powdrhound

Jason,
Your car is looking great! Just wanted to chime in here as I'm run just about every permutation of brakes on the 996 platform. For the past 7 years, I've been running the front and rear 997.2GT3 calipers with 380/350 rotors, a 996Cup ABS, and a 997GT3 cam driven vacuum pump. IF you are going to be using the 380 7.2GT3 front brakes, you will absolutely positively need the matching 350 rotors AND the larger 997GT3 rear calipers. Don't even bother with 330 rears as the brake bias will be totally off on the car and horrible for any aggressive driving. It will probably be worse than with the 330/330 brakes. Don't bother with the smaller 996GT3 rear caliper even with 350 rears. 10 years ago I tried the 350 996GT3 rear calipers with the 7.2GT3 380 fronts and the pistons are simply too small with too much bias still in the front. It's doable but still not right. The 7.2GT3 rear calipers have almost 50% greater piston area and are a must with the 380 fronts. Porsche really got the brake balance correct on the 997.2 GT3 and Cup. If you are using a street ABS, it still helps to use a slightly more aggressive rear pad with the 7.2GT3 front/rear brakes (i.e PFC 08 front / 11 rear). With a Cup ABS this is not necessary as the ABS is biassed more aggressively to the rear. The Cup ABS is a game changer. The 380s are nice but not much if any appreciable advantage over the 350/350 GT3 brake set up on a moderate HP 3000lb car as you are adding unsprung mass which not ideal. Adequate air flow directed to the rotors is paramount with any brake set up, the 380 included. Don't know if you had already done so, but swapping to the larger 27mm 997GT3 brake master cylinder really helps to firm up the brake pedal especially with the larger rear calipers which move substantially more fluid.

Anyway, sorry to jump in on your thread, just giving you my $0.02 so hopefully you can sidestep a few landmines as I had been down this road about 12 years ago. Good luck on your journey. Cheers!

zbomb

Thanks for the input John - since you're speaking from experience, I have no choice but to listen even if what you're saying doesn't fit with what my plan was So... back to the drawing board on braking system.... Better to find out now.

ETA - it seems as though your various setups did not include moving a front caliper to the rear. I don’t know enough about the proportioning system to say anything intelligent, I am inferring based off of the little trustable information I have read the the F to R proportioning functions primarily as a result of F to R piston area and subsequently to a lesser degree rotor size and pad compound.

I am going to start the exercise of looking up various piston sizes between the F and R calipers - 996 C2, GT3 and Turbo, 997.1/2 C2, GT3 and Turbo and try to learn about the ratios used in stock form.

Regardless, The change to a 350MM rotor, likely, not a huge deal. Sourcing and funding a set of 7.2GT3 rear calipers is something I need to investigate as well as any mounting hurdles that may exist to bolt them to my C2 uprights.

As to your question on MC, I'm running a 996 Cup MC.

De Jeeper

Jason if u get a chance measure the pistons in the front c2 calipers. I know they r the same size as the rear gt3 calipers but im not sure if they use the same pistons. Im curious since they r front calipers if they r closer to the 997.2 rear caliper pistons?

One of these days ill introduce myself to Powderhound, he seems to be a race porsche encyclopedia.

zbomb

Yup… was kind of hoping / wishing that to be true. But I am fully prepared for porsche 996 race encyclopedia @powdrhound to crush my hopes and dreams on that front.

My other thought was - install a manual proportioning valve… which will likely come with a boatload of its own issues I have yet to learn about, but will.

powdrhound

Hi Jason,
You are 100% correct in that the caliper piston area front to rear is primarily responsible for front to rear brake bias. Brake disc diameter is secondary and pad compound rounds out the pack. ABS programming also plays a part but that's not really relevant in the conversation here.

Let me start by saying that I am not well versed at all in the wheel carrier architecture on the C2 cars but I'm intimately familiar with all the Mezger variants. Same goes with the brakes. Going off memory, so don't quote me on the #s. The piston caliper area on the 996TT/GT2/GT3/Cup is the same across the board. All the fronts, whether 4 piston or 6 piston are the same with the same total combined piston area. 6 pistons are 28/32/38, 4 pistons are 36/44. The rears are all 28/30. Again, don't quote the #s. In the 997.1GT3, the standard brakes were front 6 pistons (same as on 996.2GT3s) but the rears were increased to 4 piston 34/34. This helped remedy the complaint of too much front bias on the 996 cars. On the 997.2GT3/Cup, Porsche increased the front brakes rotors from 350 to 380 but decreased the piston area from 28/32/38 to 28/30/32. Rears remained at 34/34. This served to further shift the brake bias to the rear as the smaller piston area in the front more than offset the increase in diameter from 350 to 380. The 997.2 PCCB calipers were externally identical to the GT3 cousins but has larger pistons. I am leaving those out of the conversation here. Hope this is making sense. Finally, I do not believe you can bolt front calipers to the rear in any of these applications but I've never tried that. I'm virtually certain you can't due to the simple fact that the front calipers are designed to use substantially thicker rotors, typically 34mm while the rears use 28mm.

