It allows more airflow. It is a complimentary mod. If should be on of the last mods after headers, intake and some sort of manifold/cam combo. Then the added flow will be more useful.
Again, it isn't a magic mod, but it is a complimentary mod for cammed cars. On cars without cams, or a manifold or heads even, the TB will give you minimal, unnoticeable gains.
SOLD - 06 GT 371WHP/332WTQ
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Sold - 2001 Audi S4