Why....?, Brake pressure regulator Options

[Speedo]

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)?

[GregAmy]

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.

[brant]

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

[Superhawk996]

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.

(IMG:style_emoticons/default/agree.gif)

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 (IMG:style_emoticons/default/happy11.gif) ). 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.

[Superhawk996]

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.

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.



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

[Chris914n6]

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

[Speedo]

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

[Superhawk996]

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.

[bbrock]

ABS ahead of its time (IMG:style_emoticons/default/naughty.gif)

[windforfun]

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.

[Rob-O]

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

[bbrock]

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

(IMG:style_emoticons/default/av-943.gif) I just said that to get under @Superhawk996 's skin. (IMG:style_emoticons/default/happy11.gif) But I still say that regulator is intended to prevent the rear brakes from locking... therefore... anti-lock brakes (IMG:style_emoticons/default/lol-2.gif)

[Superhawk996]

(IMG:style_emoticons/default/av-943.gif) I just said that to get under @Superhawk996 's skin. (IMG:style_emoticons/default/happy11.gif)

It was working beyond your wildest dreams. (IMG:style_emoticons/default/slap.gif) (IMG:style_emoticons/default/laugh.gif)

[PanelBilly]

Just another reason to love these cars.

[kanata914]

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?

[GregAmy]

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.

[bbrock]

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.

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 (IMG:style_emoticons/default/biggrin.gif)

[Superhawk996]

Not cool like a 914 with ABS, but still neat (IMG:style_emoticons/default/biggrin.gif)

(IMG:style_emoticons/default/whip[1].gif) (IMG:style_emoticons/default/laugh.gif)

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.

[914e]

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.

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.



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

@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.

[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.