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3.2 p / v at 55 bars on powering the brakes

Hi

From what I find then most of the 3.2 didnt come with bias brake valves for the rear circuit with the exception of 80 to 85 years ROW and in those years there could be two types for different pressures To

Because I have a car 84 491 TL with braking system 3.2. It has been said it has an exotic brake bias ratio valve where it cuts down the pressure to the rear brakes even earlier than has positive 3.2 p / v at 55 bars on powering the brakes. front. My car is lighted up about 2500lbs with quite hard and shock absorbing tb. I felt it locked the brakes very early.

Who knows the # portions of the valve p / v and knee bar specifications for the four types known to be used on a G car?

Thank you
Old 09-06-2018, 02:18 AM
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Quote:
Originally Posted by linhnv231 View Post
Hi
Welcome!

Quote:
Originally Posted by linhnv231 View Post
From what I find then most of the 3.2 didnt come with bias brake valves for the rear circuit with the exception of 80 to 85 years ROW and in those years there could be two types for different pressures To
The 3.0 SC's didn't have P/Vs at all. The SC, and all 911's before it, had a natural front/rear bias relationship enforced by the two different master cylinders built into the combined unit, together with the relationship of the piston sizes in the front/rear calipers.

For the 3.2's, the diameter of the rear brake pistons increased disproportionately. Apparently so that the front/rear pads wore at a more even rate, due to customer complaints. This was the yuppy era... In order that the rear wheels did not lock prematurely with the over-sized rear pistons - e.g. in poor traction conditions with soggy factory suspensions, sways and stock tire sizes, with, shall we say, non-enthusiast drivers? - a P/V was used to limit rear pressure in the rear circuit..

After the 3.2 was the 964. That car had a usable front/rear bias as-was - but it, and all the cars that came after it, had anti-lock brakes. The later cars are much more front-biased than early cars; because the ABS takes care of it. The turbo variants much more so.

There are many, many threads on this topic in the 911 section; look for information from Bill Verburg; he's the brake guru, upgrade or otherwise.

Quote:
Originally Posted by linhnv231 View Post
Because I have a car 84 491 TL with braking system 3.2.
If you have an option 491 aka the "Turbo Look", then you would have 930 calipers, rotors, master cylinder, suspension pickups for anti-squat/dive, banana arms etc - essentially a turbo car, except fitted with an N/A engine by the factory.

If you have a 3.2 with wide wheel arches, then you have a wide-body 3.2. This would (probably) have 3.2 brakes, and a 3.2 proportioning valve. Although, frankly, by now anything could have been fitted, I'm just saying what is most likely....

That would be a 911 with wide arches; very different from an option 491 car (which didn't have a P/V and shouldn't have one, because 930 brakes are close to ideally balanced front/rear without one).

It would be important for you to know which of the two you have. Do you have 3.2 caliper, or 930 calipers? How thick are your rotors (this is a good clue). Is your P/V factory fitment?

For almost all things related to a 3.2, you'd get many more responses in the 911 forum; even a 491 Turbo Look is an N/A 3.2 - and there's probably 10000 x as many people reading the 911 forum as this one...

Quote:
Originally Posted by linhnv231 View Post
It has been said it has an exotic brake bias ratio valve where it cuts down the pressure to the rear brakes even earlier than has positive 3.2 p / v at 55 bars on powering the brakes. front. My car is lighted up about 2500lbs with quite hard and shock absorbing tb. I felt it locked the brakes very early.
The factory defaulted, for stock cars run on the street, to front-bias. Because, while less efficient, this results in less law suits from unskilled or inexperienced drivers locking the rears and spinning.

They always used (much) more rear-bias on the race cars. But those weren't bought from a showroom and driven on public roads by anyone with money.

Stiffer TBs, sways, lowering, wider/stickier rear tires and an asymmetric plate-type LSD all increase the amount of rear-bias the car can tolerate safely.

For safety, you should stop short of the rears locking up first under any conditions - the factory bias limit for the RSRs - adjustable, of course, via a dial on the dash - was 1.26, IIRC. Of course, the skill level of the drivers is also a factor there - and those cars are pretty different to street cars...

Which end do the brakes lock up? At the front? So you're complaining that it seems excessively front-biased with your suspension mods?

