I took apart my brake pressure regulator to clean and restore it before re-installing on my 6. Got me wondering why the 914s have these and the 911s don't? I assume (probably incorrectly) that based on the design, the regulator attempts to keep pressure in the rear half of the system. Kind of like the water pressure tank in my basement. It keeps water pressure in the system at all times without demanding the well pump become operational.
Just wonder why the 911s don't incorporate these? Maybe it is because the 914s are a whopping 2 inches longer and a device is required so that the rear pressure doesn't drop as a result of being so far from the reservoir? Or is it because of the design of the dual purpose 914 rear brakes (ebrakes and disc)?

The brake valve on the firewall is a "knee valve" that changes the pressure rise in the rear brake system. Its purpose is to reduce the ratio of rear-to-front brakes at high pedal force.

Basically, it's to keep the rear wheels from locking up.

Why doesn't the 911 have one? Purely a guess, but given the higher rear weight bias it probably doesn't need one, you're more likely to lock up the fronts rather than the rears.

agreed....

the rear lump of weight in a 911 creates additional weight-bias-traction

where as the closer to 50/50 weight of a 914 causes the rear to lock and the car to exit the road 180 degree's from forward.... so the brake valve reduces rear brake pressure in the interest of safety on slick or dirty roads

Correct. Weight transfer off the rear axle under breaking is greatly reduced with rear engine.

There are still instances where 911's suffer from rear wheel lock which in part contibutes to their notorious handling (both good and bad depending on how you drive ). Check various forums and you'll note people putting proportioning valves into early 911's trying to address rear wheel lock prior to front wheel lock.

Nope. Not at all like that. As noted, it limits pressure to rear in proportion to the front pressure.

Here is the curve that shows how it works.

Click to view attachment

Keep the proportioning valve, it's there for a reason.

911s have one, it's just normal size and not adjustable.

Well there you have it! Thanks guys. It was pretty nasty. A quick resto was in order.

Sounds like you've already done the basic cleaning.

I can't recommend PMB's restoration service strongly enough. Cleaned, replated hardware, tested, and knee point set properly to your application.

Mine is currently set for /4 but when I get to /6 conversion, it will go back to be set to the /6 knee point.

ABS ahead of its time

Consider the sticker price of a "73 1.7.....

Then, consider this "new" technology.

First with:
unibody
fully independent suspension
sway bars
air cooled mid-engine
fuel injection
anti-lock brakes

And consider the competition:
none of the above?
no built-in roll bar & targa top
no 15" Italian alloys
fixed rear axles
water cooled
BUT, MORE HORSE POWER!!!

Please chime in with comments &/or corrections.

Not anti-lock brakes. It just ensured that the fronts locked up before the rears.

I just said that to get under @Superhawk996 's skin. But I still say that regulator is intended to prevent the rear brakes from locking... therefore... anti-lock brakes

It was working beyond your wildest dreams.

Just another reason to love these cars.

On my 72 914 1.7, I have the 320i front calipers with a 911 19mm master cylinder. My front pads are the Hawk composite street type. My rear calipers are original with Mintex pads (the ones from Pelican). Current tires are Goodyear Eagle Sport 195-65VR15. This is probably a pretty standard restomod setup, I imagine. I replaced the brake balance valve with a simple JWest tee. I've never experienced rear wheel lockup, more likely front instead, even on our amazingly beat-up frost damaged pavement. But, if I were still using the stock setup, I would stay with the original brake balance valve for safety. Any other comments or experience on the 320i setup?

These types of valves are very common in pre-ABS cars. For example, I remember my Chrysler mini-van had one mounted on the rear trailing arm, with a spring attached to the chassis; loading more weight in the back would reduce the spring's tension, thus increasing the rear brake pressure ratio.

My 80's Dodge cars had them too (we'd diddle with them in the Showroom Stock racers to get more rear brakes and to help rotate the cars).

Porsche may have been a bit ahead of the curve though. Anyone old enough to remember the late-70s/early-80s Chevy Citation X-Car rear brake lockup? GM replaced the prop valve with less-aggressive ones but still never got it right; it cost them a ton of money.

