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There was a lot of discussion on this over on the other board - most of those sedans/sports sedan's rotors are the size of the mk1s wheels!!! And they also have more weight to stop, and a lot more horsepower. Mercedes/BMW/Porsche are all cars designed for the autobahn too, and need to routinely stop from 150+ mph, whereas I've never had my mk1 NA over 130 in extreme cases. Also, most of their rotors are also thicker, and designed to be drilled.
If you consider that our mk1 rotors will warp just by having one wheel lug tighter than the others, removing material just makes them more prone to cracking, especially if you get them really hot, such as in track racing. I have found fine cracks in my stock Raybestos rotors after a good spirited... well, more than spirited... run over a closed forest service road with Porterfield R4 Race pads. Besides the speed and weight, we don't really have any kind of brake ducting to make the drlled rotors effective at cooling anyway. Also, the front rotor is already designed to move air through the core. Venting/slotting on the other hand can work, but compared to the thickness of porsche performance rotors our rotors are like a potato chip; if you take too much material out, you're going to severly weaken the structure of the rotor. Imagine just slotting 1/16" into a rotor face that is only around 1/4" thick - you just gave it a nice score line to break on. The brakes on many of the cars you mentioned are also designed to drilled/slotted rotors, and cost thousands of dollars. Ever hear how much a full brake job on a Porsche GT3 w/the race brake package costs?? Last I heard pads alone were $400...
Back to the question on hand. On the mk1, the rears are solid, so drilling really offers no gain for venting (if the pad is engaged over one side, the gas cannot escape between the two halves - it only hits the other pad). The slotting can work if you have pads that actually need to be vented such as Porterfields, and you actually use your brakes hard enough to warrant their use. Ask most people who track race their mk1s though. Considering that after one good race, the rotors are most likely going to be warped, you could spend $250 for decent quality slot/drilled rotors, or $60 for stock Napa rotors. I'd be willing to bet that Matt will never ever push his mk1 hard enough to warrant drilled or slotted rotors regardless of whether they were stock rotors, or a full custom brake package.
I hope this post makes sense - it's been an hour or so of occasionally writing while I'm at work
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