71porsche

Guys
Rebuilding my brake system including regulators…..
any issues with using later 951 355 305 01 VS 928 355 083 02 original
thanks for advice & comments
regards
frank

worf928

The 951 part is an 18 BAR bias regulator which was OE for 87+. Earlier 928s had higher rear bias.

Without knowing the model year of your 928 it is not possible to opine on the correctness of the 951 part.

A lot of folks with 87+ swap out the 18 BAR regulator for something higher like 33 BAR.

71porsche

Dave
Thanks, I have 79 and would like to improve braking beyond the rebuild/ replacement of components including the regulators.
looking for suggestions for regulators
regards
frank

worf928

You should swap out the regulator for peace of mind since the can leak or get clogged when they get old.

I don’t know off the top of my head the rating of the OE ‘79 unit. I had a ‘79 in the shop last week, but it’s home now so I cannot go run and check.

Your old unit should have the bias stamped in the body. It will be of the form “1” <down arrow> “XX” where XX is the rating in BAR.

Once you know the rating then it will be possible to determine the replacement.

71porsche

Reading various posts…
Early 928 were 33bar
S4 & later 18 bar
930’s 55bar
964’s 45 bar

Per Porsche……930 part# Has been replaced by 964 part number

Seems you can improve by going to 964 version ($209) also less cost than the 928 version ($290)

Interested in comments those who have changed
Thanks
Frank

worf928

The a$$-draggers with their rear weight bias need a lot more rear brake pressure.

The 928 is slightly nose heavy and too much rear brake pressure will cause the rears to lock up before the fronts. And that's bad juju.

A bunch of '87+ owners (myself included) 'upgrade' the 18 BAR regulator to the 33 BAR unit with good results.

I would advise with sticking with a (new) 33 BAR regulator. The 911 parts have too much rear pressure.

I have a 45 BAR regulator on my '89 and it's too much. The '91 and '94 have the 33 BAR units and it's perfect.

I've swapped-in the 33 BAR unit in a lot of client 928s. I won't install a more aggressive unit.

71porsche

Dave
Great summary & experience presented….will stay with the 33
thanks
frank

Bulvot

Before changing the brake proportioning valve, it's best to look at the braking system and driving style holistically. If you don't do any serious performance driving (e.g. on the track), you might get little to no benefit from more braking force in the rear. Plus, the rear/front tires really need to have the right traction differential in order to make use of additional braking force. And, the calipers need to be matched properly for the additional force.

For reference, the numbers on the body of the valve mean:

• The first number is how much additional pressure, in percentage times 10, that the rears receive once the system pressure is above the second number.
• The second number is the brake system pressure, in bars, up to which the front and rear brakes receive the same amount of pressure.

Calculate rear pressure when above the limit:
((Current bar - bar limit) * Addition Factor * .1) + Bar limit

For example, 5|18 would mean that the front and rear receive the same pressure up to 18 bar (261psi). Above that pressure, the rears receive 50% of whatever the fronts receive. If the brake system pressure is 24, the fronts get 24. The rears get 21.
((24 - 18) * 5 * .1) + 18 = 21

5|55 would mean that they are the same up to 55 bar (797psi), and then the rears receive 50%. At 61 bar, the fronts get 61. The rears get 58.
((61 - 55) * 5 * .1) + 55 = 58

This is all based on brake fluid pressure. So, the size and number of caliper pistons, and the size of the pads and rotor diameter, are all critical factors. As is the contact patch, or maximum traction, for each tire. Even the weight bias, alignment and suspension are factors.

The reason why you may want less pressure in the rear sooner, is so that the fronts are more likely to lock up sooner and produce understeer instead of oversteer. If you have to choose just one of those (understeer or oversteer), the average driver will do better with understeer. They can just ride it out straight, or they can let off of the brakes and try again. With oversteer, a lot more finesse and skill is required to maintain, or regain, control of the car. Understeer was intentionally engineered into the 928's, and most other cars', braking system. Supposedly. I'm no expert on automotive engineering, so I'm just repeating what I've read about understeering being preferred in cars. The logic makes sense, though. Most people are not comfortable with correcting braking induced oversteer at high speeds and in tight corners.

Anyway, with all of that in mind, consider the following setups for my personal car:

Pirelli P-Zero 225 front and 275 rear tires. 5|55 brake proportioning valve. Upgraded "big red" front calipers with 322mm rotors. Upgraded 993 Turbo rear calipers with 322mm rotors. Standard semi-metallic brake pads. Alignment was a standard "middle of all settings" setup, no corner balancing done. When racing, I still had serious understeer under extreme braking. It required a lot more braking finesse to get around the course quickly. Once I was comfortable with that, I was able to run the courses well, but it always felt like a compromise. The difference in front vs. rear traction was too great, even with proper trail braking.

Yokohama A052 255 front and 295 rear tires. 5|55 brake proportioning valve. Upgraded "big red" front calipers with 322mm rotors. Upgraded 993 Turbo rear calipers with 322mm rotors. Hawk HPS 5.0 brake pads. Alignment is adjusted specifically for my driving style and intended use and tires/wheels, and it has been corner balanced. When racing, I have yet to experience understeer or any lock up of the fronts. The car is very neutral and easy to manage. The braking is extremely strong, and easy to modulate, so I rarely have the ABS activate. This has turned out to be an ideal setup for me and I'm still learning the limits with various surfaces and conditions.

My personal recommendation is to stick with the stock proportioning valve for your car, unless you're doing serious performance driving. Even then, if it's all on the street, I wouldn't recommend more than 5|33. If you're getting on the track, then get what works best for your brake/tire setup.

Just be aware that more is not always better. And changing the brake proportioning valve shouldn't be done in isolation from consideration of the rest of the whole picture (brake calipers, brake pads, tire type/size, alignment, suspension, driving style, intended use, etc).

Good luck and let us know what you decide to do and how it works out for you.

71porsche

Bulvot
thanks for such a comprehensive overview & recommendations.
I am now committed to maintaining the original 33 bar
regards
frank

IcemanG17

I ran the early regulator in all of my 928 race cars with good results. Never tried on a street car

Bulvot

Came across the attached in the WSM today. Thought it might provide some useful data on the proportioning valves used in the 928's.