Waterguy

The 944 series came stock with three different sets of brake caliper/rotor diameter combinations and two different master cylinders, which resulted in five different distributions of front/rear effective braking torque. These same brake biasing valve may not have the same results on different cars.

Starting in 1987, all cars (except the 924S) got a revised master cylinder which had a larger piston in the rear only. This would result in a firmer pedal but less braking force to the rear. The force of the braking booster was increased to compensate.

When you run the numbers of M/C piston area versus caliper piston areas/rotor diameters, you find that the 944 series had the following inherent front/rear brake force distributions:

944 82-86, 924S: 1.34:1
951 1986: -------- 1.11:1
944 87-89, 944S: 1.57:1
951 87-88, 944S2: 1.30:1
951S, 944S2 M030: 1.53:1

I can see why the 1986 turbo needed a brake proportioning valve, but I don't see why the 944S did.

You then add in three more caliper/rotor combos (BR front/stock rear; BR front, stock front moved to rear; BR front and rear) and mix and match pads front and rear, then you have a situation where there is definitely more than one correct answer.

Oddjob

I didnt buy into it at the time, I was just frustrated that others didnt chime in to help me argue the benefit of ABS. Ive been tracking 944s for 12 years and know all too well that my right foot cant beat bosch, but I learned that some others out there think they have that much talent...

But speaking of learning, I had never put much thought into swapping front calipers to the rear before. A quick calc shows that the front calipers (36/40mm pistons) will exert approx 1.7 times the clamping force of the rears (28/30mm).

My biggest concern with the ABS retrofit is finding the rest of the parts (I have about 80% of the components) at a reasonable price. Dealer costs are prohibitive. Installing the complete ABS wiring harness up to the fuse box may also be problematic, but it may turn out to be straight forward.

Sam Lin

Chris, I'd be interested in any ABS install details as well, I thought it would be far too difficult, so your mention of it being relatively straightforward provides some hope.

Sam

Danno

" At a high level, the back completely closes and it all goes to the front. "
"33,40,55,66 etc = % to the back when pressure applied > 5bar."
"The 5/18, 33, 45, and a 55 bar (the designation is a reduction ratio pressure in bar, not a %)."

Uhhh, no, no, and no. First let's go over what the various brake-proportioning valves are and what they mean:

5/18 bar, 951.355.305.01
5/33 bar, 928.355.305.01
5/45 bar, 964.355.305.01
5/55 bar, 930.355.305.01

First, the "5" part refers to the reduction-ratio of 50%. ALL of these valves have a 50% reduction in brake-pressure going to the rear brakes once their preset-pressure has been reached. The second part "18" refers to the set-point (knee-point) where the valve starts to reduce pressure by 50%.

So on the stock 951 5/18 valve, brake-hydraulic pressure below 18-bar (261psi) goes evenly to both front & rear brakes. At pressures above 18-bar, only 1/2 (0.5) of the pressure-increase goes to the rear. So here's how it looks at various pressures:

0-18 bar: F=18bar / R=18bar
22 bar: F=22bar / R=20bar (front gets +4bar from 18, rear gets half of that +2bar)
26 bar: F=26bar / R=22bar (+8/+4)
30 bar: F=30bar / R=24bar (+12/+6)
34 bar: F=34bar / R=26bar (+16/+8)
38 bar: F=38bar / R=28bar (+20/+10)

So from 18-bar onwards, the rear brake gets only 1/2 of the pressure-increase as the front. Let's say we upgrade to a 5/33 prop-valve, we'd get the following response from our brakes:

0-18 bar: F=18bar / R=18bar
22 bar: F=22bar / R=22bar
26 bar: F=26bar / R=26bar
30 bar: F=30bar / R=30bar
34 bar: F=34bar / R=33.5bar (+1/+0.5)
38 bar: F=38bar / R=35.5bar (+5/+2.5)

So you can see with the 5/33 prop-valve, you have more pressure going to the rear brakes because the point where the valve starts to reduce pressure-increases is later, even though the reduction-ratio is still the same. Here's how it looks if you were to graph the data:



Notice that the slope of the pressure-curve are all parallel once the different valves start their pressure-limiting. That's because they all have the same 50% reduction ratio. Just the preset knee-point where they start the limiting is different.

For the price of the Porsche valves, you might as well get an adjustable one because you'll have a lot finer adjustments of the knee-point (reduction-ratio still the same no matter the adjustment):


BTW- AlanC. does his F/R brake-proportioning by juggling his master-cylinder to brake-caliper cylinder sizes , like that table Don put up. He does not have front-calipers on the rear; you'd need the GrandaddyKingOfBigReds in front with 60/55mm pistons to balance that out...

turbite

Danno, Thanks!

I was hoping you'd jump in on this thread as I remember reading you explain the porportional valve numbers way back.

Chris Moon

OK, I'm maybe taking this thread in a slightly different direction, but I've got two questions.

First, I don't see a brake bias valve on my master cylinder, why? Is it just missing? Any hard lines just connect directly to it. The front pads wear about twice as fast as the rears. I assumed this was normal.

Second, based on the table that Don posted, if I have the wrong master cylinder in my car, would that account for a slightly soft pedal? I've been chasing this for quite a while without any improvement. Here is what I've done to try to get a firmer pedal: flushed and bled the system, replaced front and rear pads with Porterfield street pads (as they wore out), replaced front and rear rotors (as they wore out), replaced master cylinder (ordered from Pelican), replaced flexible lines with DOT stainless lines from Lindsey. The brakes seem to work fine even in the few track events I've done, it's just that I'd like a firmer pedal. Shortly after I got the car, I could bleed the system so that the pedal would be rock hard. For the last few years that hasn't been the case.

Any ideas?

Thanks,
CM

Oddjob

If your car has ABS, the proportioning valve will be on the hydraulic/pump unit inside the right front wheel well (to the rear of the wheel behind the plastic fender tub).

If you think you have the wrong master cylinder, just compare the part numbers of the one you ordered from Pelican to the numbers that are posted above. It would be very unlikely the OEM installed one is incorrect, but may be possible if the original was improperly replaced by a previous owner.

Ive chased soft pedal problems before and it seems to always be a bleeding/air in the line issue. Especially anytime that you are removing and replacing lines and master cylinders. Just make sure you are very thorough with the bleeding afterwards. Ive known guys that will jack up either end of the car when bleeding to help the air bubbles move to the highest point.

turbite

Good article:

http://www.stoptech.com/whitepapers/...erformance.htm

Danno

Good article Dirk, thanks!

Nosing around that sight, brought this article up: MONOBLOC Versus Two Piece Calipers . I guess I won't be putting Boxster calipers on the front o fmy 951 after all... at least not without drilling through the bridge and putting a steel bolt in.... or beryllium...

turbite

Wow, didnt even know that problem existed.

I guess the Boxster (non-S) calipers are monobloc?