mikymu

If you look at the street 7.2 GT3 front rotors and compare with 2010 Cup they look almost identical except for more cross drilled holes on the Cup and some sort of powder coat on the dome of the hat on street rotor. The parts number are only off by the last digit as follow:

Street rotor: 99735140991
Cup rotor : 99735140992

The minimum thickness spec on both rotors are off by 2 mm. Street calls for 32 mm and Cup has 30 mm stamp on the hat. I would assume they are made the same way with same material and has different requirement for minimium thickness when it comes to street vs race use?

I walked around the paddock this pass weekend at Laguna Seca ALMS race and look at bunch of Cup car. Some 7.2 Cup are using rotors from 7.1 Cup and some 7.1 Cup are using rotors from 7.2 Cup. The 7.2 Cup still use street 997 TT rear rotors as shown below.

So, my question is will the street 7.2 GT3 front steel rotor fit on the 7.1 and 7.2 Cup?? My guess is yes. I have a set of street 7.2 GT3 front steel rotor coming and it will be interesting to see how it fits. Any one with first hand experience?

Street 7.2 GT3 front steel rotor



997 Cup front steel rotor



2010 Cup rear rotor is the same as street 997 TT rear rotor

DJN

Hi Mike,

I am not sure I would put the 997.2 GT3 street rotors on your super "cup-mobile" (even if they fit with no issues). Note the drill hole difference, much bigger on the cup rotor. I found that the street rotor holes filled with brake dust very quickly.....effectively, cough-cough, choking the rotor's cooling ability.

Something to think about......

However, if the street rotors are a lot cheaper, you might want to drill the holes out bigger to "cup" size.

Have fun!
Doug N.

mikymu

lol

It is curious why there are more cross drilled hole on the Cup rotor compare with street. I much prefer slotted rotors over drilled since slotted rotors have less chance of forming cracks around the drill holes. Honestly I don't know the purposed of drilled holes and some how I don't think it is there for cooling since slotted rotors have no over heating issue.

Yes, street 2010 GT3 front rotors are about $300 cheaper a set. I have Porsche motorsports account and getting the Cup rotor is no problem but PMNA does not accept credit card and sending them a check, pay sales tax then shipping takes a long time. I can get Suncoast to ship me a set of street GT3 front rotors and the whole transaction can be done in 15 seconds. I figure if the street 7.2 GT3 rotors can stop a much heavier car then it should be fine on a much lighter Cup ...

24Chromium

The cross drilled holes serve two purposes on a rotor disc. One - it gives you more surface area, and therefore greater ability to radiate heat. The caveat, obviously, is that when cracks occur, they spread rapidly from hole to hole. Two - it allows the gasses that build up during braking a way to vent, and therefore makes the pad more efficient at slowing the car.

The slotted rotors are equally effective at item two, since the slots are machined so that even with pad contact, they can vent out beyond the pad surface.

24Chromium

It is interesting that the cup rotor and the street rotor are different in thickness by 2mm. Maybe the cup rotor is the older of the two and the common thickness then was 30mm? Or, maybe the factory wanted 30mm thickness to allow 1mm more pad thickness for each pad?

Anyone know the history? (sorry for being such a geek)

rlips

I think I must respectfully disagree. It seems that the more material you drill out, the LESS surface area is contacting the pad, and the less effective heat transfer from the pad to the rotor will be. I am not sure that these serve to ventilate effectively either, as they quickly get choked up with brake dust.

They are, however, very good at allowing cracks to form on the rotor. Of yet, I've never had the pleasure of wearing away a drilled rotor, they ALWAYS are taken out of service due to crack propagation well before they reach minimum thickness.

I guess the old saying is applicable... "crack kills!!!!" (rotors prematurely)

Cheers,

Ron

mikymu

Very cool insight into rotor design! You know this is why I call you professor right

If you look closely at the street 7.2 rotor, it has many drilled "dimples" that does not go all the way through which will serve to radiate heat as you mentioned. I spoke with Bruce at BR and he said the minimum thickness between Cup and street rotors are likely due to European standard difference. Most European rotor has 30 mm minimum thickness as shown on Cup rotors which does not have to comply with US street regulation

24Chromium

I didn't mean to imply that you got more surface area in terms of contact with the pad - obviously not. I mean more surface area on the rotor disc itself. Think of it in terms of a heat sink, you know, the type you might see on the head of an engine on an old-school motorcycle, with all the fins. Or, like on a heat sink for a computer chip. They have tons of thin fins in order to create the maximum amount of surface area. That's how they work to cool those things, surface area and airflow.

In simple terms, the rotor disc gets hot thru contact with the pad during braking. The heat built up in the rotor gets shed only by conduction to other parts (the hat, spindle, caliper, fluid, etc.) and via convection (radiated thru the air). Porsche uses air ducts to provide the necessary air flow to increase the efficiency of the heat convection.

Hey kids, listen up for my PSA! Don't pull the parking brake when you pull into your paddock fresh off the track. You'll likely warp your rotors.

TRAKCAR

PFC dimples only, nothing drilled through; No cracks after 14 days

mikymu

Holy crap! No large cracks like the one found on cross drill rotors. Street 7.2 GT3 rotors look promising despite of less drilled holes

TRAKCAR

My OEM front rotors after 15 days; I figure they are good for 20 days, but I changed the first set after 15 days so they would be a good backup set and the second set also after 15 days because PFC insisted I give their pads and rotors a fail try starting with both new. So now i have 2 backup sets, each good for a weekend or 2 or 3. I do plan to finish off one set (Really crack them) once the PFC's are done, probably in another 5-15 days.

TRAKCAR

OEM rears are good for at least 20 days, probably more like 30, so I stick with OEM for the rear.
These are 15 days, no cracks to speak of:

24Chromium

It's my understanding that these shallow dimples are for gauging rotor disc wear. If you've worn the rotor down so much that the dimple no longer shows, then you've worn out the rotor.

mikymu

Rear held up real well which is the same on my Cup car. Any difference in stopping power between OEM and PFC?

That's what I figure too. Just courious why the Cup rotors have so many more drill holes compare with street. I understand a race car may generate more heat in a track event but so does many street GT3 driven hard on the track and it is much heavier than Cup which means more heat will be generated ...

911rox

Hey Mikymu, those cup rotors on your car don't seem to be cracking (great thing!) whereas after 5 track days (2000kms) my 997.2 rotors are already showing some 5mm+ cracks on them... Admittedly I'm pitting only for fuel so they are doing 30-40 laps continuously... If you aren't getting cracking on cup rotors, I'd say they are the way to go...

I'm looking at PFc slotteds as a replacement at the moment... Just needing to identify what to order so I can continue using OEM pads for now...