Here's something i pulled off autospeed
AutoSpeed: What are the advantages of drilled discs?
Steve Borg: They dissipate water and dirt, degas the pads (giving better braking distance) and also stay moderately cooler. But that's not an issue - I don't think - for street use. We guarantee our drilled discs to up to 700 degrees C - and they will withstand more than that - but you'd never exceed 450 degrees on the street. On the racetrack you'd probably get up to 600-700 degrees. Again, though, our rotors aren't sold as racing discs. And, of course, our drilled discs look the part - especially behind mag wheels. The slotted discs perform exactly the same and do exactly the same job - getting rid of the water, dust, gasses, deglazing the pads.
AutoSpeed: What are the disadvantages of drilled discs?
They are a little bit noisy - you do get a whirring noise. And, with our drilled and slotted rotors, you do lose about 10% pad life. In saying that, you gain in rotor life because the pad won't bite hard into the disc. If you let the pads glaze up, their material becomes harder than the cast iron of the rotor, and it then starts to bite in and create grooves. But, with slotted and drilled discs, that doesn't happen because you've always got a nice fresh pad surface running over the disc.
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AutoSpeed: We have seen several sets of cracked drilled discs - how do you explain this?
Steve Borg: Look, I keep myself fairly fit, but I'd be crazy to jump out of bed and do a sprint without warming up. It's just like you don't take an engine and go racing without warming it up. It's the same for rotors. They're cast iron, they expand when they get hot, and you really need to warm them up before working them. Abuse them when they're cold and they'll crack. You don't have to warm them 'till they're red hot, so long as the heat sink is right through the rotor and it's not just the outside surfaces getting hot, it's fine.