The Edelbrock 2737 Throttle Body Adaptor is an 80mm LS1 to 90mm Elbow. This thin plate is similar in design to our 2732 carburetor adaptor plate. It's needed because the large 90mm opening on our 3837, 3848, and 3850 elbows exceeds the sealing area on LS1 style throttle bodies. It is included with the 3847 elbow.