924RACR

Hmmm. How much is this then changing your driving style? Almost seems as if with ABS you were just overusing the brakes.

If you're cooking all 4... well, ABS can after all only reduce pressure to individual wheels, not increase. In other words, ABS optimizes the braking efficiency of each wheel individually. Optimal efficiency means maximum transfer of energy, from kinetic to thermal. You, as the driver, were requesting max braking (through your pedal apply). ABS was ensuring each wheel operates at and remains at peak effectiveness.

I won't outright criticize your driving style, as I'm not there to assess, and our data collection (walkaround of the car, without brake temp monitoring) is a bit sketchy.

But it sounds like perhaps you were just driving well beyond the capability of the braking system you had at the time, and were in need of upgrades.

Once you disable the ABS, you can't get the most (apparently) out of the rears, as the fronts are overpowered first, and so you have to ease back on the brake usage, limited by what the fronts are doing.

Note that all the above is assuming your assessment of what's going on is 100% correct. With your description of mods, sounds reasonable - other than pads, have you done any other brake upgrades? Maybe now it's time to look at that?

Apex996

>> driving well beyond the capability of the braking system

Aww comon man... This is a Porsche! LOL jk .... but it is a 996 with the same brakes that were used in Grand-Am Cup and by PCA club racers like Karl Poetl who are going a LOT faster than I am. Same mass being decelerated by the same swept area, same cooling, same pad material, etc etc... Pretty sure it's my ABS. The ABS might well be operating incorrectly, but the driver isnt.

In three laps the "dragging" ABS roasts the brakes because apparently (and acceleromter data supports) it wont release on corner exit and therefore wont let them cool. BTW, I'm not suggesting that it's a Porsche design flaw. There could well be something wrong with my ABS. I've read in the factory repair docs that even changing a brake line or altering the tubing bend could require the whole system to be recalibrated by the factory tool... it's that sensitive. This is something that I have not done and REALLY should have done. I will after I plumb in my Tilton.

>> have you done any other brake upgrades?

Nothing significant. Lightened car 100-200 lbs, ATE or Motul fluids, GT3 ducts, Race Pads .... that's all.

>> Once you disable the ABS, you can't get the most (apparently) out of the rears, as the fronts are overpowered first

That's a good point. And one I had considered...but I was thinking about it in terms of bias moving toward the fronts because of what I was told by a GA team. If as you suggest, the rears are giving up to create the front locking issues I'd think they would either wear faster in the rear or be heating up more. No? With the ABS disconnected I don't see evidence of that.

EDIT: or ... are you saying you think the rears never get to perform their function because the fronts lock and I must release pedal pressure ?
Recall that w/o ABS I don't seem to have an issue at higher speeds when the weight is still more evenly distributed. It's really at the end of a braking zone and especially if bumpy that the front lock & I have to jump off the pedal and then start "nursing" the brakes to keep from locking.

Good discussion.
And please let's not lose site of the OP's initial questions. Sorry OZ !

utkinpol

speaking of heat - proper GT3 calipers have special ceramic 'pucks' on top of each piston to isolate pads from piston metal.
also, gt3 standard ducts are useless, you really need bigger gt2 (cup?) ducts to get air on rotors and need in rear those gt3 rs ducts that blow at lower part of a rear rotor (you need to trim metal shield to make it fit).

PFC pads are vey hot. on my car i got brakes overheated same way you did on yours with them. pagids RS29 do not run that hot and so far worked excellent with gt3 stock ducts and stock C2 calipers (which are same as 996 'S' imho).

so best way is to get stock gt3 calipers imho if you want to run PFC pads as you do need those ceramic inserts in the calipers to isolate heat away from a piston.

see post #26 in the link below.
https://rennlist.com/forums/997-gt2-...ut-them-2.html

924RACR

Well - there's no ABS function to do what you're describing, to have more brake pressure than driver's pedal apply. Unless there's other non-ABS functions in there, but I'll take your statement at face value that no, you've confirmed already there are no such things.

Pressure data would certainly confirm, in your data, if there's a restriction or such keeping pressure from releasing. If I had that, I could probably speak with more certainty 'bout what's happening.

Re: recalibrating ABS for brake line changes - damn, I hope not. Things would really have to be screwed up for that to truly be necessary, IMO. Maybe they're just running the standard line of overly-conservative practice...

Yes, I did mean to say that it seems like the rears don't get to do their job because you have to release pressure due do fronts locking - your second interpretation. Sorry for the confusion. Didn't mean to suggest rear fade or such.

Define "higher speeds" - could this be an aero/downforce distribution variance?

Or could it be a matter of long braking zones and substantial pad temp difference at beginning of stop vs. end of stop? For example, do you have the problem, say, if you go to the brakes at 50mph (initial brake apply speed), but not AT 50mph IF you start braking from 100mph?

Apex996

Forgot to mention that I do now run Ti heat isolators under my pads.
Not planning to run PF pads.
Unless you have hard data to support...Saying the GT3 air ducts are useless is a bit of an overstatement.
I noticed an improvement when I switched to them. Think that was back when I ran the Hawk blacks.

Agreed, the GT3 brakes are nice but I want to get the C2 brakes working as designed. They worked for GA Cups teams, they should work for me. Brakes should easily be sufficient for the mass of this car. And considering I can lock them, the pressure is certainly there. Just need to figure out what the issues are.

