I just bought a 93Z, and had to buy a TB for it (Thanks 383TransAm). Unlike my TPI TB, which has no hole below the Throttle blades, the LT1 TB has a hole. It's like a bypass to the IAC it seems. I was wondering why. The 95 and 93 tb both have this, so I'm guessing all LT1 TBs are like this, but why?

Thanks,

D.J.

 

So they don't die when the blades are closed.

 

You can consider the bypass as a "baseline" idle airflow. It is sized so the IAC (Idle Air Control) stepper solenoid can help stabilize the idle (through little idle air passages drilled in the intake manifold).

This "bypass" hole is normally enlarged when a larger cam is installed and manifold pressure instability ensues. It allows the IAC to help "fine tune" (a relative term depending on the severity of the cam) the idle characteristics.