3.2 has a pressure limiting valve for the rear brakes?

[GaryR]

In my ongoing SC to Carrera conversion this has me purplexed. I'm told I have to install a limiting valve as it is on the 84-89 cars (not on SC's), is this the item?



Anyone have a picture of the booster area with this valve installed? I assume it's on the MC in fron of the rear brake line.. It's part #92835530502

[OffCamber00]

Here you go.

[chrisbruck]

If I remember correctly it's only needed if you are using the Carrera REAR calipers. They have larger pistons than the SC's.

Bill Verburg is the brake expert with all the answers.... he helped me immensely in understanding my brake system and changes.

[GaryR]

Originally Posted by chrisbruck

If I remember correctly it's only needed if you are using the Carrera REAR calipers. They have larger pistons than the SC's.

Bill Verburg is the brake expert with all the answers.... he helped me immensely in understanding my brake system and changes.

I will have Carrera brakes all around, have to try and find that in PET, it isn't what I found.. thanks for the pic, anyone have a close up?

[chrisbruck]

This is what I ordered from our host to replace mine that was removed by some PO.

928-355-305-02-M100

For my stock Carrera.

[GH85Carrera]

My understanding is the stock Carrera brake system has no limiting valves at all. Many cars have a proportioning valve that can be swapped out or adjusted. The stock 1980s Carrera does not. I have thought about adding a little more pressure to my rear brakes but I may just adjust the brake bias with different pads front and rear.

[GH85Carrera]

Originally Posted by chrisbruck

This is what I ordered from our host to replace mine that was removed by some PO.

928-355-305-02-M100

For my stock Carrera.

OK, I see that part and now I wonder if that is what I need for more brakes to the rear.

[Bill Verburg]

Originally Posted by GaryR

I will have Carrera brakes all around, have to try and find that in PET, it isn't what I found.. thanks for the pic, anyone have a close up?

3.2 Carrera come oe w/ a 33bar p/v as pictured above
the reason is that the Carrera was equipped w/ larger rear pistons than SC, due to the demands of all the doctors, lawyers and indian chiefs that felt that the rear pads should wear out at the same intervals as the fronts

brake torque SC @70bar 1444/969NM, bias ratio 1.491
brake torque 3.2 @70bar 1444/1183NM, bias ratio 1.220 w/ the 33bar p/v it goes to 1444/639NM, bias ratio 2.26

is the p/v required by rules?

[uwanna]

I have a spare 928 355 305 02 M100 if you need one. List is $129, you can have mine for $75 shipped. Looks perfect. PM me if interested.
UPDATE: I also have the two OE hydraulic lines that hook it up $20 extra. total $95 shipped

[Bill Verburg]

Originally Posted by GH85Carrera

OK, I see that part and now I wonder if that is what I need for more brakes to the rear.

no, a p/v can only reduce line pressure to the rear and only above a threshhold pressure called the knee

here is a summary of most of the p/v used on the air cooled cars
p/v info
951.355.305.01 .5/18 '86 944t
964.355.305.00 .46/45 '89-91 964 C2
964.355.305.10 .46/55 '92-94 964 C2, all C4, all 964RS, all 993tt
965.355.305.01 .46/60 all 964t
993.355.305.01 .46/40 all 993 C2, all 993RS
993.355.305.00 .46/45 all 993 C4
928.355.305.02 5/33

most of us are happiest w/ bias ratios in the 1.425 to 1.5 range, the lower you can comfortably use the better for track use as it gets the rear working and rlieves the thermal loads for the front.

to get lower ratios comfortably you want a lower, stiffer car w/ an effective lsd, an asymmetric that is more effective on the decel side is best

[GaryR]

Bill - so the short answer is I have the correct part, yes? The P/V is part of the conversion to Carrera so yes, it is part of the rules. Without it I believe I will have too much rear bias anyway, correct?

