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Before we design a brake kit, we always like to take a look inside OE set up first, and this is no exception for 991 GT3 that come with PCCB - Yellow Calipers.
Here are some pictures that I hope can help you understand better about your OE calipers, what you can expect from performance & maintenance standard point, and any improvement you like to have.
Front 6-pot Caliper - Front View
Rear 4-pot Caliper - Front View
It seems the most heard complaint is the calipers do not allow pad replacement from the top unless the calipers are detached from the spindle.
Front Caliper - Top View
Rear Caliper - Top View
We looked further by removing the pads and noticed these pads are held in place by retaining/hold down springs; 3 piece in the front, and 1 piece in the rear. These springs require considerable efforts to break the tension and push them off the calipers, and replace them back after pad is replaced - Not an easy task.
We were also little puzzled with how these calipers are designed in two particular areas:
1. The size of pistons on these calipers. As you can see from the pictures. All 6 pistons in the front are same in 30mm, and 4 in the rear are 34mm - Contrary to common understanding of a high performance brake caliper is supposed to have staggering (differential bores) piston sizes to avoid the pad tapering.
Front Caliper Pistons (30x30x30)
Rear Caliper Pistons (34x34)
2. The piston area ratio is also odd, normally it's 2:1 ratio (2 front and 1 rear) or ~65% front and 35% rear. But in this set up it's 54% front and 46% rear. Much more rear biased than usual.
Front and Rear Caliper Pistons
3. We have a collection of Porsche calipers including Boxster, 996, 997 etc. and here are two earlier models' ratio:
997T: 58%/42%
996T: 65%/35%
If it were so designed because of rear engine (heavy), so the car needs more brake in the rear, then shouldn't it be more effective to put a larger rotor (for more leverage) in the rear like NSX?
Those GT3 owners who have already tracked their cars might want to chime in and share their feedback and insight as to how the OE system run so far, and any shortcoming they like to address, in hope that we can develop a brake system whether it's for PCCB or non PCCB; the way you like it to be built.
I also like to take this opportunity to thank Michael Filippello of Advance Performance in Tampa FL.
for his generous help in lending us those brake components towards this brake development project.
First thought I have is WOW! What the heck is Porsche doing? First the impossible and impractical center lock wheels and now the brake calipers that's completely seal off to prevent a quick brake pad change? One reason why they are design this way (not for efficiency) is to make owner take their car to the dealer for service. Funny how they also run the sealed off PFC caliper on 991 Cup and only when you move up the pecking order to "real" racing where race cars actually have pit stops and time and speed for component change is life and death matter do you see the Brembo calipers equipped with brake pads quick release access from the back shown below on the 991 GT3 RSR
I have faith that Warren will come up with a much better designed caliper with easy brake pad access. Taking calipers on and off for any car is not fun and required for brake pads change is just backward in engineering
look forward to your new calipers.
agree with most ur points.
but not so sure 2:1 F to R caliper size is bad. most inexperienced drivers are too hard on brakes. if rear were any bigger, you likely lock up rear entering turns and spin. so easy way to avoid that is be front biased. not the perfect soln, but a reasonably easy one.
look forward to your new calipers.
agree with most ur points.
but not so sure 2:1 F to R caliper size is bad. most inexperienced drivers are too hard on brakes. if rear were any bigger, you likely lock up rear entering turns and spin. so easy way to avoid that is be front biased. not the perfect soln, but a reasonably easy one.
I think the OP's point was the opposite one - the new brakes have MORE rear bias via piston sizing, not less (46% at the rear rather than 1/3 or so traditionally). I am just guessing, but that may have to do with reduced weight transfer to the front at braking in the new chassis. Or maybe something else. I drove cars with too much rear bias, where rear loses traction before front does, and any steering while braking causes a handbrake-style turn or spin, and 991 GT3 definitely does not feel like that - very stable on braking, actually a bit understeery.
I might be on an island on this but aside from the excellent engineering review, on the track and in stock form I was astounded by feel and performance for a factory setup. I think they will be further tweaked by different pads (and maybe discs for that matter) once a wider selection becomes available. I'm certainly griping about the lack of quicker pad changes without caliper removal but as far as performance goes (both steel and ccb), my initial thoughts are they are the best factory calipers/pads I've ever driven on track.
Besides the hydraulic bias mentioned above, rotor size and pad effectiveness bias and installed p/v affect bias.
I like to use a single # the front to rear ratio to sum up bias
for instance a 2007 GT3 street car has a hyd bias of 1.931 which goes to 2.084 when rotor bias and pad effective area bias is factored in, I didn't run the numbers for the effect of the p/v
For my own cars(both w/ 993RS brakes) I have run the #s
hyd 1.472, w/ rotor/pad bias 1.426 w/ p/v 2.970, one runs w/o a p/v the other w/ stock 993p/v's
I also ran the #s for a 2007 GT#RSR w/ dual master setup
hyd 1.504, w/ rotor/pad bias 1.613, no p/v but w/ cockpit adj. dual master bias control max 1.875 min 1.260
Any new setup needs to design this out. I wouldn't look to what Porsche did as any indicator of what to copy necessarily. New calipers should be top loading for pads and have differential bores. As far as brake bias, that is another issue. Warren, we need to get the piston sizes from the steel equipped car to compare. We need to see if the bias is the same or if this ratio is unique to the PCCB setup. The 991 already upped the rear rotor from 350 to 380 on the steel equipped car without changing the front rotor size. Not sure if that change was also accompanied with a decreas in piston size to offset the extra leverage of the larger rotor. If not, more rear bias. Lots going on with the new cars. Rear steering, traction control, new suspension.
Too bad they didn't go with the PFC set up used on the new Cups. Super stiff, differentiated piston size - big step up in performance from prior gen brakes. Another place where the street car has diverged from the Cup car.....
Look forward to seeing what you guys come up with.
[QUOTE=Nizer;11655885]Too bad they didn't go with the PFC set up used on the new Cups. Super stiff, differentiated piston size - big step up in performance from prior gen brakes. Another place where the street car has diverged from the Cup car.....
We can probably thank VW for that. Forcing people to go to dealer for service. (Ie: CL and closed top caliper). On the PFC setup for track, the closed top caliper probably isn't a big deal. Not a lot of pad changes going on during a race anymore After the race, who cares when you have help and are going to go through everything anyway. On a street/weekend track car that requires switching from street pads to track pads and back again, this becomes a real inconvenience. This is why a stiff open top caliper will be a nice option for guys who do HPDE events and like to work on the cars themselves.
Now we just need to get a 5 lug conversion going. Working on this now.
09-17-2014, 01:26 AM
Ideal Brake Kits for 991 GT3
After the race, who cares when you have help and are going to go through everything anyway. On a street/weekend track car that requires switching from street pads to track pads and back again, this becomes a real inconvenience. This is why a stiff open top caliper will be a nice option for guys who do HPDE events and like to work on the cars themselves. 