I've heard a lot of people say the BBK units are crap. Do yourself a favor and get the MAC unit if you are going to get one. And get a fenderwell one while you are at it. To me it would just be worse if the filter was right next to the engine sucking up all that hot air. Just an opinion though.
If you already have a K&N you probably won't notice any difference in power, but to me it's worth it to have one because they make the engine bay look so much cleaner.
Go here and check out their price on the MAC unit...
http://www.mustangwarehouse.com
That's a site I did for a guy out of New York and he has some pretty good prices on stuff.
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Indiana Stangers Association
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