Thanks
Gary

Originally Posted by Bill Verburg

no, a p/v can only reduce line pressure to the rear and only above a threshhold pressure called the knee

here is a summary of most of the p/v used on the air cooled cars
p/v info
951.355.305.01 .5/18 '86 944t
964.355.305.00 .46/45 '89-91 964 C2
964.355.305.10 .46/55 '92-94 964 C2, all C4, all 964RS, all 993tt
965.355.305.01 .46/60 all 964t
993.355.305.01 .46/40 all 993 C2, all 993RS
993.355.305.00 .46/45 all 993 C4
928.355.305.02 5/33

most of us are happiest w/ bias ratios in the 1.425 to 1.5 range, the lower you can comfortably use the better for track use as it gets the rear working and rlieves the thermal loads for the front.

to get lower ratios comfortably you want a lower, stiffer car w/ an effective lsd, an asymmetric that is more effective on the decel side is best

[Bill Verburg]

Originally Posted by GaryR

Bill - so the short answer is I have the correct part, yes?

Thanks
Gary

yes, 928.355.305.02

[GaryR]

Thanks Bill, and everyone else!

[Elombard]

Gary, just to be sure we are all on the same page removing the valve can be a bias improvement for tracking (If I am reading this correctly), i.e. more rear braking above the knee. I guess you need the pv for the rules?

[GaryR]

Originally Posted by Elombard

Gary, just to be sure we are all on the same page removing the valve can be a bias improvement for tracking (If I am reading this correctly), i.e. more rear braking above the knee. I guess you need the pv for the rules?

That is correct, if it came on a Carrera it has to be on my car.

[uwanna]

Gary,
Got your PM and will respond with Paypal addy. Here's a pic from the '86 PET showing
the parts you will get. Numbers 10,11,12,13. The Pet pic is a little confusing, but I think
when you have the parts in hand and look at the routing of the hydraulic lines in your car,
it will make sense.
Grant

[GaryR]

Perfect, thank you Grant.

[javadog]

Originally Posted by GH85Carrera

OK, I see that part and now I wonder if that is what I need for more brakes to the rear.

Originally Posted by Elombard

Gary, just to be sure we are all on the same page removing the valve can be a bias improvement for tracking (If I am reading this correctly), i.e. more rear braking above the knee. I guess you need the pv for the rules?

Porsche changed the piston sizes in the Carerra rear brakes to make the rear brakes do more of the braking work at lower deceleration rates, eg. around town and other normal driving. It also helps on low coefficient of friction surfaces. The bias is too rearward for maximum braking, so they added this valve to change the slope of the rear brake pressure curve above a certain value. It gives reduced rear braking above a certain value.

JR

[Flat Six]

Originally Posted by Bill Verburg

no, a p/v can only reduce line pressure to the rear and only above a threshhold pressure called the knee

here is a summary of most of the p/v used on the air cooled cars
p/v info
951.355.305.01 .5/18 '86 944t
964.355.305.00 .46/45 '89-91 964 C2
964.355.305.10 .46/55 '92-94 964 C2, all C4, all 964RS, all 993tt
965.355.305.01 .46/60 all 964t
993.355.305.01 .46/40 all 993 C2, all 993RS
993.355.305.00 .46/45 all 993 C4
928.355.305.02 5/33

most of us are happiest w/ bias ratios in the 1.425 to 1.5 range, the lower you can comfortably use the better for track use as it gets the rear working and rlieves the thermal loads for the front.

to get lower ratios comfortably you want a lower, stiffer car w/ an effective lsd, an asymmetric that is more effective on the decel side is best

Thanks for posting this, Bill -- haven't seen it before.

Sorry to hijack, but the ratios you list next to the part numbers (e.g., .46/45) -- do they refer to the rate of rear brake pressure reduction? For example, does .5/18 ('86 944T) mean that this particular p/v reduces pressure to the rear brakes by 50% (.5) starting at 18 bar?

Thanks in advance for any insight, and as always I appreciate the wealth of information I find here.

[Bill Verburg]

Originally Posted by Flat Six

Thanks for posting this, Bill -- haven't seen it before.

Sorry to hijack, but the ratios you list next to the part numbers (e.g., .46/45) -- do they refer to the rate of rear brake pressure reduction? For example, does .5/18 ('86 944T) mean that this particular p/v reduces pressure to the rear brakes by 50% (.5) starting at 18 bar?

Thanks in advance for any insight, and as always I appreciate the wealth of information I find here.

Yes, the first # is the reduction factor the second is the knee
928.355.305.02 5/33
should read
928.355.305.02 .5/33 '84-89 3.2 Carrera
the effect can be seen here