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Different types of fuel systems


Information about the different fuel systems used. The most common type used is electric fuel injection (EFI) with top-mounted injectors that sit in the intake.

In order to be able to compare better, we divide the different systems into categories:

  1. Electronic fuel injection (EFI)
  2. Direct injection
  3. Carburettor


Electronic fuel injection (EFI)

Electronic injection is a category for several different types of systems that are controlled electrically. The biggest difference between the different systems is the type of injector and pressure used. Usually up to about 5 bar pressure is used, but sometimes more.

TBI = Throttle body injection.

An old system that uses up to about 1 bar in pressure. The injectors are located in the plenum of the intake. From here, the fuel follows the runner to each cylinder. The risk of uneven distribution is big.

Top mounted injectors

Used on European cars with pressure up to approximate 5 bar. Injectors are placed in the manifold runners close to the head. This way, the fuel is distributed evenly between cylinders.

Side fed injectors

Used on Japanese cars with pressure up to approximate 5 bar. Injectors are placed in the intake runners. This way, the fuel is distributed evenly between cylinders.


Both top-fed and side-fed injectors can, if the control system (ECU) allows, be controlled individually per cylinder. In advanced installations, several injectors per cylinder can also be used.



Direct injection

Direct injection actually belongs to the category of electronic fuel injection (EFI), but since the components differ significantly, this gets its own category. Direct injection works with a pressure up to about 140 bar, but sometimes more. The injectors are mounted in the cylinder head and inject fuel directly into the combustion chamber, hence the high pressure. There are often two fuel pumps, a low pressure pump that sits in the fuel tank and feeds a high pressure pump that supplies the injectors with the high fuel pressure. The high pressure pump is most commonly driven by a camshaft.




Carburetors are the oldest of injection methods. It sits like TBI injection on the intake plenum / inlet. From there, the fuel follows down the runners to each cylinder.

Several different varieties of carburetors are available with the "EFI carburetor" becoming popular as a replacement unit.
In such a unit, top-fed injectors that are electronically controlled are mounted. This way you get the control of an EFI system by just replacing the carburettor.