OZ951

I'm running a C2 track car and I have recently put 350mm/6 piston calipers on the front and moved my C2 front calipers to the rear and spaced for 330mm rotors.

It is still running the C2 Brake booster (3.85) and C2 23.8mm Master cylinder, although GT3 booster (3.15) and 25.4mm MC are both already in the mail to me.

I have tracked the current setup and whilst it worked, it could do with some fine tuning, obviously the MC & Booster are a part of that but the other element of fine tuning that I am thinking about is the brake bias. The C2 front caliper which is now on the rear has much larger pistons than the GT3/turbo/C2 rear caliper so that and the larger rear rotor (330mm vice 299mm) has the effect of increasing the rear bias.

The rear end waggled a little under braking, but not as much as I might have expected to be honest. My 40/60 LSD is also freshly rebuilt & doing its bit so that is not a contributor to rear end instability.

Given the initial results I suspect I may only need a minor reduction in rear bias and I wish to continue using the PFC-06 pads I have installed there. This led me to thinking about the GT3 bias. If I have read correctly the GT3 has no rear bias valve - can someone confirm if that is correct ? (C2 has a 5/55 rear bias valve).

If there is no rear bias valve in a GT3, is it the case that GT3 rear bias managed by the ABS unit ? (I would have thought there would be some form of proportional reduced bias to the rear in a GT3)

Has anyone measured front & rear rotor temperatures after hot laps to compare relative temperatures (relative bias) in a mk II ? Some information of this nature would be helpful in establishing a baseline before I change the bias in my car.

Given the larger pistons in my rear calipers I'm thinking of trying a 5/45 or possibly 5/33 bias valve to reduce rear bias a little.

OZ951

I found the answer to the GT3 proportioning valve question in a workshop manual. There is no rear brake proportioning valve fitted to the 6 GT3. That implies that the GT3 uses the rear brakes a lot more than the carrera and may also be why many GT3 owners have commented that their GT3s respond well to a LSD refresh to stabilise the back end under hard braking.

IPguy

However, it is my understanding that a fair number of 6gt3 owners also increase the rear rotor size to 350mm to increase the rear bias which is relativley low compared to the 7gt3 (i.e., the rear brakes on the 6 are not doing too much work, which is why the pad change interval is usually greater than 2 front for every 1 rear).

OZ951

I found a resource for calculating brake bias based on tyre/master cylinder/piston/booster parameters. 6gt3 numbers produced a bias of roughly 65%F/35%R from memory.

My recent changes evened that balance out somewhat but a bias valve doesnt factor into the worksheet so I can't say what the resultant balance for my car is, just that in practical terms it was a bit less planted than it was before.

I have ordered a Tilton adjustable brake proportioning valve so that I can fine tune my particular setup - driven primarily by the fact that I am using rear calipers with bigger than standard pistons.

Dervish if you happen to be reading this I wouldn't mind measuring/comparing your rotor temps when we do Spa in a few weeks.

Dervish

No problem.

911SLOW

Interesting thread.

Subscribed.

I will address my PCCB bias theory / ABS kill switch, sometime later this summer so any hard data would be appreciated.

OZ951

Thanks Stuart, (Captivating new signature BTW).


No problem, hopefully should have some reasonable data by mid summer.

aussie jimmy

i got stuck firing 200 rounds

spare tire

This type of set up will never be optimum. Your bias changed to the rear because the 6 piston calipers eat more hydrolic pressure compared to the fronts. The volume of piston fluid displacement is not balanced any longer. If you push the brake pedel down hard enough for the fronts to be where you want them then the rears have too much fluid pressure. Or you brake just right with the rears as your guide and pat youself on the back because your front brakes never overheat anymore. But your stopping distance has increaced!! Try your set up with you original rear caliper and the Gt3 fronts. It might be better. A bias valve or different compounds in the pads will never eliminate the problems at all the brake pressures you will use. If you get things working at hard braking, then they will be a little off at medium braking.

OZ951

Thanks for your input Spare, your comments are most welcome of course. What follows is the logic I have used and the experience I have had with the system so far.

Whilst some will hold the view that optimum is defined as absolute stopping distance, it isn't the benchmark that is of most concern to me. That is not to say that that the change I have made results in an increased stopping distance and I doubt that it does - but I haven't measured it. The car is primarily a track car and optimum to me means brakes that enable me to get around the track faster with spare thermal capacity that reassures me they are not going to fade and are ideally with slower wear than I was experiencing before.

I am aware of the limitations of proportioning valves based on knee points & rates, so I get the point your making about performance variations between hard and medium braking. Since every system (OEM included) is designed to operate across a range of conditions it has to be compromised to suit someones definition of optimum. Hence my system (once bias is finally tuned) will also be optimum for one set of conditions (hard braking) and relatively compromised in another area (medium braking). I see that as different, not wrong, kind of like the way that some people modify their GT3s because they would like them to operate in a different way to what Porsche decided was optimum, be that suspension, brakes, aero etc.

I am unable to retain the C2 rear calipers with 330x28mm rotors as the C2 rear calipers are sized for 24mm rotors. If I really had to I could find some GT3 rear calipers but the car has already been tracked with good results so I don't anticipate needing quite such a drastic change.

After it's first track day with the new brakes I was left with the impression that the car did well, the brakes seemed much less stressed, I was able to stay on throttle longer and pull up more rapidly than I had been doing in the first 5 track days I'd done this year. By the end of the day I took nearly 2 sec off my lap time so I'm confident that the brakes 'work' and I'm pleased that the early signs look like they will wear more slowly.

dan212

Spare Tire is absolutely right: It isn't even a matter of brake bias (although that helps).

