Porsche Rotors

The Porsche literature says Porsche does cross drilling of rotors for better water dispersal. There is some internet "opinions" that the holes are not cross drilled but in fact cast ... or maybe partially cast and then drilled completely through later. Does anybody have definitive proof, one way or the other (a pic would be nice). Or, can it be assumed it proprietary information and is not going to be released to the public and hence, the truth will not be known? Hearsay or internet opinions don't count. Definitive references would be nice.

I seem to remember an article on the matter, sorry can't give you the reference, that the rotors are not cross drilled but cast that way. If I remember correctly, the article said that drilling would actually compromise the structure of the rotor . :drivingskid:

Yes, if your read though p-car forums searching on brakes there are many "opinions" that the rotors are cast-in, but forum opinions are just that ... opinions. I'd like to find a definitive source that the holes are truly cast Same about the water dispersal thing. I've read that a bunch of times but no clear technical description of "why" its better.

Cannot say how Porsche does the brakes, but can shed a little light on why the holes are there. The holes are not for water escape, but to prevent a build-up of plasma (super heated gas) caused by friction of pads, which is a cause of brake fade under heavy use conditions. Pads will float on plasma and not get full grip on rotors without some sort of escape path (holes or grooving or both together). As for integity issues of drilling over casting, done correctly, which one might be able to assume Porsche knows how to do, drilling will not harm the structure of the metal.

But that is supposed to be only true for pads years ago. The use of cross drilled brakes for outgassing is long gone. Here is one reference Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." ...It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads. and another Rotors were first ‘drilled’ because early brake pad materials gave off gasses when heated to racing temperatures – a process known as ‘gassing out’. These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses ‘somewhere to go’. It was an effective solution, but today’s friction materials do not exhibit the same gassing out phenomenon as the early pads.

I believe holes these days are cosmetic. I have read from trusted sources that drilling does induce cracks and that is why they went to cast holes. From Cryogenics International (granted they're pushing cold treating disks): Drilled Vs. Slotted Some drilled holes in rotors are not really drilled at all. They are cast holes. This is done to help minimize the effects a hole has in creating a stress riser in the surface of the brake rotor . A drilled hole goes directly through and interrupts the grain structure of the metal. Where a cast hole has the grain structure formed around it in an uninterrupted flow. All holes in a brake rotor will eventually show signs of stress cracking. A drilled hole will crack much sooner than a cast one. Admittedly, a “drilled” rotor will cool better than a smooth rotor , and has slightly better “bite”. However, the person who blindly goes ahead and drills his stock rotors is asking for trouble, especially in high heat or severe brake conditions. It’s not unusual for these rotors to eventually crack completely across the surface, causing a dangerous situation. Therefore, it is not recommended to drill unless it is for an under-stressed condition or, for a street car just for looks. Study on Crack Propagation Mechanism due to Thermal Stress around Small Holes in One-piece Brake Disc Science Links Japan | Study on Crack Propagation Mechanism due to Thermal Stress around Small Holes in One-piece Brake Disc

That is good stuff I am looking for a definitive source that Porsche rotors are, in fact, cast in. A porsche website would be nice. All I hear is hearsay.

Although the porsche site under brakes for the Cayman , says little, if you go to the same section for the 911 , it says. "Superior braking in wet weather is facilitated by cross drilling the 911 Carrera ’s massive brake discs to dissipate water vapor pressure. Each disc is also internally vented for improved heat dispersal and reduced fade. Brake cooling is enhanced even further by an aero-dynamically optimized underbody lining and larger air ducts at the front of the car that redirect the jet stream directly onto the brakes." Assuming the brakes are not dissimilar, they are claiming dissipation of water vapor and not brake plasma, perhaps reflecting that modern pads do not have this problem as did the old. Knowing that Brembo makes our brakes, I reviewed their site. They refer to the disks as "cross-drilled". While I thought this might be a description of the look rather than the manufacturing process, and they do not precisely answer it one way or the other, I found this tidbit in their FAQ. "Are discs with cast-in-place holes better than cross-drilled discs? Brembo has extensively studied and tested cross-drilling versus casting the holes in place and found no significant effect on performance or durability."

