V2Rocket

Hi,
Would like to solicit some experiences using the 944S brake proportioning valve on a 944NA.

The NA brakes work well enough, but the "S" prop valve is supposed to put more pressure to the rear brakes to make the stock calipers work better and stop harder.

I would like to try this before any more ambitious mods to the brakes (the stock pedal is sort of soft despite having a newer booster/master cylinder and being properly bled, I'd like it firmer aka full-manual brakes?).

Oddjob

N/A's don't have a prop valve at all, so adding the 944S 33 bar prop valve would decrease rear braking pressure on an N/A.

V2Rocket

I thought it worked the opposite, to decrease the fronts but increase the rears to more evenly balance the F/R braking power?


***looks like I misunderstood the function of the valve. this should summarize it now...right?

without a valve, the F/R circuits have more or less even braking pressure.

with the valve, they F/R are even pressure until the line pressure reaches a certain point (18 or 33 bar depending on valve), at which point the R pressure increases at a fraction of the F pressure past that point.

e.g. 5/18 valve with say a 0.5 ratio means that F/R are equal until line pressure gets to 18 bar. then any increase over 18 bar at the front, about half that increase goes to the back. so if front pressure goes to 22 bar (18+4) then rear goes to 20 (18+(4/2))



so basically the proportioning valve, at high brake pressure (hard stop) situations actually shifts more pressure to the front brakes.

rlets

When you brake hard, vehicle weight is transferred to the front axle. Traction on the road is a function of the tire's grip and the weight on it, so more weight on the front axle means more traction and thus more potential braking power up front and less out back. Hence the proportioning valve which reduces rear line pressure relative to front to prevent rear lockup and allow more stopping power in general by using the available front traction.

I found adding a prop valve to be key on my 944 n/a. With good rubber on the track, I would lock the rears very easily which caused me to spin a couple times. I experimented with the various prop valves available and found the 5/33 to be best with M030 brakes.

Rich

fasteddie313

Why not this one right next to your shifter? $45 ish


That is my plan, eventually..

V2Rocket

the only "adjustable" thing i want on my car is the DME tune.

i don't want to ever have to change the suspension or brake setups, i just want them to work when required. so the passive porsche prop valve would be my choice.


not to mention that adjustable **** would be hard to blend in on a street car interior

Oddjob

Sorry - getting back to this one late.

Your math is correct on how the prop valve works. It doesn't shift more pressure front (the front circuit pressure is not effected by the prop valve), it reduces the rate of pressure increase to the rear.

The master cylinder piston sizes for the front and rear circuits are not the same size, so front and rear line pressure is never indentical. Differ by the ratio of the piston area. Then the force on the pads is the line pressure x the caliper piston area. And the torque generated is the pad force x the rotor radius (or a mid point of where the pad force is acting on the rotor). So there is a dramatic front brake bias even w/o a proportioning valve.

Because of the weight transfer and effect on traction that Rich describes above, brake balance is needed to keep the rear wheels from locking before the fronts. Since the coeff. of sliding friction is lower than rolling friction, if you locks the rears first, the back end now starts going faster than the front (or is slowing less than the front), so the back end gets really unstable, starts dancing, and can cause the car to spin - which is very unnerving at best, and can be rather dangerous in either a panic stop situation or on the race track. Note, normal street driving stops will generally not generate enough weight transfer and braking force to lock the rears and create this unstable condition.

A stock N/A with stock brakes does not need an additional proportioning valve to keep the rears from locking first, or so the factory decided. Later cars with slightly different weight balance, different brakes, shocks & springs, etc were equipped with brake bias/proportioning valves from the factory. The S came with a 5/33 bar valve, the Turbo/S2/Turbo S used the 5/18 bar valve. And there are several other bias valves available from porsche, used on other cars. There is a 45 bar and a 55 bar that come on different versions of 911s. All done just to keep the rear end of the car safely stable during braking.

Ideally, you want as much rear brake as you can get away with, so the least amount of rear line pressure reduction. When you modify suspension, move weight around, change the brakes - this all changes how the car responds under heavy braking, and you may need or want to make some changes to the brake balance.

When racing a 944S (stock weight, stock brake calipers, upgraded suspension), I could use a 45 bar prop valve instead of the stock 33 bar. A 55 bar unit caused the rear brakes to lock. On Turbos and Turbo S cars, you can swap the 18 bar unit for the 33 bar valve w/o risking rear lock up.

Its also not uncommon to play with pad compounds to tune brake balance. Many use more aggressive compounds on the front than rear.


The goal is to maximize overal braking performance. So point is, if you are not locking the rears under heavy braking, you don't want to add a prop valve, as it will slightly reduce your overall braking performance. If you are locking rears first, then start with one of the higher pressure prop valves and go down from there.



So you have the full Turbo S/M030 brakes on an N/A, front and rear calipers?

V2Rocket

I like to hit the brakes hard in my 944 since it's a toy now and not the daily car anymore. And doing so it does sometimes lock the rears. I don't have super sticky tires which of course compromises the brake effectiveness, and I am using oe-spec pads. So maybe the prop valve would work for me, so I can keep my cheap tires

Oddjob

Here's what happens when your ABS goes out and you lock the rears going into Turn 5 at Road America. Lucky it was the first lap of the first practice sesson, so no one was pushing hard and plenty of room between cars.

fasteddie313

This is what I thought too but it turns out to be incorrect.. The 2nd piston for the rear brakes is not mechanically linked to the actuator but rather just floats. It's job is to keep the brakes working in event of a failure on either circuit and has nothing to do with F/R bias at all. There is no F/R bias difference to the master cylinder whatsoever and both circuits are actuated off the 1 front brake piston..
There is inherent front bias due to the larger front calipers but the master does not favor either side, both sides will get the same line pressure but the 2nd piston floats allowing for automatic adjustment to the volume requirement difference between the circuits..

Read more here about how the stepped master works starting at post #8...
https://rennlist.com/forums/944-turb...-calipers.html


In order for a prop valve to be advantageous you have to have a bias problem to solve, if you do not have too much bias on one end or the other to turn down with a prop valve then installing a prop valve will only create a problem.

fasteddie313

More like reduces the rate of brake pressure rise (after a certain point) to the rear circuit relative to the front circuit brake pressure, if installed on the rear circuit..

A prop valve can only turn down the pressure on whatever circuit it is installed on..