brake proportioning valve options for 996TT brake setup

[Shuie]

I have a narrow body '86 with a 996TT brake setup. I read many of the threads about the increased front bias with this setup and how running more aggressive pads on the rear should help to even that out before choosing it.

Car was built as a hotrod for street and weekend DE/track stuff. I have no seat time in it yet so I don't have a feel for how much of an issue the increased front bias really is. I don't want to learn anything the hard way. Is there some kind of adjustable proportioning valve or a fixed valve will integrate into the front circuit easily and make the bias more ideal if running the more aggressive rear pads isn't enough?

TIA

[Uwon]

I installed the adjustable valve in the frunk on the rear brake line. I adjusted by threshold braking in the wet to make sure the rear does not lock up before the front. Same setting for street and track pads (unless experienced enough to trail brake on track).



Hope this helps.
Johan

[chris_seven]

This discussion keeps cropping up and raises several issues.

The brake bias of the car is determined by the master cylinder and caliper piston area ratios.

If the car has a tandem master cylinder with equal area then the line pressures to the front and rear brakes will be identical and the brake balance will be fundamentally determined by the ratio of the pistons areas of the calipers.

Pad material can have a minor influence on balance but in general i think this effect is overstated.

By changing calipers and increasing the front brake bias it is possible that lap times may well increase.

At the point where the front brakes lock the rear brakes will be contributing less effort so the overall braking performance of the car will have reduced.

Pedal effort with increased front bias may be lower when the brakes lock and brakes may feel sharper but they may be less effective.

The way in which 'bias' valves are described is generally misleading and not very accurate.

They don't really adjust pressure in the way we imagine.

These valves typically have a fixed ratio between input and output pressures. The seem to vary between 30 and 45% depending on the individual manufacturer.

The adjuster just changes the pressure at which they operate by effectively introducing a 'knee'.

When fully open they will typically allow 90% of the pressure at the outlet due to flow restriction.

Up to the 'knee' the inlet and the outlet pressure will be linear.

https://tiltonracing.com/wp-content/uploads/2013/07/98-1261-Brake-Proportioning-Valve.pdf

It is conventional to fit these bias valves into the rear brake line and as described above they are used to prevent rear brake lock up.

If you fit them to the front brake line then you will almost certainly increase the pedal force needed to lock the brakes but up to the set point 'knee' the balance of the braking will still be front biased.

I believe that this means that once you pass this set point you will need to increase pedal effort quite significantly.

I don't really believe these devices adjust brake balance effectively and an Adjustable Pedal Box is the only real way.

AP Racing suggest using these valves to control rear braking and generally using them with cars that are overbraked at the rear.

https://www.apracing.com/product_details/race_car/actuation_products/proportioning_valves/screw_type_prop_valve/cp3550-14.aspx

[3rd_gear_Ted]

996 cars have an ABS system, your butt is now the ABS.
Starting to learn the cars braking capabilities from scratch on the track is probably a good thing though. Having tracked and older 911, you will know about the need for some quick feet already to prevent lock up, nothing new, just adapt as needed.

I always get a kick out of the folks who have never driven a non-ABS car before.
(can you say flat spot )

[Shuie]

Okay, thank you all for the help. I need to drive it in a controlled environment first and see if it is actually an issue. This will be the first air cooled 911 I've driven since selling my early cars off 10yrs ago and the first I've ever driven with a brake booster. First 3.6 too so I have a lot to learn. I'll just go slow until I get a feel for it

[HorstP]

An adjustable valve should not be used in the front brakes and putting them to the rear will just make things worse.

I have 996 (non TT) brakes in my 87 and can live with the too much front bias pretty ok.

[Bill Verburg]

Originally Posted by Shuie

I have a narrow body '86 with a 996TT brake setup. I read many of the threads about the increased front bias with this setup and how running more aggressive pads on the rear should help to even that out before choosing it.

Car was built as a hotrod for street and weekend DE/track stuff. I have no seat time in it yet so I don't have a feel for how much of an issue the increased front bias really is. I don't want to learn anything the hard way. Is there some kind of adjustable proportioning valve or a fixed valve will integrate into the front circuit easily and make the bias more ideal if running the more aggressive rear pads isn't enough?

TIA

p/v can only be used to reduce rear line pressure

too much front bias isn't a safety issue, that;'s why most late Porsche exhibit an over abundance of front bias. It's an efficiency issue, you just aren't getting the most from the brakes.

[Shuie]

Okay, good, thanks again for the help & education.

[RagingBull]

FWIW I have an '87 3.2 Carrera with 996TT brakes as well. I installed a Tilton Brake Proportioning Valve (to the rear as it reduces line pressure, as Bill mentioned). However, I run the p/v wide open, so not contributing anything. The fronts will lock-up but only under hard braking at low speed (as expected). No issue at all at high speeds (aerodynamic downforce?).