The best replacement filter without a doubt is the BMC F1 guys. It is a direct replacement. NO MAF problems because they don't slop on the oil from the factory like the K&N guys do. Adds a few Hp and increases the intake sound. I've put nearly 2k miles on mine since spring and it has been nothing but a slice of heaven. This is one of the best upgrades for the $$$ you can do for your boxster. MSRP $130. Did some searching and found this site that sells it for $70! I am very happy with the product and service.
http://www.eagleday.com/pobmcpefi.html
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