the stock throttle body and port it out a little more than 1mm on this one in particular. They polish the lip for ultimate flow. I use the stock butterfly as to not take apart the rest of the throttle body (cause it is a bitch to put these back together without those brittle plastic piece going to crap.) I usually get around an 7-10hp gain on a stage III NA. I'm hoping for a little more on Tims (ie-thebigtman) TB's. They came out very nice.