Fuel injectors are available in various styles and designs. They can be divided in two specific categories: top feed and side feed. The designation of top feed and side feed indicates how and where the fuel enters the fuel injector.

Side Feed Fuel Injectors:

Side feed fuel injectors are injectors where the injector body is surrounded by fuel. The fuel is fed through a hole or holes in the side of the fuel injector. This category can again be sub divided into two groups: throttle body and manifold.


Throttle body injectorSome of the older vehicles made use of throttle body fuel injection. Also known as TBI. The fuel injector is located in what looks like a carburetor right above the throttle plate. Depending on engine size and performance delivery, manufacturers made use of 1 or 2 of these TBI’s. Some of the aftermarket high performance setups used as much as 4 throttle body injectors.
Side feed fuel injectorIn general the fuel injectors are located in a common fuel rail which in turn is located at a point on the intake manifold close to the cylinder head. There is a top and bottom O-ring that forms a seal between the fuel injector and the fuel rail. There is also an O-ring or seal between the fuel rail and cylinder head or intake manifold.

Side feed injector setups tend to be more compact thus can be installed in a smaller space.

Top Feed Fuel Injectors:

Top feed fuel injectors are fed from the top of the injector and are divided into three basic sub categories. Hose end, normal top feed and direct fuel injection injectors.

Hose end fuel injectorSome of the earlier vehicles were equipped with a hose that connects the injector to the fuel rail. These hose ends were connected by means of a hose clamp to the fuel rail side, and a ferrule on the fuel injector side. Some hose end injectors had hose clamps on both the fuel rail and the fuel injector side. Age will cause the hose to become brittle and crack.

An O-ring or seal forms a seal between the fuel injector and the intake manifold.

Top feed fuel injectorTop feed injectors are the most abundant and make use of a common fuel rail. Although there is a wide variety of top feed injectors they all operate on the same basis.

The top of the injector is pushed into the fuel rail. An O-ring forms a seal between the injector and the fuel rail. Sometimes the injector is secured to the fuel rail with an injector clips. The outlet side of the fuel injector is normally located at a specific angle into the intake manifold. An O-ring or Viton rubber seal, forms an air tight seal between the fuel injector outlet and the intake manifold.

direct injection fuel injectorIn principle a direct fuel injector or GDI injector works the same than normal or manifold fuel injectors and that is where it ends. Direct fueling injectors also make use of a common rail and use a highly specialized O-ring setup to seal between the fuel rail and the injector.

Unlike manifold or throttle body injectors that inject somewhere in the intake tract, direct injectors inject under very high pressures, highly atomized fuel directly into the combustion chamber.

Side feed or top feed injectors operate at fuel pressures ranging from 38 Psi and up to 100 Psi in the event of some very high performance turbo charged applications. Direct fuel injectors operate at pressures ranging from 450 Psi up to 3000 Psi. Due to the environment direct  injectors operate in, they are more prone to carbon buildup than normal fuel injectors.

Marine, Inboard and Outboard fuel injectors:

Marine, inboard and outboard fuel injectors are divided in the same categories their automotive counter parts. Inboard fuel injectors range from the older throttle body fuel injector style to the latest top feed direct fuel injectors.

Outboard fuel injectors consist of side feed, top feed and direct injection fuel injectors. In addition to the standard injector group there is also the Ficht Ram Fuel injection system and the Evinrude E-TEC injectors.

Optimax air injectors

Some of the outboard and other personal water craft applications make use of the Optimax direct injection system which consist of an Optimax air injector and a fuel injector. In this system fuel is injected via a side feed fuel injector into the air injector. The air injector in turn injects additional air with the fuel directly into the combustion chamber.

Due to the environment they operate in, Marine inboard and outboard fuel injectors are prone to become dirty and or clogged. There is really no difference between 99 % of marine based fuel injectors and those used in the automotive, motorcycle or related industries.

Visual Inspection and flow testing is the only sure way to confirm that the fuel injector cleaning process is a success, anything else is only guess work.

Call us today at  1 (800) 381-2065 for all your fuel injector cleaning needs.