Function

• Throttle bodies are essentially air valves, and can take a number of forms. Most are butterfly valves, but barrel-type and pinch valves have also been used.
  
  

Precedents

• Prior to fuel injection, air delivery was modulated by a set of butterfly valves set into the base of an engine's carburetor.
  
  

MFI Throttle Bodies

• Most engines use Multi-Point Fuel Injection (MFI.) In these systems, the throttle body is responsible only for modulating air delivery.
  
  

TBI Systems

• Many older engines use Throttle Body Injection (TBI.) These throttle bodies resemble carburetors, and utilize one or a pair of fuel injectors mounted atop the assembly, which shoot fuel through the butterfly valves and into the engine.
  
  

Idle Air Control

• On most engines, the throttle body also contains a secondary circuit to control airflow at idle. This channel is called the IAC (Idle Air Control), and utilizes a computer-controlled valve to vary airflow.