I have found these references saying that the stock crank pulley circumference is 454mm and the alternator pulley circumference is ~166mm (taking the 53.9mm diameter and converting to circumference). This would give a ratio of ~2.7:1.  
This was the best that I could find. 
 Mantis versus BBI pulley size
Amazon.com: NEW ALTERNATOR PULLEY PORSCHE 911 SERIES BOXSTER 0-124-515-073 996-603-012-02: Automotive
		 
		
		
		
		
		
		
			
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				1999 996 C2 - sold - bought back - sold for more 
1997 Spec Boxster BSR #254 
1979 911 SC 
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