I would expect that's entirely correct.

You still always want the fronts to lock up before the rear (because if the rears lock, it'll swap ends), but if the car is excessively front-biased, then you're leaving a lot of braking efficiency on the table. Because you can't brake any harder once any wheel locks.

Several ways you can go; adjustable P/V. Remove the P/V entirely. Or go to a dual master cylinder system with a balance bar - which allows you to tune front/rear bias.

There is the RSR system (have to cut holes in the cabin), the SC/RS system (all fits in the cabin) - have to lose the brake booster fro both of these) or something aftermarket like the Fabspeed; believe they do a boosted system too. Doesn't look very factory though...

This thread Turbo Brake Upgrade has discussion of brake bias.
And this one Carrera brake rotor advice, please...

And this post discussing bias specifically Brake Caliper ID Help

Here's some SC/RS pedal boxes to look at SCRS pedal cluster in a 67

Quote:
Originally Posted by linhnv231 View Post
Who knows the # portions of the valve p / v and knee bar specifications for the four types known to be used on a G car?

Thank you
I looked at this once. 55 bar sounds in the ballpark for the factory P/V - thought there was only a couple of types, but it was years ago. I won't guess.

Try an adjustable proportioning valve; they're cheap. StopTech sells good equipment. If you're complaining about what I think you are, you probably want to increase the threshold knee pressure on the P/V so it takes more pressure to activate the P/V on the rear circuit, then reducing the pressure to the rear circuit less for any given input.

Here's a detailed discussion of proportioning valves - including why they never go on the front... Brake bias adjustment
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Old 09-17-2018, 06:39 PM
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With a street car do your brake bias setting adjustments on wet roads and leave it. The rears lock up way earlier when wet so you have to leave a safe margin there.
Old 09-18-2018, 06:51 AM
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Quote:
Originally Posted by spuggy View Post
Welcome!

If you have an option 491 aka the "Turbo Look", then you would have 930 calipers, rotors, master cylinder, suspension pickups for anti-squat/dive, banana arms etc - essentially a turbo car, except fitted with an N/A engine by the factory.

If you have a 3.2 with wide wheel arches, then you have a wide-body 3.2. This would (probably) have 3.2 brakes, and a 3.2 proportioning valve. Although, frankly, by now anything could have been fitted, I'm just saying what is most likely....

That would be a 911 with wide arches; very different from an option 491 car (which didn't have a P/V and shouldn't have one, because 930 brakes are close to ideally balanced front/rear without one).

It would be important for you to know which of the two you have. Do you have 3.2 caliper, or 930 calipers? How thick are your rotors (this is a good clue). Is your P/V factory fitment?

For almost all things related to a 3.2, you'd get many more responses in the 911 forum; even a 491 Turbo Look is an N/A 3.2 - and there's probably 10000 x as many people reading the 911 forum as this one...



The factory defaulted, for stock cars run on the street, to front-bias. Because, while less efficient, this results in less law suits from unskilled or inexperienced drivers locking the rears and spinning.

They always used (much) more rear-bias on the race cars. But those weren't bought from a showroom and driven on public roads by anyone with money.

Stiffer TBs, sways, lowering, wider/stickier rear tires and an asymmetric plate-type LSD all increase the amount of rear-bias the car can tolerate safely.

For safety, you should stop short of the rears locking up first under any conditions - the factory bias limit for the RSRs - adjustable, of course, via a dial on the dash - was 1.26, IIRC. Of course, the skill level of the drivers is also a factor there - and those cars are pretty different to street cars...

Which end do the brakes lock up? At the front? So you're complaining that it seems excessively front-biased with your suspension mods?

I would expect that's entirely correct.

You still always want the fronts to lock up before the rear (because if the rears lock, it'll swap ends), but if the car is excessively front-biased, then you're leaving a lot of braking efficiency on the table. Because you can't brake any harder once any wheel locks.

Several ways you can go; adjustable P/V. Remove the P/V entirely. Or go to a dual master cylinder system with a balance bar - which allows you to tune front/rear bias.

There is the RSR system (have to cut holes in the cabin), the SC/RS system (all fits in the cabin) - have to lose the brake booster fro both of these) or something aftermarket like the Fabspeed; believe they do a boosted system too. Doesn't look very factory though...