Regarding brake master cylinders, I'm not sure about the C2 without looking at PET, but all the 996TT/GT2/GT3/Cup used the same MC with a 25mm piston. On the 997GT cars, Porsche increased the MC piston area to 27 to firm up the brake pedal which became softer due to the fact that the rear calipers not had significantly larger 34/34 pistons which naturally moved more fluid. The upgrade to the larger 27mm MC is straight forward as its identical externally and cheap at around $300 new. This was a very popular mod on our endurance race Boxsters and can be done on any 986 and 996.

Yes, a proportioning valve would allow you to set front and rear bias and I know it is not compatible with PSM which is present on some of these cars as it will trigger a failure mode. I have not experimented with a proportioning valve but that may be a work around to this in your situations. Certainly worth looking into as 997Cup use this arrangement but they do not have ABS. I can not say if a proportioning valve would mess with the ABS logic or not.

Bottom line, Porsche has had good brakes on their cars but they incrementally made them better and really got them right on the 997GT cars. They essentially did the R&D for us on this so it generally does not pay dividends to reinvent the wheel here. My experience confirms this. I will also mention that all of the calipers mentioned above can be spaced out radially EXCEPT the 996GT2/3/Cup front 6 piston versions. This is due to the design of the mounting arrangement on those.

Hope my rambling helps you some here.

zbomb

I hear you on not reinventing the wheel - unfortunately, due to budget and my lack of ability, I’ve been known to modify the wheel to mixed results.

It looks like, the plan as it was, would mean, taking only into account the role piston area impacts proportioning, the bias would be shifted well rearward compared to what the factory has done.

powdrhound

Are you saying you can bolt bolt a 996C2 front caliper on the rear wheel carrier? Doesn't the front 996C2 caliper use a thicker front rotor than what is used on the rear?

I am not sure how you are arriving at your piston area calculation above. For example, for the 997.2GT3 I get:

28/30/32: 21.27cm2 (53.9%)
34/34: 18.16cm2 (46.1%)
Total: 39.43cm2
Front to rear ratio: 1.17

On your example of:

28/30/32: 21.27cm2 (48.3%)
36/40: 22.75cm2 (51.7%)
Total: 44.02cm2
Front to rear ratio: 0.94

De Jeeper

[QUOTE=powdrhound;17887604]I will also mention that all of the calipers mentioned above can be spaced out radially EXCEPT the 996GT2/3/Cup front 6 piston versions. This is due to the design of the mounting arrangement on those.
/QUOTE]

can u expand in this? I was looking for some 2 piece floating 380 disks for my gt3 uprights and i was gonna space out the caliper. Also my uprights r the larger rs versions. It looked like a simple 15mm spacer and longer bolts would work?




Jason can u add the 997.2 gt3 front, c2 front (as rear) on your spead sheet. Judging by your sheet my set up with the 996gt3 front and c2 front(as rear) has the closest frknt to back bias. Am i reading that right? May also explain my softer pedal. May be time to go to the 997gt3 mc.

zbomb

Yes, in theory. The plan has been to use the stock C2 front caliper with a 996GT3 rear rotor.

https://rennlist.com/forums/996-foru...kes-on-c2.html

https://rennlist.com/forums/996-gt2-...ture-long.html

Aaron - the table on the right is 7.2 GT3F front and C2F in the rear.

ETA - ok in the ratio, was not sure how that is measured.

zbomb

K… trying again. Looks like keeping the stock rear C2 would keep the proportioning very close to how it comes on the C2 but overall piston area is less than stock…

De Jeeper

yes as jason explained it. I am running like this now with a 10mm apacer and using a turbo panamera rear 350mm disk.

De Jeeper

Just so i dont have to do the math how about including stock 996gt3 f&r and front 996gt3 and rear c2(fronts)

powdrhound

[QUOTE=De Jeeper;17887699]The 996GT3 (997.1) 350mm 6 piston calipers can not be spaced out radially due to their asymmetric mounting lugs. When you try to space it out the bottom of the caliper will contact the rotor. It't hard to explain but it you take a look at the caliper it will make sense.

Justin76

Sounds like a Brembo kit is easier than trying to piece together Porsche parts for a C2...

zbomb

Well, its a turnkey product, so I'd hope so. But... for what I would want excruciatingly expensive.

Hopefully when I'm done and sorted folks can use my setup as a resource to follow (if it works )... It's not always pretty watching the sausage being made but I find the process to be part of the fun so I share the highs and the lows.