OZ951

I have exchanged a couple of emails with an engineer at Tilton regarding the use of one of their lever adjustable brake bias valves in my 996, my concern was related to possible side effects of plumbing an M10-3/16ths valve into the M12-6mm brake line of the 996.

He was of the view that plumbing the M10 valve into the M12 circuit was OK, but the surprise was that he commented that their valves are not recommended for use in ABS equipped cars due to high pressure pulsations from the ABS possibly damaging the seals in the Tilton valve.

I am guessing that there must be a few race/track cars out there with ABS and a Tilton bias valve that are working OK ? If so please let us know.

Just for completeness I will add that I discovered the hard way that the older OEM bias valves I am more familiar with (5/33 5/18) are M10 valves but the 996 5/55 valve has M12x1 input/output ports.

Veloce Raptor

I don't know about Porsches, but I have driven a full-race Corvette Z06 with ABS that has fully adjustable bias. Not sure whether it was a Tilton bias adjuster, however.

924RACR

So the 996 doesn't have electronically-controlled proportioning? Sounds wrong... but I'm not an expert on the setup in the 996, so couldn't say for a fact...

924RACR

Just curious, what year?

OZ951

A 99 C2 without PSM has a 5/55 bias valve. If I recall correctly there are about 8-10 versions of the ABS pump depending on the car model. I know the 6.GT3 does not use a rear bias valve.

Veloce Raptor

I dunno, 07 or 08...but the bias adjuster was added when it was built into a race car. Not a factory part.

KaiB

This has me questioning also.

I'll try to call Tilton today and try to get a better answer than "possibly damaging" and definitive pressure numbers.

onefastviking

And for what purpose are you trying to add the valve while keeping the stock ABS ? Or better stated, what problem do you feel you have that the valve will resolve ?

924RACR

OK, that makes more sense... wouldn't be too useful from '09-onward...

Likewise re: the 996 ABS-only version...

Back to that original question, I'm surprised that they'd have a concern with the pressure modulation... but probably better to express concern than not, JIC...

docwyte

No problems with mine in my 89 951...

OZ951

I have replaced my 4 pot/318x28 mm fronts with GT3 6 pot/350x34mm fronts and I have relocated my front calipers to the rear with 330x28mm discs (vice 299x24mm).

The calipers I am now running on the rear share the same pad size & rotor diameter as the GT3 but the pistons are quite a lot larger (36/44 vs 28/30 for GT3), hence more than desirable rear bias. The Gt3 doesn't run a rear bias valve but the C2 does (5/55). I have tracked my setup and it is a little - (emphasis on little), unstable under hard braking, hence my desire to reduce the rear braking bias by a small amount. I have a 5/33 & a tilton bias valve on hand but they are both M10 fittings, hence my discussion with Tilton.

FWIW I originally ran the C2 3.85 booster & 23.8mm master cylinder but have since swapped to the 3.15 GT3 booster and 25.4mm MC in order to regain a better pedal feel & aid in brake modulation (consistent brake modulation had been a bit more difficult to achieve with the higher boost and smaller MC)

OZ951

Thanks Doc, how long have you been running a Tilton bias valve ?

onefastviking

Why not just change to the GT3 calipers with the more appropriate piston size to match the front ? Yes, that would be a little more money but it's better designed than the band aid you are trying to do with the valve. And for what you can sell your old C2 front calipers for the difference would not be that bad.

onefastviking

Do you have 3 channel or 4 channel ABS on your specific 996 ?

OZ951

Two reasons, 1) I already have the C2 calipers on hand so I wanted to see for myself what the results would be (and as I mentioned it's only a little too much rear bias), 2) The Gt3 crowd with no rear bias often reports that there is inadequate rear brake.

In effect I can reduce the rear bias with from my 36/44mm C2 calipers on the rear axle with a less agressive proportioning valve, but there would be fewer options to increase rear bias without sacrificing front brakes if using the standard GT3 rear caliper & no bias valve. (I hope that makes sense.)



I have LSD / Traction control, so 4 channel ABS. (4 output ports on ABS unit)