Someone with more mechanical intelligence than I may be more eloquent than me here but -

Front calipers will have larger pistons than rear even when the piston count is the same. . Moving your old fronts to the rear sounds like a nice way to balance our your new bigger front calipers, but as Spare Tire says, this will never be optimum.

For example: GT3 front brakes and rear calipers . The GT3 rear caliper and the Boxster/S front calipers both even use the SAME brake pads. Should be a good result right?? - Same brake pad too!!.. But it isn't.. The two calipers are NOT the same, , because while the GT3 rear calipers and Boxster/S front calipers do take the same pads, the Boxster/S front calipers have much larger pistons than the GT3 rear calipers and draw more fluid than they should and this throws the entire system off. The system is sending too much fluid to the rear and there are proportioning valve fluid volume caps that will can make the results uneven. Can throws ABS off too.

If you think about it. The reason that front brake calipers will have larger pistons than similar looking but designed as rear calipers of the the same piston count and rotor size is pretty obvious given the difference in load that front brakes take compared to rear and the difference in fluid volume required.

I guess if you install something like Tilton dual masters you can get around this, but its much easier and more effective to just put the right calipers by volume in the rear - which means not just appropriate rotor size and piston count, but piston size as well .

I know this from painful experience.

It really throws the boost off. You can turn the boost off altogether and things won't be as bad (without boost you get nice feel, but its still not optimal and some people aren't happy with no boost), but relatively correct caliper/rotor and piston size for appropriate rear brake fluid volume is going to be better. The system is designed to want things that way.

pete95zhn

996 C2 front pistons are 36 & 40 mm. Below a list of 996/7 turbo/GT3 piston sizes. As you mentioned, all those models use 25.4mm master cylinder. Now, when you use bigger pistons with small MC you definitely add more barke force, and in your case also bias to rear. Just physics...laws of hydraulics. Using bigger MC the front is aligned with GT3's (just more boosted with that booster), but you still have more force at rear even compared to 997 GT3. Smaller brake disc may compensate that a bit though.

Fronts:
996 turbo & GT3 -'02 36-44mm
996 turbo PCCB & GT2 & GT3 '04- 28-32-38mm

997 GT3 -'08 & turbo 28-32-38mm
997 GT3 '10- & GT2 & GT2RS 28-30-32mm
997 GT3 PCCB 28-30-32mm
997 turbo PCCB 28-32-36mm

Rears:
996 GT2/3 & turbo 28-30mm
997 GT2/3 & turbo 2x 34mm

More info here and here.

OZ951

Dan, as per my quote from my initial post I am aware that the piston sizes are larger and the effect on bias. Perhaps it's worth reiterating that I'm not theorising about this system, I've tried it and I know how it behaves. It is a little unstable in the rear hence my post about needing some minor reduction of the rear bias (as opposed to a significant cut). This statement should be taken in the context of the fact that the car is currently fitted with a 5/55 proportioning valve. Considering it is only a little unstable at present it is my expectation that a 5/45 or 5/33 proportioning valve will reduce the rear bias to the point where that small amount of instability I am currently experiencing is eliminated.

Whilst I don't mind engaging in a discussion about my setup, the point of my original post wasn't really to canvas opinions about the viability of my brake setup (because I know how it behaves), it was to state what I had done and ask some specific questions about GT3 rotor temps & bias valves.


I'm not sure I quite follow you about throwing boost off? perhaps you can elaborate on that further.

With respect to brake fluid volume the C2 & GT3 master cylinders both use an 18mm stroke for the front & rear circuits so it can move an equal amount of fluid in either circuit.

Thanks Pete I hadn't noted the size of the 7.gt3 rear calipers. By my reckoning that makes the F/R bias figures stack up like this:

C2 64/36 Plus further bias reduction via 5/55 PV
6 GT3 65/35 no further bias reduction
7 GT3 58/42 (? not sure about the presence of a PV in these cars)

The same spreadsheet has my brakes at:
53/47 Plus further bias reduction via 5/55 PV

When I first did the calculations & came up with the 53/47 bias for my car I thought that was pretty aggressive rear bias - that is why I was surprised that the car only felt slightly unstable. No doubt things would be worse if there were no bias valve or no LSD. Considering the 5/55 bias valve is at the aggressive range of bias valves that Porsche has used (18-60 bar - that I am aware of) - I think that IMHO, a reasonable next step based on the way the car is feeling, is to use a less aggressive bias valve ie 5/33 or 5/45.

Since Porsche has gone to larger rear pistons for the 7.gt3 it would seem that there is scope to employ larger pistons than the 6gt3, but I will be the first to admit that 36/40 is too much without some degree of taming with a proportioning valve.

Since were on the subject.. I've installed a 25.4mm MC and 3.15 Brake booster today. I still have the 5/55 PV installed though as fitting the Tilton PV hit some snags with fitting sizes.

spare tire

Another not too expensive easy fix is to just shim your stock rears out with bars of aluminum with 2 holes drilled so the caliper is farther away from the center of the hub. And use longer bolts to attach the caliper to your wheel carrier. Some 996 GT3 guys do this and it gives them a little more bias. The 996 GT3 ceramic brakes are 350mm front and rear. The 996 iron brakes are 330mm in the rear. All it take is a 1 cm shim. Cheap to try and easy to undo if it doesn't work. Or a stack of fender washers and longer bolts. I would only use that for a session, and if it works good then get a proper machined shim.

spare tire

The better you get at driving , the less you use your brakes hard. The fast guys allways say brake less!!
The best club racers i know in older 911s run the stock equipment, upgrade to bigger brakes, get a little faster and swich back to stock and have no problems.