I didn't mean to imply this. The F/R balance (bias) is changed by the removal of the ABS (fuse pull).

Check out the 996 Factory Service Manual. Its at the end of the section on ABS and TC. I can do a cut & paste later if interested. I too was surprised to see the comments about moving a brake line. But they are actually quite specific about which lines.


120-130mph

Not sure what the technical term is.... perhaps "Brake Torque" ?... the force on the rotors overcoming the pad friction is greater "at speed" than at a slower pace .... I'll guess there is no real significant "downforce" so we can probably assume the cF of the tires (vertical load) is relatively constant through most of the deceleration?...

Something I could certainly test.
I have some brake paint and temp labels around somewhere from my racing days.

utkinpol

i do not argue with that. still it depends very much how lower plastic plates affect ventilation on race cars and stock cars and a lot of other variables.
if you are still boiling liquid even with ducts - well, you can go gt3 calipers, stoptech calipers, either way bigger ducts and bigger rotors should help.
taking away ABS you just stopped using car`s stock braking power to the max, so, it resulted in less heat but it did not eliminate origin of the issue, IMHO.

i run stock C2 calipers with gt3 master cylinder and it works fine for me, with GT3 ducts and lower heat pads like RS29. but with PFC01 it is not able to cool down properly no matter what, so, there is no solution other than to go bigger and add on ventilation, IMHO.

Apex996

The GT3 MC is giving you more line pressure to the C2 calipers than I have. So that kind of supports my contention that the C2 calipers are fit for the job.

Interesting point about the PFC pads vs RS29. I'm assuming then that the PFC have a significantly higher CoF than do the RS29 ?

I really need to get the temp labels and paint out before the next test day.
Thanks for the info.

utkinpol

pressure wise all calipers have more than enough, they just not large enough to dissipate al the heat they get, especially rear ones. those are different matters as, well, as soon as they have boiled your liquid it does not matter much anymore what pressure pedal makes.

i suppose gt2 ducts in front and those RS car rear ducts would probably improve cooling considerably but for me RS29 pads work just fine and i do not want to invest $600 more into stupid plastic ducts. look at RS and cup cars how they have lower plastic liner done - they have channels there to move air to rear calipers. street cars do not have anything like that and this is imho that makes most of the difference.

jrgordonsenior

Great and informative thread, thought I'd share my experiences after 3 years of racing my 99' 996 in 2 different series each year. For those reading this that are considering changing their OEM 996 brakes to GT3 or TT calipers & rotors you should know that there's a heavy weight penalty involved. We weighed all the parts once and IIRC it around 10lbs. per corner. That's one hell of a lot of unsprung weight to add at the corners of a race car.

I run stock calipers and rotors with RS29 pads front & rear. I run 3" cooling ducts from the front grill thru to the rotors and although I sometimes smoke em, usually I'm good. I use Endless or SRF fluid and have changed it completely only once but I do bleed it at the beginning of each season. I've never had fade.
I've changed the front calipers once using junk yard street C2 calipers. The results? I can brake with any Cup I've raced with be it 996 or 997 variant. I'm light at around 2600 /driver and that certainly makes a difference. My experience has been that the OEM C2 brakes are more than adequate for racing these cars.

PS: I have a front set of Stoptech 355mm 4 pad calipers and rotors w/floating hats if anyone's interested.....

Apex996

Thanks JR! Good info and that's what I've heard form those who race the 996 C2. The brakes if well prepped are more than sufficient.

Here is that info from the Service Manual I mentioned earlier.....

Important instructions for ABS and ABS/TC

If certain brake lines are replaced, e.g. at the intermediate piece in the front left wheel housing, a system check also must be performed. The hydraulic allocation could
be incorrect in spite of the different threads (M12 x 1 and M 10 x I), due to unintentional bending of the brake lines.

Weird huh?

KaiB

JR, you use brakes????

OZ951

I believe what that quote is saying is that the plumbing may have been done incorrectly (cross wired so to speak) and the system should be checked for correct operation, it is not a re-calibration as such.

OZ951

The bigger GT3 MC actually produces less brake force at the caliper for the same amount of brake pedal pressure. Net result is that the pedal feels firmer.

Bill in answer to your initial post it certainly sounds like you have TC/ABD type behaviour/symptoms, and if your LSD is worn that wouldnt be helping either.

The OEM proportioning valve in the 996 is a 5/55 valve so below 55 Bar brake line pressure the front and rear brake line pressure is the same but above 55 bar, rear line pressure increases at a slower rate than front brake line pressure as the brake pedal is pressed harder.

I haven't fitted an alternative bias valve yet as I am waiting on delivery of a bubble flare tool so I can make some new brake lines. In the mean time I've done a track day at Spa, braking from about 140 down to about 68 and the rear end hasn't been terribly unstable with my current bias. I do still intend to plumb in the bias valve and see for myself how it affects things.

utkinpol

GT3 MC is 24mm bore, stock C2 is 22mm bore. So with GT3 you move more liquid, efficiently it results in slightly shorter pedal travel and much more precise modulation of brakes as you do not have to drop pedal to the floor at the end of the straight anymore. heel&toe is also simpler with it, at least for me.

utkinpol

JR, what do you use for rear ducts? i adjusted front GT3 ducts to fit on racing arms i got there in rear but it does not seem to work well enough, imho.