What Is a Throttle Body?

What Is a Throttle Body?

Internal combustion gasoline engines require three things to operate: oxygen, fuel and a spark. On modern engines, fuel delivery and spark timing are handled by a computer controlled system of pumps and actuators. Air delivery is a bit more analog, which is where the throttle body comes in.

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.
Man working on engine
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Precedents
Prior to fuel injection, air delivery was modulated by a set of butterfly valves set into the base of an engine's carburetor.
Open hotrod engine
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MFI Throttle Bodies
Most engines use Multi-Point Fuel Injection (MFI.) In these systems, the throttle body is responsible only for modulating air delivery.
Car engine
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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.
Mechanic working on engine
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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.
Mechanic working on engine of car
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