses 23.8mm DoT twin master. This means that the 2 pistons are both inline and are the same 23.8mm size. This setup doesn't alter bias.

SC/RS uses a racing twin m/c, This setup has dual side by side m/c. These are individually swap-able for different sizes. SC/RS came from the factory w/ 17.78 and 15.78mm m/c. this moves bias to the back, hence the 1.244 spec.

On the older 3.0RSR they used essentially the same calipers and rotors and also used different m/c combos to tweek bias to suit the driver, circuit and conditions.

panzerfaust

So what would be the most effective way to move some of the bias to the front on the m491 some where close to the NB 3.2 cars without resorting to twin MC and no booster? Does my car have a proportion valve that I can swap with another 911 or tinker with like the 964's? I don't want to over do it cuz u can spin like a top without warning if the rears lock up first.

-nick

Thanks for bringing up this discussion. I'm putting it on my to-do list. Anyone with aftermarket springs/shocks is losing braking ability due to too much front bias. Less weight transfer to the front and the front tires will break loose more quickly. My wet weather braking is truly terrible.