Basically, an electronic fuel injection system (EFI) works by delivering high pressure fuel to an electrically operated valve called an injector. Where the electronic wizardry comes in, is delivering the right amount of fuel, at precisely the right time for every engine operating condition, across the rev range. The electronic control unit (ECU) receives information from a variety of sensors, in and around the engine and makes the appropriate adjustments to fuel delivery to maintain perfect, preset air/fuel ratios.

The problem with original equipment EFI is that reprogramming for nonstandard applications involved creating a new chip for the ECU, a process beyond the capabilities of most engine tuners. Even if a modified chip was available commercially, it was still not possible to “tune” the injection system for further engine modifications. In the late 1980s a number of programmable aftermarket injection system ECUs arrived in the US, such as those manufactured by SDS, MoTeC, Electromotive, Haltech, DFI, Felpro, Split Second or Field (available through Group A Autosports). These systems can be programmed “on line” by connecting them to an IBM-type personal computer. They allow the tuner access to the fuel maps for idle, transition, wide open throttle, cold start, turbo boost enrichment, and in some cases, ignition timing.

Tim Suddard, owner and editor of Grassroots Motorsport Magazine, writing in an article about EFI states the following major advantages:

• The extreme accuracy of fuel delivery by the ECU, at any load or RPM, provides the engine with air/fuel mixtures that fall within a tiny window of accuracy required for maximum power, or maximum economy.
  
  
• EFI systems are not subject to the usual fuel surge and frothing associated with floats and float bowls in carburetors. One of the limiting factors in race car lap times has been the ability of the fuel system to deal with G forces. Gravitational forces in both horizontal and vertical planes have no effect on EFI systems.
  
  
• ECU control of air/fuel ratios allows racing engines to safely operate nearer to the “ragged edge”.
  
  
• PC programmable EFI can easily be adapted to suit future engine modifications as a vehicle evolves. Adjustments to fuel and ignition curves being as simple as making a few keystrokes on a PC.
  
  
• EFI generally permits greater flexibility of intake manifolds designed to achieve higher inlet air flow rates and consistent cylinder to cylinder air/fuel distribution.
  
  
• More efficient, higher compression ratios are usable, due to accurate fuel metering. This is especially the case with EFI units incorporating ignition control.
  
  
• When converting to forced induction, turbocharging, or supercharging, EFI will enable the user to program boost-relative enrichment easily, usually leading to substantial power increases as a result of accurate fuel delivery.
  
  
• Most EFI systems compensate automatically for changes in altitude and ambient temperature. Calibrating a fuel system for a specific race venue is hardly necessary with EFI, if adjustments are to be made, a few keystrokes on a PC are all that is necessary.
  
  
• Some EFI systems also have a provision for a cockpit-mounted mixture control with which the driver can vary the air/fuel ratio. TWM's HALMETER AF30 is particularly useful for this purpose, providing a visual, onboard read out of the air/fuel ratio with its 30 LED display.
  
  
• The solid state electronics in EFI systems are not susceptible to the mechanical failures associated with carburetors. Tuning parameters remain as programmed, with never any need to adjust for wear.

More explanations: Throttle Body FAQ