We have examined the physical hardware of the injection system itself but not actually covered the induction system. With carburettors they are one and the same thing, but with injection systems they are separate. There are two basic types of induction systems used with injection - plenum-based systems with a single throttle body, and multiple throttle body systems that do not use a plenum but supply the inlet ports directly.
A plenum is a large chamber on the engine side of the throttle body that helps to even out the pulses in the inlet tract by providing a buffer of incoming air. This in turn can help economy and emissions and also provide a longer effective inlet tract which can help torque generation. The plenum is a convenient place to mount airflow sensors and vacuum sensors since it is at the confluence of all the inlet runners. When the engine is running, the throttle body determines how much air will flow into the plenum and therefore the engine. The plenum is generally in a condition of partial vacuum.
The EMS can maintain a good and clean idle by allowing more or less air into the plenum via a bypass valve called the Idle Air Control Valve. This together with a special idle routine in the EMS allows a perfectly controlled idle (and emissions) regardless of ambient conditions. The IACV works independently of the throttle body and bypasses its operation.
A throttle body is no more than a tube or barrel that regulates air into the engine's inlet manifold, plenum or inlet port. It is normally of tubular construction with a butterfly or throttle plate that opens and closes to regulate the airstream. Some throttle bodies have provision for mounting of fuel injectors, but others do not - it depends entirely on the application. The type of throttle body that feeds a plenum is normally a single body and it has no provision for an injector pocket. Throttle bodies are essentially like carburettors but without the float chamber or jets/venturis. Their configuration is often similar to carburettor configurations in that they are generally available as individual throttle bodies or twinned as dual bodies.
Individual throttle bodies
Performance induction systems normally involve the fitment of individual throttle bodies for each inlet port/manifold runner. Individual bodies can be aligned precisely with the inlet ports and this can give advantages. A system that provides individual bodies to each of the inlet ports should maximise the airflow potential for each cylinder and therefore help to improve the engine's performance. Sometimes these bodies are designed to bolt straight to the cylinder head for a particular application and can be designed to taper to an exact fit on the inlet port.
Dual throttle bodies
These perform the same function as the individual bodies but have two single bodies which are joined together with a fixed spacing between the individual barrels which may not be absolutely in line with the inlet ports. These are not unlike Weber DCOE or IDA carburettors in appearance. Often the difference in alignment between barrels and ports is negligible and so does not affect the performance of the engine. A set of dual throttle bodies is normally substantially cheaper than a set of individual throttle bodies. Dual bodies can often be fitted directly in the place of existing carburettors utilising the same manifold, air filters etc., which can bring down the costs considerably.