Jeremy Himsel

Not the case at all on a 951 if you are using a decent street pad. I can see this being an issue using Hawk blacks or some other pad that have no business being on the street but ice cold just pulling out of my driveway I can easily lock the fronts with a 255mm front tire. The Big reds/blacks are a bit much for a street car as it only takes medium pedal effort to lock up a street tire.

333pg333

Interestingly I just read somewhere that our cars had the 5/33 in there anyway? Anyone know for sure?

95ONE

I read on here somewhere that the rest of world got a different valve than the US because of ride height regulations screwing up the balance. so, it might be correct for other countries.Ollie might be correct for Finland, but not the states?

Kool

So the 928 valve puts more braking force to the rear?

kdjones2000

Here is what the factory manual has to say about this stuff:

944 S: 33/5
944 S2/Turbo: 18/5
924S/944: None

So the changeover pressure is different for the different models (18 bar on Turbo, 33 bar on S), but the "Reduction coefficient" is the same across models (5 = 0.46 reduction).


For the 928, it also varies:
-83: Diagonal pressure reduction used
84 models: 33/5
85 models: 33/5
86 models onward: 18/5

So there you have it.....

Are people here saying that the 33 bar regulator will make the rear brakes provide more braking force than will the 18 bar?

Cheers,

Keith

kdjones2000

Who knows about these Braking-Force Boosters, and can explain the difference between the 33/5 and 18/5 units?

TIA, Keith

Oddjob

PET shows the same proportioning valve numbers for North America and RoW cars, so that would indicate that all Turbos (incl. Turbo S, S2s, 968s) came with the 18/5 valve.

The prop. valve reduces the increase of pressure to the rear after the pressure passes the designated set pressure. The 5 is for approx 50% reduction and the 18 is for 18 bar. So the valve will decrease pressure to the rear brakes by approx 50% once line/master cylinder pressure has passed 18 bar.

For example, You stand on the brakes and put out 30 bar pressure from the master cylinder - the front brakes see 30 bar at the pistons, the rear brakes see 24 bar [18+ 50%(30-18)].

If you had installed a 5/33 valve, for the above example at 30 bar, both your front and rear brakes would see a full 30 bar pressure.

60 bar at the MC, the pressure at the rear brakes would be 39 bar with an 18/5 valve, and 46.5 with a 33/5 valve.

TurboTommy

I had some extra brake parts from another car.
So, I ended up putting on the MO30 brakes on the front, and taking the front calibers from a 951-non-S for the back. This way you get the extra clamping force from the extra front caliber piston ( I think this is right) for the back. I stayed with the regular 5/18 bias valve, however. I'm not sure if my wrench also changed the 951-non-S front rotors to the back. Does anybody know if the rotors are interchangeable front to back (non-S, we're talking here)?

mortymower

Front rotors are not interchangeable with the rear.

kdjones2000

Thanks, that is what I was wondering. So it looks like the 33/5 would provide more rear braking power, which could lead to less front diving, but also might cause lockup....

Keith

333pg333

I've not heard of anyone who switches to it having lockup issues.

Oddjob

Someone enlighten me, I don’t remember the std turbo/S2 brembos having the same mounting bolt spacing front and rear, i.e. you cannot bolt the front calipers on the rear (?).

Regardless, you dont want to do that. The front calipers have significantly larger pistons than the rears 36 & 40mm versus the rears, 28 & 30mm. The effect is that the force on the pads is 1.72 times greater for the larger pistoned caliper. This will throw the braking bias WAY off if you put the front calipers on the rear, regardless of what front calipers you have on the car (unless you put on some GT3RS or Carrera GT ceramic brake kits...).

And you dont want the rears to outbrake the fronts, the car will want to rotate ends fast!





Yes, it’s a subtle increase in straight line braking power. You will use a little more rear brake, and therefore a little more rear brake pad. I doubt you would notice much difference in front end dive, since that is due more to the location/height of the vehicle center of gravity and the spring/damping rates. In fact, if you increase the de-acceleration rate by using more rear brake, it will actually increase front end dive.

With the 33/5 valve, there is nearly no concern with causing rear lockup on a car with 928S4 fronts. With the standard Turbo front brakes and 33/5 valve, the car may be a little more sensitive to rear alignment (make sure you have some toe-in in the back), and may be slightly unsettled if trail braking hard. If youre not a heavy trail braker, then its also of no concern. I have played with higher bias valves, 45bar and 55bar, and you can get the rears to lock first, which is not what you want, and nearly dangerous.

NOTE: these comments are for track use. I don’t think you would notice any real difference (or need) in street use. And I would not recommend wasting the time and $ swapping out bias valves unless its for a track duty car.

sawood12

However I guess it stands to reason that if you upgrade your front brakes, as I have to Big Blacks, then the 33/5 bias valve would be useful. I have noticed that since upgrading my rear discs are getting virtually no use as I replaced them at the same time and they look untouched - even showing signs of mild pitting due to lack of use - OK i'm not tracking the car much at the moment, but the roads around where I live give you plenty of opportunity to use the brakes so they don't exactly get an easy time. I feel in this situation the 33/5 is a good move even for street use.

TurboTommy

Oddjob;
I appreciate what you're saying.
It came about like this:
On a previous car, I had the big brakes on the front, and I felt that the rears were hardly doing anything. You could swipe your finger on a back wheel and there was practically no dust and the front wheels were black dusty in very short time. Also, I felt straight line braking should be much better. Then I switched to the 5/33 bias valve. Braking improved some (I really think with the S4 front brakes, it's a must to get the 5/33 bias, and I don't know why Porsche didn't do it from the factory like with the 928); however, I felt that front lock-up would happen way sooner than the rear. I know that that's the way it's supposed to be, but I figured there might be some room for more rear brake. I called around and many said, like you, that the front calibers on the rear would give too much rear bias. However, I felt they were just theorizing and didn't know for sure.
One wrench/racer said it would work, so I tried it with my new build-up.
I've never had better braking; the rears grab better right at the onset of pedal pressure. With the standard 5/18 bias, you're still gonna get more going to the fronts with harder braking.
I haven't had rear lock-up in 2 years with threshold braking on the street and autox's and such.
I could imagine somebody not 100% comfortable with their car, with this set-up, making a mistake and braking too hard mid-curve on the interstate, might lead to disaster; so I'm not saying everybody should go out and do this.