mds

Greg you are correct. The 996 GT3RSR brake cooling system is very different. I looked at a car up for repair on a lift this morning.

There is a large carbon fiber duct with an elliptically shaped cross section taking exhaust from the center radiator well back into the wheel well to a point roughly adjacent to the eye. From there, two flexible tubes run the air directly into two separate eye ports. The top tube is about 4" in diameter, the bottom tube is smaller, about 2". Exhaust from the side radiators exits along the side of the car outside the wheel well.

The tech at the shop says there are a couple of issues with the system. First, turning radius is poor because the ducts limit steering lock. Furthermore, this limited lock makes spin recovery more difficult - sometimes there is just not enough lock to catch the spin.

macfly, the rotors on the car were cracked even though they are slotted.

Bob Rouleau

MDS - Colm, Bob-Dallas I 'm with you guys. I try and use my brakes as little as possible and use the better balance to carry momentum. I'm not sure I should say this here because Colorchange (ABS to the Apex) will be all over me yet again [chuckles].

MDS I would also be interested in a better source for GT3 rotors. My wife toasted hers because she did not break-in the yellow sport pads enough. The dealer replaced them but I can see myself needing more in the coming season.

Best,

mds

I received an email from Andreas Boehm at Pagid Motorsports. He says the following:

It gets only dangerous if there is an open crack (not only surface but through the whole wall thickness) that goes from a vent hole to the very end of the outer or inner diameter. Using the ABS less would help to avoid bad cracking.

The cracks on my discs are only 1mm deep where they start at the vent holes and so are very shallow relative to the wall thickness. So the discs are OK to use after all.

Karl S

I would just add to Andreas' response that you also don't want the cracks to connect bwteen two vent holes.

Karl

PogueMoHone

Mike,

It would still be interesting to see if Porsche would replace the rotors.

enthusiast

Guidance from Porsche AG on 964 Cup Car Cast Iron Disks.

"Ventilated discs will develop small heat cracks with race use. These cracks will have no effect on braking. Discs must be replace (sic) when three or more adjacent radial holes are connected by cracks or the outside edge of the disc is cracked."

I have been following these "crack" replacement criteria for years with no disc failures. The holes are weak points in the disk. The heat and stress cause the cracks. The air cooling fixes will help, but they won't eliminate the cracking. Also not the inside of the disk has less cracks than the outside because the air flow enters there. The maximum cooling takes place while the vehicle is moving.

mds

Thanks for the quote. The cracks on my discs are oriented radially and bypass adjacent vent holes due to their circumferential offset around the disc. Had there been no vent hole offset then all of the holes would have been connected. Also, the cracks don't continue out to the edge of the disc. So I think based on all input they are fine to use. My digital camera will not focus well on the cracks so I haven't been able to post a picture.

enthusiast

Correct, those are still good disks. They will form cracks from hole to hole, but most crack from an outer hole to the outer edge of the disc first.

macfly

macfly, the rotors on the car were cracked even though they are slotted.

mds, these cracks, how bad were they, and do you think they would be as bad on a more road / track dual purpose car using slotted rotors?

mds

macfly, the cracks were longer, but not quite twice as long as those on my discs. Note that these discs are a floating design which offers better thermal stability I understand, and the cooling system is probably superior too, yet they still were cracked. Like my car the pads had relatively little wear. So slotted discs may offer some advantages but they are still going to crack eventually.

macfly

So slotted discs may offer some advantages but they are still going to crack eventually

Were they Porsche'sown comp discs, or were they some after/race market special thing?
Do you think they'd have the same wear out rate, or last a little longer?

What I want to know is with the same use-abuse as the racing discs you saw how would the drilled rotors have looked? Logic says the holes give the cracks a lot more places to start out from, but I haven't seen the discs you've seen, nor do I know what they've been through. Did you ask the team why they don't use drilled discs?

Greg Fishman

The GT3RS's come with Alcon slotted rotors. They are designed to last for 24hr races so I think it would be safe to say they have a better longevity than cast drilled rotors.

When I first started tracking my car, I was concerned with some cracks in my rotors and my mechanic just smiled and said that meant I was actually using them, nothing to be concerned about.

Johninrsf

Mike,
Here's a picture, though not a very good one, of my rotors after 650 track miles. You can see one pretty good crack, even though the focus is not great. How do these compare to yours?

mds

John, most every vent hole on my discs has two cracks, one going inward the other outward, each with a length similar to the bigger one on yours. There is also very fine, narrow cracking on the disc surfaces that do not connect to the vent holes, but appear in the space between them. Some of these are 15mm long. The outer parts of the discs, between the outer row of holes and the edge, are in good shape with little or no cracking at all.

Johninrsf

Mike,
FWIW, I only have a few cracks per wheel and braking style has included ABS pretty heavily at end of long straightaways.
I will say I'm pretty fastidious about doing a slow cool down lap without using brakes, though it's amazing how hot the rotors still are after the cool down lap!!