Finding cracks between holes at the track last fall, I replaced my front with cross-drilled OEM rotors. Now, looking for rear rotors, I found that TireRack states that cross-drilled rotors are not recommended for track use, presumably since the holes tend to start and propagate cracks. I could go with ATE solid rear rotors with no holes. The holes quickly fill solidly with brake dust anyway, so they may not be real effective in removing water vapor, but why then does Porsche still use them on turbos, GT2s and GT3s?

Bling. Try out the Girodisc rotors from RSS, I have an article on them coming very soon.

Two quite credible sources: 1) From eBay Guides, as submitted by Brake Motive (an aftermarket brake component supplier), which is a summary of a 2006 SAE paper by 2 GM engineers after they tested drilled vs non-drilled rotors, and 2) AP Racing as submitted in a Formula SAE braking summary. 1) Interpreted by Brake Motive, from SAE paper 2006-01-0691 “The Effect of Rotor Cross-drilling on Brake Performance”: There are many claims as to the benefits of drilled vs slotted rotors on stopping power. This guide is intended to provide some facts about drilled and slotted rotors. As a member of the SAE (Society of Automotive Engineers), I was pleased to see a paper "The Effect of Rotor Crossdrilling on Brake Performance" by two GM engineers published in 2006. They examined three vehicle platforms with cross-drilled rotors vs standard rotors to measure convection cooling capability, fade characteristics, wet braking, pedal feel and lining wear. The result is summarized as follows: For the sports sedan, the coefficient of friction was 21% higher for drilled rotors than standard front rotors at 340F and higher using 15 brake snubs at 62mph. The track simulated 124 mph fade test showed 37% better brake output for drilled rotors. The drilled rotor brake temperature was about 150 degrees cooler. For the performance car, the coefficient of friction was significantly higher for drilled rotors especially at high temperature. Wet braking at high pedal pressure was the same for drilled or standard rotors. Wet braking is not significantly improved by drilled rotors. Pedal force was much more consistent with drilled rotors over the brake temperature range. That is, to stop at the same deceleration rate, the driver does not need to modulate pedal pressure based on different brake temperatures. This reduces driver fatigue and improves brake response. The authors also reported that drilled rotors prevent pad resin glazing on the rotor . So we now have solid evidence that drilled rotors have benefits over standard rotors. However, I have not found any published paper to show how slots affect brake output. So I reviewed inertial dynamometer tests using ISO NWI 26867 from Link Testing in Detroit with slotted rotors vs standard rotors. The results showed no significant difference in the coefficient of friction during the fade sections, hot stop section or pedal sensitivity portion of the test. My hypothesis is that slotted rotors do not contribute to rotor cooling whereas drilled rotors improve convection heat transfer to cool rotors and reduce brake fade. I should also point out that the pad lining wear for the slotted rotor was very severe during the test, i.e. the pad was chewed up over 20% more than the lining with stock rotors. While I believe that slots will help remove gas and debri from under the pad, I am not sure that this has a significant effect on brake torque for normal street driving. Perhaps the effect of slotted rotors is more significant on the race track, and conversely, I believe that drilled rotors are better for street and highway driving. For most drivers, I recommend drilled rotors over slotted rotors, and this conclusion is supported by the fact that Corvette, Ford GT, Porsche , Mercedes and BMW come with OEM drilled rotors. 2) This from AP Racing: Grooved discs. 1. grooves clean the pad surface and increase the brake faces (the leading edges of the grooves) so in theory the more grooves the more faces. 2. grooved discs can be used with any pad material as the disc is more durable than a drilled version. 3. discs life is longer than a drilled equivalent. Disadvantages. Disc is heavier than a drilled equivalent. Drilled discs. 1. Lighter weight than a grooved equivalent 2. Good brake bite due to: A forcing pad material into the holes B increased braking surface (many leading edges in theory) 3. allowing gases formed during the braking cycle to escape from the disc braking face. Disadvantages. Discs can be prone to cracking unless care taken when bedding in The use of hard race pads (high friction level) can cause cracking giving the disc a limited life. You will find today that most forms of car motor sport will use grooved discs.

Flying Lizard use grooved, enough said.

Seeing that my earlier reply didn't answer the OP's question: are they drilled or cast - I am going to see if there is definitive proof of that too.

I have looked for that answer for years and no one has yet to prove that anyone actually cast the holes. One aftermarket brake kit company is even offering to pay anyone that can prove that there are rotors with cast holes. It seems to me that it would be a nightmare to produce the molds necessary. AFAIK Porsche rotors are drilled and always have been.