Here are some pictures of how I modified an aftermarket 58 mm throttle body. This first picture shows what the throttle body looks like. Looking downward at the IAC transfer port there is no bleed hole. In this design all idle air must be metered by the IAC valve and the throttle blades. As you can see in the rear view, the IAC valve port is opened up to the main throttle blade area. Consequently all idle air is transferred into the intake plenum.
  This idle air design conflicts with GM's engineering. The stock throttle body seals the IAC air from the main throttle blade area. The stock throttle body also has a "bleed hole", or a calibrated orifice. This hole decreases the dependency of the IAC valve by allowing a certain amount of air thru the IAC passage. Most performance throttle bodies are missing this orifice. Honed 52 mm throttle bodies retain their stock design.
  My theory is this promotes better idle air distribution and therefore better idle quality. The front of the LT1 intake has three holes, two for the main air inlets and one small one for the idle air. This idle air goes to a common area, which in turn distributes to each port an equal amount of air. This eliminates the front cylinders from "getting" most of the air at idle and can also help atomization. The calibrated bleed hole allows the throttle blades to be nearly closed while not expecting the IAC valve to flow all idle air. With this in mind take note of the various size orifices in different engine displacements. The Caprice with the 4.3 L99 has a very small orifice compared to the manual 5.7 LT1. A high performance engine with a raised desired idle will need the calibrated hole enlarged to keep the IAC valve working as designed. Different idle RPM needs will necessitate attention to the orifice.
  On aftermarket throttle bodies I have modified them for better idle quality. All I have seen are missing the air bleed. Some throttle bodies have the IAC passage sealed from the throttle blade area. This one in particular needed both issues resolved.

To see my modifications, CLICK HERE.