4"OD to 3.25" ID

This reducing insert adapts a 4" ID intake hose to a 3-1/4" OD tube (or throttle body). You install it in the end of the 4" rubber elbow or hump hose, and then the 3-1/4" intake tube fits in the hole. Actual ID is 3.23", so it will be a snug fit and may require a little additional force to install.