Pretty much after that rear brakes on cars became significantly less effective as manufacturers (and their lawyers) worked to avoid the same engineering mistakes.

BTW, you can add a longer bolt to the stock 914 valve to increase the rear pressure ratio. I've done it.

I have a 93 Nissan Pickup with the same type of mechanism. Always thought that was pretty neat. Not cool like a 914 with ABS, but still neat

Someday in the far distant future, when all my other projects like the chassis rustoration, /6 conversion, steel flare and repaint, are all done, I'd like to actually do a real ABS system for a 914.

I've been mulling over what it would take. It's doable but lots of obstacles like where to put wheel speed sensors and having to accept the calibration of whatever base ABS module you start with. I've done it before on a race car as an experiment but it had warts that I'm not sure I'd accept in a street car.

Hoping someone else beats me to it.

@Superhawk996
Am I reading the graph correctly a six knees at the lower pressure than a 4? IT seems like it should be the other way around. Since my car will have the same weight as a six I think I should use that setting.

@914e

/6 had 38mm rear caliper pistons on a rotor that was roughtly same OD as the /4 (actually 4mm bigger OD). /4 piston size is 33mm.

/6 therefore makes more brake torque per unit of line pressure input. Therefore the knee point is set lower for the /6.

Yes the engine is a bit heavier on a /6 and that would potentially allow a little more rear brake bias than a /4 but I'm sure it is offset by the larger piston diameter which develops 33% more brake torque (assuming equivalent pad friction).

The main advantage of the larger rear brake is earlier in the stop (toward initial pedal apply). Before weight transfer has fully occurred, the heavier /6 engine, and the 33% more effective rear brakes net you better stopping power via the bigger rear brakes. By the time weight transfer has begun, now you need to limit the more effective rear brakes to even more than a /4 to prevent rear caliper lockup.

If you are at /6 weight but running /4 brakes, I'd stay with the /4 set point.

Having a little more weight on the rear with the /4 brakes and the /4 set point is the more "fail-safe" combination that is less likely to lock up but still sends enough pressure to the rear to not lose rear brake effectiveness like you would with /4 brakes and the /6 set point.

Having said all that. I have no idea where your batteries are, Cg location, etc. What you don't want is a high Cg and/or batteries in the front. If this is the case, you probably would want to think about using the /6 set point. More than anything, those sort of major weight bias and Cg changes would ideally want some real design work done to figure out what the right brake set up is.

I will say this....

I have Boxster brakes on my car. I took the proportioning valve off and replaced it with a T fitting. The Boxster brakes are already balanced for a mid engine car, and that valve is just unnecessary with the Boxster brakes.


Clay

I highly recommend getting 2 friends in a parking lot... one on each side of the car.
drive through at a reasonable speed and lock up your brakes...
have your friends determine if the front or the rear are locking first in a full on brake situation....

this is a good first step to tuning a proportioning valve, and a very good test for a T fitting too.

There is no longer a separate proportioning valve in modern cars. The functionality is now handled by an algorithm called Electronic Brakeforce Distribution (EBD) within the ABS control module that montitors wheel speed and and has a proportioning function built into the controls to limit rear brake pressures even in the event of wheel speed sensor failures.

Boxter brake system would not pass FMVSS regulations without this EBD functionality or a proportioning valve . . which you've removed. To assume that the Boxster brake hardware alone as-sized for a Boxter is sufficient to ensure balance and prevent rear wheel lock is a false premise.

It's nice that your happy, but, I can assure you that there are braking scenarios that can put you and others at risk of having a rear wheel lock event. OEM's don't give away parts that aren't needed.

Please don't take this as a personal attack. It's not. It's just that the advice to replace the proportioning valve with a T is not based in solid autmotive engineering and brake system design principles.

Just want others to have the full story of what has been removed from the proposed system.

Note: Early Boxter was a bit more convoluted with both a proportioning valve and ABS module.

. . . but that is only a good start.


Don't forget to do that on split mu surfaces so bring a water truck.

And do it with various load conditions like front trunk fully loaded w/o passenger and still having vehicle at GVW. Front trunk fully loaded increases brake capacity on front axle and simultaneously reduces load on the rear tires making rear wheel lock more likely.

