trackdaycareracer

I've run 996 GT3 Mk1 front calipers with stock 996 GT3 Mk1 discs for three seasons now. That's for all practical purposes the 4-pot front calipers and matching discs for the 996 Turbo.

For the first two seasons I ran with the stock 968 non-M030 front calipers in the rear.

This season I reinstalled the 968 rear calipers in the back. Too much time wrenching and too little driving gives no real data to compare the two setups.

The reason for the swap was that the brake pedal was mushy as the master cylinder couldn't push that much fluid in a jiffy. This could of course have been rectified with relocating the pushrod connection to the brake pedal, but for a street legal car it didn't seem right.

To increase brake pressure to the rear with the reinstalled rear calipers, the plan was to install the 5/33 bias valve I have had laying around for three years. Being a gynocologist on more of a hobby basis than a professional, I gave up trying to remove the stock bias valve and will have a garage do the job.

Reading up on Rennlist, it seems I should go with a 45 bar bias valve instead, perhaps a 55 or even a 60. I guess a 45 bar would be a good place to start, and then move up to a 55 or down to a 33 trying to find the maximum amount of rear bias without locking up the rears.

The stock 968 front calipers have 36mm and 40mm pistons, whereas the rears have 28 and 30. The optional M030 968 (928 S4) front calipers have 36 and 44, while the rears are still 28/30. Sharing the same ABS unit, I trust the ABS handles the change in front to rear clamping pressure.

Interestingly, the 993 turbo (968 Turbo S, 928 GTS) and 996 turbo have the same piston sizes in the rear, 28/30, and 36/44 in the front. This means the clamping force is the same as for the M030 968, as is the volume the MC has to move.

The diameter of the M030 968 front discs is 304mm, 322 for the 968 Turbo S, and 330 for the 996 turbo. This means the radius for the 996 Turbo discs is 8% longer than the M030 968 discs and 2% longer than the 968 Turbo S discs. Assuming the effective radii ("arm") of the discs are also 8% and 2% longer, the torque should increase with the same amount (Torque = Force * Arm).

As pad area is not relevant for the pressure-torque, it seems that the 8% increase in pressure-torque over the M030 968 front brakes should be within the limits that the ABS unit is programmed to handle, or did I go wrong somewhere?

Coming back to the brake bias valve, the amount my 968 is lowered affects the front to rear brake bias, as do the springs and dampers. Running semi-slicks on an essentially trackday 968, the stickier tires should add more to the front bias. Aerodynamics come into play as well; the car has a 968 TRS front splitter and wing. So, what do you track-guys think; start with a 45 bar bias valve or a 33/55/60? (An adjustable one from say, Tilton, is not an option).

odurandina

i'm running a 5/33 with 928 GTS calipers and rotors and stock in the rear and it's still not enough.

i still need to move more braking to the back running these 275mm tires. this is a street only car.

333pg333

My guess is there is no way of really knowing until you try them on the track. Not the answer you'd be looking for but you can only go so far with calculations.

To digress, has your brake pedal always been mushy? What other parts to your brake system? Pads, rotors, cooling etc...This is on a 968, yes? You might try this question on the 944turbo forum and even the Racer one too.

zeusrotty

Why not get a cabin adjustable bias valve?

968workaholic

Start with the 45, P/N 964.355.305.00. The 33 is good for M030 setups, but when using the even larger front brakes, the 45 seems to work better. I think I have one if you have trouble locating one.

mattipuh

I have 996 Turbo calibers + discs, and tested 5/33. was too much rear bias for me. This all depends on the suspension setup + your driving style. Adjustable bias valve would be best IMHO, you could dial in the level that is best for track / driving style.'

I have extra 5/33 bias valve if you're interested, used for 2months.

trackdaycareracer

Thanks, that's valuable feedback as it is close to my own set-up. Looking through the below responses and old threads it seems like the solution is a 5/45 bias valve, although the suspension, tire width and other facors come into play as well.

trackdaycareracer

If I could only find a way to change that bias valve quickly, I could do a few laps with the stock 18 bar, the 33 bar that I already have, the 45 bar, and the 55 bar on the same track day (under the same conditions) and look at the data output afterwards. Regarding the calculations, they were mostly to convince myself that the ABS unit would be still functioning as originally designed instead of being overpowered by increased volume and torque and thus making the braking distances longer. Not always braking in a straight line, it could be that the right bias valve for me is not the one with the shortest braking distance (even with ABS I could risk swapping ends going into a corner while braking).

The brake pedal was mushy after installing larger piston sizes front and rear, or more correctly the pedal travel was long. Running Pagid RS29s, braided hoses, and Castrol SRF I was never able to overheat the set-up even on tight tracks with little cooling while going from full to empty tank without breaks. The GT3 calipers haven't changed color either like they do on GT3s (Mk1s), which I think is related to the 968's brake cooling system - supposedly being effective from 45mph upwards. Changing back to stock piston sizes in the rear cured the long pedal travel problem.

trackdaycareracer

I'm not sure if the adjustable bias valves are street legal, but I'll check it out as it is the easiest solution.

trackdaycareracer

Thanks for bringing that issue up: What is the difference between the 965.355.305.01 and the 964.355.305.00 ? They both pop up as 5/45s when scrambling through Rennlist and other forums. I found this somewhere: "The 45 bar bias valve was used on the 90-91 C2, the 55 bar bias valve on the 92+ C2, all C4's, and Carrera RS. The 60 bar bias valve is used on the Turbo." Looking through PET, 965.355.305.01 is (964) Turbo/Look Carrera 2, while 964.355.305.00 is one of two regulators for (964) Carrera 2

trackdaycareracer

Hi Matti,

I already have the 5/33 laying around, thanks for the offer

Do you have the 4-pot 996 Turbo calipers and discs up front (not the 6-pot?), what about the rears, are they stock 968 or are they 996 Turbos as well? It is interesting that you get rear lock up with a 5/33. Do you recall during what situations that happened? (I know that with the high-compression 968 engine I get some tire squeal in the back when letting the clutch carefully out after downshifts.)

docwyte

I went with a Tilton 7 way adjustable rear proportioning valve. I haven't had a chance to test it yet as it's winter, but I can easily reach it in the cabin of the car...

The issue I was having was way too much rear brake bias since I did the LS1 swap, as I'm now using a Ford Mustang hydroboost brake setup.

mattipuh

Yes my setup is 4-pot 996 Turbo calibers with 330mm disc front, rears are original 968 calibers. rear lockup happened while braking late, actually spun once on wet conditions when braking from 95mph to 55mph, luckily stayed on tarmac still, but wasn't nice experience. might be that there's still some air in the system, cause the balance seemed to be ok when starting to race, but turned to be more rear biased after 5 laps or so.

968workaholic

Alot of different factors come into play with the bias valve in the 911's. The only one I have used is the one that I gave you the part # for, and it works really well. In regards to easily switching the bias valve for testing, you can easily add a piece of line where the valve is now on top of the ABS unit and put the valve off of the master cylinder. Just like they did on the non ABS 944's

trackdaycareracer

That's an excellent tip, thank you