OZ951

GT3 is 25.4 & C2 is 23.8mm but agreed the pedal modulation is easier with the stiffer pedal/less travel.

jrgordonsenior

Just in the pits


I don't use anything other than what the gets directed there by the undertrays. Again I use the RS29 pads F & R and I swear they just don't overheat or fade. I probably should duct some air to the backs of the rotors since my rears are 2 years old and cracking. My fronts are about 1.5 years old and look great. Ducted air works quite efficiently.....

Bill Verburg

996 GT3 & Cup used 25.4mm m/c

997 GT3 thrue 2007 also used 25.4mm

997GT3 from 2009 used 26.99mm

the bigger the m/s wrt the slave cylinders the better pedal you get, to a point, as w/ a better pedal comes greater leg input.

the sweet spot for performance use is a ratio of slave/master ratios in the low 30 to high 20 range, in this range the ability to modulate and apply enough leg is optimized for most people, especially when a mild boost ratio is also used

this ratio for 996 GT3 was ~30.5
for early 997 GT3 ~30.6
for 2010 997GT3&RS ~27.6

late RSRs w/o boost use ~29.5 and are optimized for Pros in prime condition

Apex996

Argh... you are right! Fluid Dynamics is often like thinking backwards... I make that mistake a lot.

I had no diff previously.

Porsche designates these older systems as:
ABS 5.3 (3 channel, no TC) and
ABS / TC 5.3 (4 channel w/ TC)

I dont have TC (only ABS 5.3, 3 channel in my car)
So when my rears slip, exiting a corner it applies brakes to both wheels at once...including the heavily loaded outer wheel trying to accelerate out of the corner! And it doesn't release for a while either....my logger suggests its usually a good second or more after I've unwound the wheel that the calipers release.

Notice the "T" in the rear brake line of the attached schematic. This is a single line feeding both rears. Hence the "3-channel designation. LF, RF and Rear.
ABS/TC 5.3 (4 channel) has 4 independent lines. Hence the 4-channel designation.

Apex996

Guess what I did tonight!
Made bubbles!

I have to work out the bias adjuster mounting location and then just bend the lines and I'm done. Hope it will be worth the effort.

utkinpol

interesting, i just saw '22' engraved in a circle on C2 cylinder and '24' on gt3 one so i assumed that was bore diameter. good to know, thanks for the info.

OZ951

Same same, I was debating whether to mount it in the cabin or just leave it in the trunk.

I picked up some M12x1 flare/brake nuts for 3/16th pipe to take care of the upsize/downsize issue with the M10 Tilton valve in the M12 lines.

Mine is ABS 5.3 4 channel w/TC, if I forget to turn it off I know it even before I have crossed the blend line to join the circuit.

OZ951

Bill, I'm not sure what the metric is that your using for the boost ratios, but I was going to add the Carrera boost ratio in the list for comparisons sake. My manuals list the 6 GT3 as 3.15 and the 996 not GT3s as 3.85 i.e. the Carrera uses a fair bit more vacuum boost. I've since moved to the 3.15 booster as well as the 25.4 MC and I like the pedal feel a lot more.

Bill Verburg

Those aren't boost ratios, they are the ratio of slave/master.

boost ratio is the additional help provided to your leg by engine vacuum or hydro-electric pump the street GT3s certainly do use vacuum boost but most of the Cup/RSR versions do not, the race cars tend to have twin m/c with no boost and no abs. The 2005 GT3Cup was the last use of abs and vacuum boost.

OZ951

Boost ratio (actually boost factor) in relation to the brake vacuum booster is how those numbers are described in the 996 factory manual.

924RACR

(re: higher pressures than brake pedal apply) - some of your comments earlier, particularly about brakes being held on, can only be interpreted as such. Again, I don't mean to be a jerk, but that is the only direct technical interpretation - so I have to question those statements, given that you have no TCS, ABD etc.

For the record - note that this is the stuff I do in the day job (though not on Porsches).

Saw that section from your quote later. The only way this would have any significance (technical accuracy) would be by unintentionally inserting smaller lines than original, thereby reducing the flow rate potential. System would still work, but would be at some reduced effectiveness in transients.

