Injectors

Primarily injectors are grouped into two categories (although there are exceptions);

• Low- 1.7 to 3.0 ohms
• High- 10 to 16 ohms

Most manufacturers have used both types at one time or another. The trend lately is to use high impedance types in most production cars. To determine injector impedance place a digital multimeter across the two electrical connections and measure the impedance (inductive resistance).

The main advantage of low impedance injectors is the shorter triggering time. When large injectors are fitted to high output engines, idle is normally effected due to the rise time of injector, so to combat this low impedance injectors will often give a better idle quality because of their short triggering time. The reason high impedance injectors are used by the OEMs is that they generate less heat in the drive circuit and often no external resistors are used.

Nozzle Types

Pintle

This is the most common type and still the best. A tapered needle sits on a tapered seat. When the solenoid is energized, the core and needle is pulled back, allowing the fuel to discharge. This design has been well proven for over 30 years.

Disc

The Bosch disc type uses the same type of actuation mechanism as the pintle type but replaces the pintle with a flat disc and a plate with tiny holes. These work fine with a good spray pattern but are slightly more prone to deposits plugging the holes. Dirty injectors will affect engine performance, fuel economy and emissions, so making sure they are clean is very important.

The Lucas type has the disc up inside the body to reduce the mass of the assembly for a quicker response. The Lucas types normally have a very narrow spray pattern which affects idle and throttle response in some applications.

Ball

The Rochester division of GM makes the ball type injector for OE applications as well as the larger flow race types for MSD. These use a ball and socket arrangement. These have excellent atomization and a wide spray pattern but are also prone to partial plugging by varnish deposits.

Electrical Connections

For the majority of injectors, there are two types of electrical connections. The D-Jetronic type used from 1967 to 1973 or so on Bosch injectors in which the plug fits internally into the injector and the L-Jetronic type in which the plug fits over the injector offering a waterproof seal. Most injectors of all brands built after 1974 use the later type. Unfortunately, several Japanese manufactures (e.g. Subaru and Toyota) decided to make their own style connector in the late 1980s which have an oval shaped plug. The D-Jet type will fit most types but is not waterproof, so it's better to retrofit the L-Jet connectors to avoid shorting issues.

Fuel Transfer and Sealing

Fuel transfer from the fuel rails was done with barb fittings and hose on early injectors as shown on the left above or small or large O-rings as shown above on the right. The later type uses a rigid fuel rail to hold the injectors down to the manifold as well as seal to the O-rings.

There are primarily two methods to seal the injector to the inlet manifold. The older style injectors used a square section O-ring slid over the pintle cap to sit against the steel injector body. The newer style injectors use a 14mm round section O-ring sitting in an isolated groove. Some new engines use side inlet injectors, (e.g Subaru, Nissan and some Ford/Mazda engines) that are not compatible with other types.

Flow Rates/ Pressure

Most OE injectors are quite small because stock power outputs are usually quite low on production engines and metering is more precise with small injectors for better idle and emissions. Very few production engines use an injector flowing more than 500cc/minute or 50lbs./hr. For performance applications, engines often require much larger injectors to satisfy the increase in fuel flow. Often larger OE injectors can be fitted from a different engine. Sometimes aftermarket ones must be used. It is essential that you have injectors large enough to feed your engine at maximum power. Most OE systems maintain a fuel pressure of between 2.5 and 3.0 bar over the intake manifold pressure. Fuel pressure can be raised to increase the rate of fuel flow but this should not exceed 4 bar in most cases. It takes 4 times the fuel pressure to double fuel flow. Raising the pressure to extreme levels is very hard on the pump and can lead to leaks or failures in the plumbing and injectors themselves. Use the proper flow rate for the intended application. If you plan to fit used injectors to a performance engine, always have them flow and leak tested first. If they are not in proper condition, the engine will never run well.