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Both BSR/BSX classes allow any underdrive pulley with a minimum diameter of 4". I am guessing that someone, somewhere calculated the change in belt speed and determined it was a good compromise between slowing the accessories while preserving coolant velocity. They have been well tested over the last 3 years in a lot of track miles with good results.
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2009 Cayman 2.9L PDK (with a few tweaks)
PCA-GPX Chief Driving Instructor-Ret.
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