garrett376

Well, I did too!! I think I've figured it out, though, thanks to Eric K who sent me his old proportioning valve to fiddle with. Hopefully this can shed some light on this little part that is often "upgraded" when brake changes are made.

Just for a little background based on knowledge I've picked up here, the proportioning valve is part of the braking system that limits the rear brakes during hard braking by blocking the pressure in the brake line to the rear brakes when a certain pressure is applied through the lines. A C2 has a 45bar cutoff, a C4 has a 55bar, and a turbo a 60 bar. Someone can chime in here if the various model years are different - I'm not sure about that part. So, on a C4, when the brake line pressure reaches 55 bar, the proportioning valve closes and does not permit further pressure to be applied to the rear brake circuit.

An example how it works is if you jam on the brakes really hard, the car's weight shifts forward so there is less weight over the rear tires. If brake pressure were at its maximum during this weight transfer, the rear brakes would lock up which doesn't help much for braking efficiency and control. So, a proportioning valve shuts off the pressure to the rear brake circuit at a certain pressure, so only the front brake calipers are getting pressure - since the front brakes are the ones with most of the weight due to the transfer.

Here is where the proportioning valve is located on a C4 (and it should be in the same place on a C2) mounted to the ABS unit. Also, here is what it looks like removed:

garrett376

Now, you can open up the proportioning valve by actually unscrewing it - one side is a 22mm size, the other side can be held in a crescent wrench. Once it's opened, it's pretty neat how simple its construction is.

Here's a pic of the innerds once it's been opened up, and with the plastic cap removed from the brass tube:

garrett376

Here's the valve in "exploded" form:

garrett376

Here you can see how the little plastic piece with a ball-shaped end, when the piston slides down, gets pressed against the brass tube, sealing it off:

garrett376

So who cares?

Well, the 964 cup cars actually had a bypassed proportioning valve (funny how many bypasses those cars had - heater, muffler, prop. valves... should have been called "bypass cars"...). Instead of a valve, they have a straight brake line that is not interrupted by the valve, like on non-cup cars.

So, what I figured out, is that you can reversibly bypass your proportioning valve by simply removing the guts of it, and putting it back together with nothing inside of it. That way you have an equal pressure between front and rear brake circuits with no cut-off pressure. The 964 cup cars used 993-sized brakes (S4 caliper up front, with the 964 sized rear pad but in a caliper with larger pistons) and had this valve bypassed for racing. So, if any of you guys are considering such an upgrade (I'd think Big Reds would qualifty), this might be a consideration to improve rear brake grip.

Hope this is helpful - I think it is neat to see how these things work!

ttAmerica RoadsterAWD

Hmmm. Neat!
Buut, so if the rear locks up during a turn with this bypass, you would have loss of adhesive forces to the rear and you might wind up facing 180deg the opposite way, No??

However, I can see how it would be advantagous if you were braking in a straight line. Seems a little risky if you were going just a little too hot around a corner in town, hit a wet spot/sand......

Jaime

JasonAndreas

Garrett are you going with the Cup "less is/costs more" Car solution?
We need to figure out how to hook this up to our ABS so more braking power is transferred to the rears when the fronts lock up!
Thanks for posting the photos I had no idea the part was so simple!

garrett376

Hi Jaime - you're right about the risk of locking the rears, especially in a butt-heavy 911 - but I think it's all about finding the right balance. Braking in a turn with a 911, especially if you really jam on the brakes (where the proporitioning valve makes a difference) would make you spin even if you had a proportioning valve! My deduction is that if the cup cars can run predictably without a proportioning valve, I'd think it would be beneficial to have the rears help stop the car more to reduce the work the front is having to do...

Anyone out there racing, or even driving their 964's with no proportioning valve? I think Cupcar does with his 964 Cup Car, but I'll have to check with him...

garrett376

I had heard of people "drilling out" the proportioning valve so I wanted to see what the fuss was all about...

Don't you think that if you can increase the rear braking force you'll decrease the overall pressure needed to stop on the fronts - therefore it seems you'd be less likely to lock up the fronts? Unless of course you're always locking the rears!

I don't know - I am not sure if I want to risk installing the "hollowed out" valve before a race weekend just yet, and end up with crappy brakes!!! Need more input!!!

burgass

Great info! Thank you !

Eric Kessel

Garrett,
Have you seen this page? http://www.proaxis.com/~tubbs/limiter.htm

Bill Gregory

The 45 bar bias valve was used on the 90-91 C2, the 55 bar bias valve on the 92+ C2, all C4's, and Carrera RS. The 60 bar bias valve is used on the Turbo.

Before anyone concludes that because Cup cars didn't use a bias valve means it's OK for a non-cup car, you need to look at the Cup Car brake system. They use Turbo 3.3 brakes, which are quite different than stock C2 or C4 brakes. In addition, they have a 40/40% limited slip differential, which influences braking at the extreme.

I know a number of people who have found the Turbo 60 bar bias valve (965.355.305.01) to be an improvement at the track, which is what I would recommend to anyone looking for more rearward bias.

Jamie Summers

As bill suggests, I went for the 60 bar valve on my '90 C2 when fitting 993 brakes front and rear. So far I've only done two trackdays on this set up (and unfortunately have no experience of 'stock' brakes to compare as the p/o had already installed 993 rear calipers - I merely evened it up by using the 993 fronts), and have only just managed to get the ABS to kick in under very heavy, very late braking (ie. much too late !). The front / rear balance seems to be fine under very heavy braking - it pulls up very straight. If the rears were getting too much bias the car would start to feel squirmy under braking. I suppose what we don't know is how hard one actually needs to brake in order to get the 60 bar valve to close - I have no idea if I am ever actually making it close, and if so whether the rear brakes could actually take some more bias before reaching lock-up.
Intuitively the best balance is achieved by reaching the point that the rears just start to lock under the heaviest brake application and then backing the balance off a tiny bit towards the front ??? I suppose the only way to do that is by trial and error using an adjustable valve (which incidentally can be bought for not much money). The balance will always be different for every track and every track condition given the amount of grip available and how heavily you need to brake. I should add that this is all entirely un-scientific - just trying to apply common sense as I see it - so please feel free to correct me if I've got it very wrong !
To sum up I think there are too many variables to think about, but for the time being, and for my use, the 60 bar valve seems to work just fine !

garrett376

Thanks Bill for re-stressing that this would ONLY be considered for a car with a different brake setup other than stock. And thanks for clarifying the model years of the valve pressures - I had an idea there was some variation to the model years!

Jamie, your experience with the rear brakes is helpful. I'd tend to agree with your thoughts about how to dial in the bias overall - it would be nice to get a variable valve - sure save a lot of brake bleeds swapping out the existing valve!

Eric - nope, never saw that site but good spot!! I didn't even think about connecting the two fittings together, although don't really like the idea of bending the brake lines....

Millemiglia

To answer your question, not really!
Mostly because I didn't know one existed...

But now that you've shown me, thank you!