Seems to me like there was a less-than-optimal translation from the German there.

There likely is downforce of significance - well, depending on how much rear wing and splitter you've added, if any. Certainly you're going fast enough.

We have a term known as Specific Brake Torque - amount of brake torque, in N-m, applied per given bar of brake pressure. Not affected directly by speed, only by brake sizing, pad/lining selection, and tire diameter.

Of course, this does bring us back around to tire sizes, are they still close to stock or at least still the same proportion front/rear with respect to diameters?

Those still don't tell you a whole lot, being only peak data. Log brake pressures and temps, we'll know for sure (and of the two, I'd rather see pressures and wheel speeds from one corner braking/accel event than anything else).

Of course, since you have figured out the workaround, it does raise the question of just how much effort is worth putting back into it. I'm quite familiar with ABS 5.3, have worked on that (not too many still in the business have, it's such old stuff... guess I've been doing this too long! LOL Also know ESP 5.3, 8 and 9 - extremely well as usual - and played a bit with some ABD back in the 5.3 days...)

@ OZ951 - If you can tell on pit-out that you've left TCS on... but are leaving ABS on... I hope your wheel/tire diameters are also still comparable to stock (at least front/rear ratio)?

OZ951

Yes that is with ABS Functional, I run a 2.6% difference front-rear circumference with R-888s at 235/40R-18 & 285/30R-18. Even with a good functioning 40/60 LSD there is enough slip during that straight line acceleration to activate the TC.

Bill Verburg

I understand what you are saying, but the subject of my post was entirely different.

add up all the caliper piston areas, that is the slave #

divide the slave area by the master cylinder area, this is the slave/master ratio which is the primary determinant of pedal feel assuming that the system has been properly engineered to reduce mechanical flex to a minimum(most Porsche brakes systems are so engineered)

the slave/master ratio has varied on 911/964/993/996/997 from the high 20s to the low 40s depending on the application, the street cars tend to be at the higher end, the race and more performance oriented models cars at the lower end. High slave/master ratios have more travel, require less leg effort and feel softer, low slave/master ratios have less travel and feel harder and areergonomically are easier to modulate

boost ratios from vacuum or electro-hydraulic pump have varied from ~2.2 to 4.4, again higher boost tends to be used on street models and lower tends to be used on the more performance oriented cars. Boost merely amplifies the effort supplied by your leg, it does not modify the basic pedal feel except to the extent that less effort is required.

the race cars tend not to have any boost or abs, they also use lower slave/master ratios and twin master cylinders.

when the slave/master ratio is too high the m/c doesn't move enough fluid to service the calipers, this occurs at slave/master ratios ~51

when the slave/master ratio is too low the human leg can't push hard enough w/o assistance, it depends on the leg but high 20s is getting there

OZ951

Thanks Bill I was wondering if we were on a different wavelength and it seems we were. Time to go and do some sums re my piston sizes to see where they lie with that metric.

Edit >> that puts my MC/Slave ratio at about 36.4 due to the large rear calipers (piston sizes) I'm running. As compared to 32.4 for a stock 996 C2.

My observation of the pedal feel is that it is firmer than before I upsized the MC & downsized the booster and brake modulation at my last event with the GT3 booster & MC was easier to regulate than with the C2 booster and MC.

Apex996

Hi Vaughn... Yup I know you are a brake engineer as I've read a lot of your posts over on IT.com

Let me try to clear up the "higher pressure" issue again. What I'm experiencing is brake activation while exiting a tight corner. Same type of thing that happens in my BMW 330i everyday on the way to work LOL

However in my 996 I'm not supposed to have TC... which means I'm also supposed to not have ASR & ABD right?

BUT! with the ABS engaged.. I can tell you I am getting brake actuation ... as mentioned, I can heat soak the system in 3 hard laps... unplug the ABS and I can not.

I'm "all ears" as to what the issue might be. Early type of ABD in play?

Yes, I'd like to log the brake pressures ... I may be able to do temps as I cobbled together an IR sensor & mount a while back when I was going to do dynamic tire temps on my SCCA race car. That stuff should all still be in a box somewhere.