This is a quick tour of a Competition Carburetion modified Holley 4412 carburetor designed for competition use. Starting with a stock 4412 carburetor Bobby Oliver performs many modifications which are outlined here. If your rule state only "4412 carburetor" and don't outline what modifications are legal, this piece shown is where you should begin. This particular example was built for just such a rules package and while it does retain the stock venturi diameter it has a modified booster and enlarged baseplate along with a ported main body and a custom taylored racing metering package.
Here is the carburetor disassembled showing all of the main components. In this picture you can see the main body, fuel bowl, metering block, baseplate gaskets and screws.
Starting with the baseplate, if the rules allow it, you must do it! In this example the rules do not specify a stock throttle diameter so a new baseplate with 1-3/4 diameter throttle blades was used. The baseplate has been bushed and sized for smooth and precise throttle opperation. New butterflies are installed using rounded capscrews. The reason for using new and not modifying the old baseplate from the core carburetor is because with the larger throttle bores the idle and transision discharge holes must be moved farther away from the centerline of the throttle bore and therefore it's not practical to just "bore out" the old baseplate to the new size.
Peering into the carburetor body from the top shows the venturi as well as the new "modified" booster. When compared to a stock 4412 down leg booster the one pictured in the Super 4412 has a much larger hole through the center. When viewed from the bottom the modified booster shows off more of it's "tricks" with the carefully machined annular step clearly visible. This helps to improve fuel atomization, signal strength as well as mixture distribution on the engine without costing valuable airflow.
The metering block is specially calibrated for this carburetor metering package. Beside the obvious jetting and power valve selection the emulsion and air correction have all been taylored to the characteristics of the modified booster and increased airflow capability of the Super 4412. Speaking of airflow capacity, a typical 4412 500cfm two barrel with no modifications flows as advertised approx. 500cfm at 40.6 inches of water depression. With the modifications outlined above the Super4412 flows approx. 600cfm again at 40.6 inches of water depression for about a 25% increase in airflow a BIG improvment.