This thread Turbo Brake Upgrade has discussion of brake bias.
And this one Carrera brake rotor advice, please...

And this post discussing bias specifically Brake Caliper ID Help

Here's some SC/RS pedal boxes to look at SCRS pedal cluster in a 67



I looked at this once. 55 bar sounds in the ballpark for the factory P/V - thought there was only a couple of types, but it was years ago. I won't guess.

Try an adjustable proportioning valve; they're cheap. StopTech sells good equipment. If you're complaining about what I think you are, you probably want to increase the threshold knee pressure on the P/V so it takes more pressure to activate the P/V on the rear circuit, then reducing the pressure to the rear circuit less for any given input.

Here's a detailed discussion of proportioning valves - including why they never go on the front... Brake bias adjustment
i had a 930 and now a 491. the 491 comes with a factory pv valve.


the TL dosent come with anti-dive like the SCRS. the factory themselves are somewhat to blame with their press releases for the misunderstanding implying? it comes with all the turbo hardware but the chassis tub where the crossmember mounts is missing the divots for a higher rear location is where it differs.

I have a dual MC, balance bar set up with an additional cockpit adjustable rear line bias valve on my racecar. its much more than i would like to add to a street driven car.

thanks for the wonderful links.


Quote:
Originally Posted by linhnv231 View Post
Hi

From what I find then most of the 3.2 didnt come with bias brake valves for the rear circuit with the exception of 80 to 85 years ROW and in those years there could be two types for different pressures To

Because I have a car 84 491 TL with braking system 3.2. It has been said it has an exotic brake bias ratio valve where it cuts down the pressure to the rear brakes even earlier than has positive 3.2 p / v at 55 bars on powering the brakes. front. My car is lighted up about 2500lbs with quite hard and shock absorbing tb. I felt it locked the brakes very early.

Who knows the # portions of the valve p / v and knee bar specifications for the four types known to be used on a G car?

Thank you
i believe all usa 3.2 spec came with a low pressure p/v valve from what ive gathered.

have you check the numbers on the p/v valve? there should be a stamp number 5/? on the pressure threshold setting. i think its 5/33 which is low. i cross reference it to a 928 part#. i cant see my #markings because there's a foam pad around it.

my car m491 is about 2550lbs stiff suspension as well. it locks up more easily with the stiffer front end and 1.5 or 2 deg camber.

i think i may remove my pv valve with a hard line but would like to progressive relieve the rear circuit first with a 55 or 65 before making such a drastic jump? last thing u want is to lock up the rears first. the higher pressure pv valves are $250 or more these days.... a crazy rip off since they share the same housing as other pv that sell for $70 with only a spring change internally.

ive been told that most 930s dont come with a pv valve except some of the middie ROW cars around 80-85ish. keep in mind despite whats been said, M491 are closer to a SCRS chassis than a 930 since they dont have the front anti dive geometry. this coupled with an additional 50-65lbs drivetrain that is also moved rearward with the longer 3.3 bellhousing, enables the 930 to utilize the rear brakes more.

let me know if you do remove it and find a shop that can do a double bubble euro type a short brake line.... perhaps we can get two or more made for those who would be interested?






60 bar.......C2 Turbo (965 355 305 01)

55bar........993tt (964 355 305 10)X2 units?

55bar........C4, RSA (964 355 305 10)

45bar........C2 (964 355 305 00)

33bar........Turbo Look cars (928 355 305 02)


regards pf
Old 09-19-2018, 07:03 AM
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Quote:
Originally Posted by panzerfaust View Post
the TL dosent come with anti-dive like the SCRS. the factory themselves are somewhat to blame with their press releases for the misunderstanding implying? it comes with all the turbo hardware but the chassis tub where the crossmember mounts is missing the divots for a higher rear location is where it differs.
Hey, I stand (well, sit) corrected - thanks panzerfaust.

Quote:
I have a dual MC, balance bar set up with an additional cockpit adjustable rear line bias valve on my racecar. its much more than i would like to add to a street driven car.

thanks for the wonderful links.
I've gazed longingly at the SC/RS pedalbox many times...
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'77 S with '78 930 power and a few other things.
Old Yesterday, 09:27 AM
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