Please include some study of brake pad temps (cold vs. hot)

Let's not forget various speeds up to and including highway speed. Please do on race track or other closed course.

And then you start to get some sense of all the scenarios went into determination of what the appropriate bias point is for a produciton car.

If anyone wants to look at the full test sequence for foundation brakes and proprotioning valves. It is contined within FMVSS 135:
https://www.nhtsa.gov/sites/nhtsa.gov/files...-135-01_tag.pdf

Since 2012 ESC has beome mandated equipment and brings along it's own set of test procedures under FMVSS 126. FMVSS 126 contains pass/fail criteria to categorize brake controls systems performance under more dynamic and more challenging conditions than FMVSS 135.

Here is the link to FMVSS 126.
https://www.nhtsa.gov/sites/nhtsa.gov/files...-126-02_tag.pdf

I agree.. its just a start.

I tune my proportioning valve on the race car each and every weekend.

depending on the temps... the weather.. for sure if its damp... I fine tune my valve on the fly during a race. it has to be close to work like this... and fine tuning only

and in this car I know my fuel weight, no cargo.. and very few variables. yet still have to fine tune the car to minimize lock up and get the best ratio...

Who said Clay was happy? I delibertly avoided a post after that.

The stock 914 brakes and that valve were purposefully engineered for a reason.

This has been semantically argued before here.

The Boxster brakes are undersized for my 4.0L engine. I have a set of 991 calipers to go on the front. That will change the brake bias. I also have an aftermarked manual proportioning valve to go inline on the rear. Because now I am mixing brake sizes, and it will need to be adjusted.

Clay

@superhawk996
Thanks, I was not aware of difference in piston size now the pressure level make more sense.

The rear batteries will be on both sides of the motor aout where the cylinders would normally be, sitting about 1/4" above stock 4 engine crossmember. If switch to a straight shift linkage and fabricate a new cross bar, I might be able to mount them an inch lower. Each pack enclosed will be around 116 pounds, a third pack will mount across the top of the other two.

Where it gets challenging, is introducing braking regen into the mix. I think setting to the 6 pressure and adding back enough regen to bring it back to 4 level might just work out. I should buy a pressure gauge.

@914e

IHMO, you can skip the line pressure gauge.

1) Adding line pressure sensors and especially a mechanical gauge screws with the brake pedal feel.

2) It won't help you much unless you were to use it to do a mapping of brake line pressure vs. tractive effort. To get the tractive effort would require wheel force transducers. Wheel force transcucers cost upwards of $100K and then you still need a data acquistion system to feed the wheel sensor inputs into. With these transducers, you could also do a mapping of tractive effort vs. regen demand to understand where it makes the most sense to put the regen to hydraulic brake transition. Cool but not really an option

Here's link to wheel force transducers if anyone is curious of what they are, what they look like, or what they do.
https://www.michsci.com/products/transducer...ce-transducers/

Tuning EV's and/or hybrids to get a smooth transitions between regen and mechanical braking has been a historical sore spot for the industry. It's gotten a lot better in the last decade but many of the early hybrids and EV's were really bad with a noticeable transition. And that was with full control over the regen AND the ability to tune the ABS module for that handoff.

Unless you have access to better equipment and instrumentation than I think you probably have, it's going to come down to seat of the pants tuning. The good news is that the transition problem was most noticable on at low speed (like 5-10 mph).

Unless you're trying to milk every last mile out of regen (you might be given your range), you could use regen harder initially (early in stop) and then just back it out completely at low speeds. Alternately, tie your regen to deceleration via accelerometer input and don't regen if you are doing a panic stop above some threshold (say 0.6G).

Either way, those are only brainstorming ideas to address a really tough problem that still isn't perfect in production vehicles.

Since you're in AZ, snow creating really low mu conditions isn't so much of an issue. What ever you settle on in the dry, I'd try in the wet just to make sure that regen isn't creating instability when road coefficient of friction is reduced.

You've got your work cut out for you but it is a cool engineering challenge!

-- I'll have to own that.

I won't argue with you.