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Early injectors use low-resistance coil windings - typically 3 Ohms. Later injectors (now the majority or production injectors) use higher resistance - typically 15/16 Ohms. If a low-resistance injector is used with the Emerald ECU then a power resistor must be put in the earth line to the ECU.

This applies to many of the available aftermarket ECUs. If needed we can supply them on request when you order a system.

Fuel Injectors

A fuel injector is a simple solenoid which controls the fuel flow by rapidly switching a tap on and off. The longer the injector remains on the more fuel is injected. The duration of the 'on' time is controlled by the ECU switching one side of the injector to earth. The longer the ECU earths the current, the longer the injector stays on, so the more fuel is supplied.

The maximum flow of the injector occurs when it is switched on 100per cent of the time (constantly fully open). Injectors are graded by the amount of fuel they flow when fully switched on, in cc/min at a given fuel pressure.

In order to maintain correct control over the fuel delivery, the injector needs ideally to be operating up to around 80per cent of its flow range.

It is worth noting early on that injectors rated at full open do not always compare like for like at 80per cent of flow. Each injector has its own opening and closing characteristics. An injector which opens and closes slowly will flow more fuel than an injector with a quicker operating cycle.

At Emerald we use the ASNU cleaning and calibration machine. It is easy to see the effect of a slow opening and closing cycle by cleaning a dirty injector. Once the injector is cleaned the flow at a given duty cycle will show less than the 'before' flow test.

The injector open and close time has been reduced because the contamination is no longer slowing the action of the pintle down.

The next biggest factor in determining fuel flow is the base line pressure of the fuel. An adjustable regulator can be used to increase the base pressure – as long as the fuel pump and lines can keep up at high flow requirements.

The ASNU machine can flow test injectors at any required base pressure which is useful for determining flow gains at given settings.

In order to make maximum power, ideally what is needed is an injector which will deliver just enough fuel at 80 to 85per cent of the available operating time. Obviously this is rpm dependent and the Emerald software shows the duty cycle as a percentage in the map information box.

A bar graph on the mapping page also displays the current duty cycle which turns red at 90per cent and flashes when it reaches 100per cent. However, if this theoretically ideal sizing is for an engine with a lot of power the injector may be too big for idle conditions.

The more powerful the engine, the more this problem of matching at either end of the power range occurs. In this situation double injectors could be considered .

Eight injectors (four cylinder engine)

There are two reasons for fitting two injectors for each cylinder. Firstly two injectors may be needed, rather than one over-large one, to cover the flow range requirement of the engine.

In this case the Emerald double injector software can be programmed to bring in the second set of injectors when the first set of injectors reach a given percentage.

This percentage could be set at say 80per cent. When the first injectors reach this pre-set limit the second set come in. If required, unequally sized injectors can be used, perhaps with smaller ones close to the inlet valve for initial running with larger secondary injectors. This is ideal for turbo or pressure-charged engines.

The second method of switching is at a given rpm and load site. If the second injectors are a long way upstream, considerable throttle opening may be needed if the fuel is to be carried on the air stream into the engine, and not dropped out along the inlet manifold.

The advantage of putting the outer injector way upstream is that power is often gained from the cooling effect of the fuel, plus a better mixing of the fuel and air on the way to the engine is achieved. On a 200bhp engine we have typically seen an 8bhp power gain.

Physical differences

Most injectors from Bosch have a similar fitting system: an O ring seal on the end-feed which locates in a smooth bore fuel rail. Some injectors have a side-entry flow system (like the Ford Zetec). These need special fuel rails and are not easily adapted to aftermarket installations.

All the injectors have internal basket filters which are replaced during routine cleaning and servicing, along with fitting new seals and plastic caps. A set of four can be ultrasonically cleaned, serviced and flow tested for £60 plus VAT.

With new injectors costing upwards of £50 each, a considerable cost saving can be made by fitting re-claimed injectors.

Injector sizing

As a rough guide to power output, take the injector flow in cc/min and divide by 5 then multiply by the number of injectors.

Example: 220cc/min divide by 5 = 44 x 4 injectors = 176bhp

If fitting of double injectors is chosen, then the figure doubles to 352bhp.